Multi-year always-on LED replacements for gaseous tritium light sources

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TritiLEDs are always-on battery powered LED glow lights for general night-time marking use. Radioactive gaseous tritium light sources (GTLSs) are allowed in the United States in several consumer product categories, including watches, compasses, and gun sights, but general-purpose markers are considered "frivolous" and are prohibited. Leveraging advances in LED efficiency, battery capacity, and microcontroller technology, TritiLEDs run from 1 to 25 years depending on battery choice, and while larger than typical GTLSs, can replace expensive and sometimes illegal tritium lighting in a variety of applications (including the frivolous). Version 2.2 of the design is complete and released under an MIT license.


This is a project born of frustration. As an amateur astronomer and astrophotographer, I often find myself stumbling around expensive and fragile equipment in the dark. A few years ago, I went looking for glow-in-the-dark markers I could attach to tripod legs and other gear to avoid costly accidents. I quickly dismissed traditional glow-in-the-dark (phosphorescent) materials because of their short half-life and the need to "charge" them before use. Chemiluminescent markers (glow sticks) are better in this regard, but are too expensive to continually replace, and end up in landfills after one night's use. Radioactive markers (especially tritium-based), with their constant glow and multi-year half-life, are technically almost perfect for this application: you can stick them to equipment and you're done. The problem with tritium markers is that they're not legal for manufacture or sale as general-purpose consumer devices in the United States (watches, compasses, and gun sights are exceptions). Even where they are legal, they are still quite expensive. This leaves electronic markers, but these often share similar shortcomings with other glow-in-the-dark technologies: constantly charging or replacing batteries, short-duration illumination, or high long-term cost. What I wanted was a small constantly-glowing LED that I could put somewhere and not have to service for a year or more. Unfortunately, most portable LED lamps are designed as flashlights - they emit a relatively powerful beam for a short duration (hours or days) per battery or charge. There's a race in the industry to make brighter and brighter lights. I wanted to go the other way: relatively dim lights, useful in dark situations, that maximized runtime.


Lithium battery powered devices are more dangerous than small tritium light sources, despite the NRC's paternalistic approach and the general public's fear of "radiation." According to the Energizer CR2032 datasheet, an ingested lithium battery can lead to serious injury or death in as little as 2 hours. Further, improperly stored coin cell batteries can short out and cause fires or burst and cause chemical damage. These batteries must be kept out of reach of children and pets at all times, and following Energizer's advice, batteries should be held in compartments with captive screws or other childproof latching devices. While the neat little glowing LEDs may look like a fun thing for a child to play with, children should not be allowed to play with these devices.

Version 1.0

TritiLED V1.0 is superseded by the much more efficient V2.x designs, and is no longer recommended. This older version uses a relatively expensive (but nearly ideal) Luxeon Z LED, and runs for a little over a year on a CR2032 battery (verified). Alternatively, it can run for 5 years (estimated) on a pair of lithium (Li/FeS2) AAA batteries, or 15 years (estimated) on a pair of lithium AA's. You can see the details in the project logs.

Version 2.x

Version 2.x improves on the earlier design by driving the LED at close to maximum efficiency using a simple inductor-based boost circuit. This design also features:

  • Selectable brightness / run-time trade-off (at program time)
  • Bright blinking modes (constant / fast blink / slow blink) with constant runtime
  • Inexpensive LEDs in many colors

Due to the higher electrical and LED drive efficiency, the V2.2 circuit will run for 5 years on a CR2032 (minimum brightness). The brightness can be increased at the expense of run time. Alternately, higher-capacity lithium cells can be used; I'm developing a derivative of this circuit in #Quarter-Century Lamp that's expected to run continuously for 25 years.

The V2.2 is considered complete, and may be the final design of the 2.x series. Details of the design can be found in the build log.


Eagle library for components used in TritiLED designs

lbr - 26.51 kB - 04/27/2017 at 19:46


JPEG Image - 1.28 MB - 09/08/2016 at 18:31



python code for modeling LiMnO2 and LiFeS2 primary cells running low-drain devices

x-compressed-tar - 4.28 kB - 09/03/2016 at 16:32



octave code and data files for calculating scotopic luminous flux for selected Luxeon Z LEDs

application/x-compressed-tar - 10.07 kB - 08/27/2016 at 15:39


initial octave code for LED current drive waveform efficiency optimization

x-zip - 1.74 kB - 08/16/2016 at 19:05


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  • V2.2 Release

    Ted Yapo07/12/2017 at 04:52 23 comments

    I am going to maintain and update this log as I add material on building these.

    UPDATE 20170716: fixed library and PCB to produce correct solder stencil

    UPDATE 20170713: added more info about the battery holder and inductor substitutions, with links to the original parts, and info about the case screws.

    UPDATE 20170712: added assembly instructions

    UPDATE 20170712: added the 3D printed case design

    I should have done this months ago - V2.2 fixed the reversed MOSFET of V2.1, and is basically a complete design, so here it is.

    I was going to re-do this in KiCAD, but I got frustrated with it again (third or fourth time), and just paid the $100 for a years worth of Eagle. The hardware and software design is in the GitHub repo.

    You can order TritiLED V2.2 boards and V2.x programmer boards from OSH Park. If you have an SOIC-8 programming socket, an SOIC-8 test clip, or don't mind temporarily soldering wires to the ICSP pads on the boards for programming, you don't need the programmer boards - they just let you connect the ICSP lines with pogo pins.

    The V2.2 circuit adds a 100-ohm resistor in the battery supply before the reverse-protection MOSFET as discussed in a previous log:

    Here's an assembled one with a Chanzon 3W cyan LED (right) next to a V1.0 with a Luxeon Z cyan LED:

    The light output from the V2.2 is much more than that of the V1.0, even though the surface of the LED on the V1.0 looks brighter - this is due to the smaller apparent area of the Luxeon LED. The V1.0 will run for a little over a year, while the V2.2 will run for over 2.5 years with the full software program. This full software includes blinking modes selected with the switch, as well as code which periodically re-initializes the PIC to re-write any RAM values that may become corrupt over time (e.g. from cosmic rays). A separate "barebones" software image can be loaded that dispenses with mode switching and periodic re-initialization, and runs at a slower 32 Hz blink rate to achieve an estimated run-time of over 5 years on a CR2032 cell. This software produces a somewhat dimmer output with noticeable flicker, but it is very usable as a marker. Both software versions are in the GitHub repo, and can be tuned to produce brighter output at the expense of run-time.

    Bill of Materials

    I need to add a BOM in the GitHub repo, but here are the components listed out

    The inductor and battery retainer are substitutions from the original parts I used. The battery retainer is a slight upgrade, being gold instead of nickel. The original nickel part is also available:

    (1) Linx BAT-HLD-001-TR CR2032 battery holder. DigKey part # BAT-HLD-001-TRCT-ND $0.73 each.

    I have not received any of the gold battery holders yet; from the datasheets they appear to be exactly the same except for finish material.

    The inductor is simply more expensive, but with the same size and specs: the old one is not stocked at DigiKey anymore. They are in-stock at Mouser:

    (1) Bourns SRR6028-102Y 1mH inductor Mouser part #652-SRR6028-102Y $0.68 each

    Again, I haven't tried the Wurth inductor on boards yet, but they appear to be identical parts from the datasheets.


    Read more »

  • CMOS Relaxation Oscillator Fail

    Ted Yapo07/10/2017 at 03:13 23 comments

    UPDATE 20170710: see below

    I have long considered trying to remove the microprocessor from the design, and decided to take a few minutes to test out some ideas. I had the parts for this experiment sitting in a box for over a year...

    The board has two 74AUP14 Schmitt-trigger inverters in SOT23-5 packages mounted on #Ugly SMD Adapters glued to the ground plane. The first inverter is used as a relaxation oscillator, while the second is used after a differentiator as a short pulse shaper:

    The 1n capacitor has a terrible tempco, so every time I read it, I get slightly different frequencies - especially if I had recently soldered the board, or touched the cap. The output of the first inverter is a nice square wave at around 65Hz (yellow trace):

    Zooming the time scale, we see the output of the second inverter (cyan trace) is a clean 12us pulse on the falling edge of the square wave:

    With a little tweaking, this would be a perfect waveform to drive the MOSFET/inductor boost circuit. Except for one thing: the circuit consumes 160 uA! This is without driving an LED, and is already an order of magnitude more than the "standard" TritiLED program running on a PIC. Craziness!

    The problem is that the inverters (74AUP1G14's) consume a lot of power when biased into the linear region, which the relaxation oscillator is always in (the culprit being shoot-through current between the high- and low-side MOSFETs). I was really surprised by this result because the 74AUP is advertised as the "lowest power" family, and the datasheet specifies 0.5uA quiescent drain maximum at 25C. With the feedback connection on the first gate broken, I actually can't detect *any* current used by the circuit with my DMM, which has 100nA resolution.

    The pulse shaper seems to consume a negligible amount of current by itself: now I'm wondering how to make a really low-power oscillator.

    For the time being, I'll stick with using a PIC15LF1571, but it's still fun to consider simpler options.


    UPDATE 20170710

    I tested the current drain on the first inverter. I removed the 10M resistor, and briefly charged the 1n capacitor by connecting it to the 3V supply. The capacitor takes about 30 seconds to discharge due to a combination of internal leakage and input current into the inverter. When the capacitor is charged to 3V, or fully discharged at 0V, I read zero quiescent current for the inverter (again, the meter resolution is 100 nA, so I assume the current is less than 50 nA). During the 30 seconds while the capacitor discharges, the current draw increases to a maximum of 220 uA, then falls back down to "zero".

    This is why they tell you to ensure than all unused CMOS inputs are tied to either Vss or Vdd. Being in-between can cause some hefty current drains.

    SECOND UPDATE 20170710

    @AlliedEnvy lent me a clue in the comments - reduce the supply voltage. The circuit oscillates with supply voltages down to 0.25V (at the temperature in my office, anyway), and seems pretty stable above 0.4V. Interestingly, the power consumption drops off dramatically with supply voltage. At 0.95V, it draws 1.0uA, while below 0.75V, the current drops below the 100nA threshold on this DMM. I'll have to drag out the good HP meter to investigate more closely.

    So, the shoot-through current in the middle of the input range is a strong function of supply voltage. To investigate this, I made this quick model in LTspice, using the N- and P- parts of and FDS9934 to make an inverter. I just chose those because it was the first complementary pair I found in the library:

    You can see how the shoot-through current gets reduced here. The top (blue) curve shows the current drawn from a 3V supply as the input voltage is swept from 0 to 3V - it peaks near 1.4V at 800 mA for these particular MOSFETS. The other color curves show the same thing for lower supply voltages stepped in 100 mV increments. By 2.3V, the maximum current is below 10mA. By 1.8V, it drops below 10 uA.

    Although there are still issues to be worked out,...

    Read more »

  • Safer TritiLEDs: One simple trick prevents catastrophic failure

    Ted Yapo06/28/2017 at 15:00 0 comments

    So, @Elliot Williams has been playing with similar ideas over on his #ATtiny45 EverLED project. There's some neat ideas over there, including the use of a different microcontroller, inductor sizing, and LEDs. One of the interesting additions he made was a pull-down resistor on the MOSFET gate to avoid the possibility of an un-driven gate being pulled high by whatever voltage fields are around. This can happen when the microcontroller output pin is tri-stated. If the MOSFET is held in a conducting state, the battery is shorted across the inductor and everything gets hot. Elliot mentioned that this happened for him during programming. Of course, the possibility also exists that during the lifetime of the circuit (which may be many years on a single cell), a cosmic ray strike or degraded flash memory could cause a similar problem with the microcontroller code, holding the MOSFET conducting, and possibly causing problems with excess heat or chemical leakage from the cell. It's very unlikely, but also very cheap and easy to protect against.

    When I re-used this circuit in #Decade Flashlight, I added a resistor in the power supply line to limit current in this case:

    The 10uF capacitor provides storage for the current pulses - in fact, the TritiLEDs run for a few seconds after the battery is removed just from this capacitor.

    The 100-ohm resistor limits the maximum current through the inductor and MOSFET. Assuming a 3V battery, the current is limited to 30mA, which will quickly drain a CR2032, but only allow 90 mW of power to be dissipated in the loop - that's low enough not to cause any damage. In practice, the internal resistance of the cell and the parasitic resistance of the inductor make the limit a little lower.

    So, it's safer now, but what about efficiency? The TritiLEDs consume from 4-30 uA or more depending on how they're programmed. Let's assume we tune one to about 12 uA, which will run for two years or more on a CR2032. The voltage drop across R1 is then V = IR = (12e-6)(100) = 1.2 mV. Since the current through the resistor and the rest of the circuit is the same, the ratio of power in the resistor to that used by the circuit is 1.2mV / (3 - 1.2mV) = 0.04%. This is ridiculously small, and essentially doesn't change the efficiency or brightness of the circuit at all.

    The addition of this resistor is one of the things I'm rolling into the 2.3 board. I've also added them to all of the versions I have made for friends. If you build some of these circuits, please consider adding a resistor in the power supply like this. The value isn't critical - in the above case, the loss is around 1% even with a resistor of 500 ohms.

    Sorry about the clickbait title :-)

  • PIC Programming Adventures

    Ted Yapo11/20/2016 at 04:40 0 comments

    I received a batch of V2.1 PCBs this week - this was the board where the reverse-battery-protection MOSFET was backwards. Rather than leave it out, I decided to use a 100-ohm 1206 resistor instead. This limits the current in case of reverse insertion, and protects against rogue PIC code holding the pulse MOSFET on. The resistor is actually a decent fit on two pads of the incorrect SOT-23 MOSFET footprint:

    This resistor is designed in as part of the V2.2 board, in addition to a properly-connected P-channel MOSFET to handle reversed batteries. I usually give boards a quick look under the microscope after reflowing them on the skillet, if only to remove the occasional solder blob, and everything on this one looked fine.

    When I went to program this board, though, I kept getting errors - the device ID was always read as 0,and if I persisted, programming failed. At first I thought that there was flux on the programming pads so the pogo pins weren't making good contact. I cleaned the pads with alcohol, then shined them with a pencil eraser, but no luck. I finally soldered wires to the pads to connect directly to a programming header - same problem.

    Then I started to wonder about that resistor. Here's how it looks in the circuit:

    The ICSP connections are shown as bubbles. For programming the TritiLEDs, I power the PICs from the programmer - they only consume microamps when they're running, so there's no problem. I think what's happening is that the Vdd to the chip takes a little while to come up, having to charge the 10uF cap through the 100 ohm resistor. This prevents the PIC from going into programming mode when the Vpp line is raised.

    After some searching, I found the MPLAB IDE setting for "Apply Vdd before Vpp" with the PICkit3. With that set, the board programs just fine. Elapsed time to debug problem: 3.5 hours. This included some very close observation of this new batch of boards - discussed below. The Vdd programming pad is on the other side of the resistor on the V2.2 board, which is why I didn't see the same problem there.

    There's another problem with these boards besides the reversed MOSFETs. The boards have a HASL finish - this can be a problem for battery contacts - with the CR2032 holders I'm using, one of the contacts is a pad on the PCB. Memory Protection Devices, which manufactures many coin cell holders, recommends nickel or gold finish for these contacts in their application note. I think I'll have to pay for ENIG going forward. It's not as good as "hard gold," the thicker coating usually used for edge connectors, but it's probably much better than HASL. Even if the thin ENIG gold were to wear off, there's nickel underneath (some thickness, anyway). Another reason to use OSH Park.

    I actually got two batches of boards in the mail this week - one was the V2.2's from OSH Park, which looks like it may be the final PCB design for version 2 - I'd like to assemble a few more of them and run some more tests before I declare victory. The other was the V2.1's from This was my first experience with this service, and they seem OK - quality maybe a little worse than iTead (not even in the same league as OSH Park), but they're cheap and they warn you over and over that their boards are dirty. You can see the mis-alignment of the soldermask and silkscreen in the image above - look at the programming pad (now blobbed with solder). They do offer your choice of soldermask color for no extra charge. Even though the electrons don't care sometimes you do.

  • These LEDs are too blue...

    Ted Yapo11/09/2016 at 01:23 0 comments

    So, in the quest for the Goldilocks LED, I decided to measure the spectrum of the two Chanzon 3W LEDs most likely to be the best for nighttime visual markers: cyan and green. My first impressions of the cyan LED was that it was significantly more blue (shorter wavelength) than other "cyan" LEDs I have seen. To get to the bottom of the issue, I dusted off my Project STAR spectrometer I bought years ago and coupled it to a webcam:

    Also shown is the fluorescent tube I used for calibration, producing the following image:

    Calibration for this spectrometer consists of sliding the transparent plastic scale to line up the bright mercury emission line at 546.6nm with the calibration mark. Additionally, you can see that the other strong lines end up where they belong.

    Here's the spectrum for the cyan LED:

    It shows a peak around 495 nm. The "specifications" for this LED state between 490 and 505 nm (no binning; you get the luck of the draw), the latter being almost ideally aligned with the dark-adapted eye sensitivity curve. I was hopeful that these numbers represented the spectrum at high current, accounting for the blue-shift seen in InGaN LEDs, so that at lower currents, they would emit longer wavelengths. Even so, this LED is still the best of the bunch for visibility in the dark.

    Here's the spectrum from the green LED:

    The peak here is around 520 or 525 nm, which as discussed before represents a good trade-off between day and night vision sensitivities.

  • Version 2.1: 5 Years on a CR2032 in 19 Different Colors

    Ted Yapo10/31/2016 at 21:05 18 comments

    Here's the latest version using the inductor boost topology, along with the matching programmer board. With a barebones program loaded, this particular unit should glow for almost seven years on a CR2032 (consuming 3.6 uA) - it runs for seven seconds after the battery is removed just from the 10uF of capacitance on the board. Adding code for blinking modes (selected with the button) and some "safety" instructions which periodically re-initialize RAM in case of radiation-induced soft errors brings the runtime down to just over 5 years.

    OK, now the fine print. Three of the 19 "colors" are infrared, so you can't see them with the naked eye. However, they work great as markers with a cheap Gen 1 night vision scope (which I'm certain is bombarding me with X-rays every time I use it). Another "color" is ultraviolet, which should be useful for charging phosphorescent materials, but I haven't tried it yet. Finally, the barebones program has a 32Hz blink rate, which is noticeable in the daytime (surprisingly, the flicker isn't noticeable in the dark, at least to middle-aged eyes). Increasing the blink rate to 64Hz eliminates the flicker, doubles the brightness (to around the level of the V1.0), and increases current draw to 9.1 uA, or about 2.7 years on a CR2032. Further brightness increases come with correspondingly shorter battery life.


    I'm currently testing the hardware and software. I know at least a V2.2 will be required, since there's a hardware bug in version 2.1; the reverse-battery-protection p-channel MOSFET source and drain got reversed somehow (I should fire my PCB designer). Interestingly, the device still works, except it offers no protection from reverse battery insertion. Unfortunately, I bought a bunch of boards already, but there's enough room on the board for a quick re-work, or the MOSFET can be omitted entirely.


    I added programming pads on the board to mate with a matching programming adapter board (see below). This is the first time I've used pogo pins, and they're working great. I thought there would be some issue with aligning the boards during programming, but it's very easy to hold them together. Right now, you need a Microchip programmer to plug into the adapter, but I'm considering using @jaromir.sukuba's Arduino-based PIC programmer idea to eliminate the need for a separate programmer. I added a hole in the top so you can see the LED during programming.


    This project was called "TritiLED" because I wanted to make substitutes for GTLS tubes. Since the 5-year version is less bright than the previous versions, it seems best to evaluate it in the original application. After all, you can just keep turning down the brightness to extend the runtime. Here it is compared with a 150 mCi (that's milli-Curie, not micro) tritium compass manufactured in 2016 and a LumiNox watch from 2015. The compass contains the brightest GTLS tubes I have ever seen, and the TritiLED is still many times brighter. It is certainly functional as a nightime marker.

    With the lights on, the glow from the compass and watch has disappeared, but the TritiLED is still visible:

    There's no need to keep the light dim, though. The Maxell CR2032 datasheet lists the "nominal" discharge rate at 200uA (1000-hour rate), which is about 40x as bright as the one shown above. With larger cells/batteries, this circuit can produce a maximum of about 2000x as much light, which would be around 10mA drain (about 350 hours from 2x lithium AA's).

    Circuit Design

    Here's the design for version 2.2:

    Q2 protects the circuit (and battery) in case the power is connected incorrectly - it only allows current to flow with the proper polarity. Unlike a simple blocking diode, the MOSFET introduces almost no voltage drop in this circuit, maintaining high efficiency. The PIC12LF1571 sleeps most of the time, periodically awakened by the internal watchdog timer to pulse the LED. GP4 and GP5 are paralleled to provide more current drive to switch...

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  • Intermission

    Ted Yapo09/03/2016 at 23:09 0 comments

    I'm taking some time off from this project, and mostly wanted a placeholder so I can remember where I left off - I have a bad habit of "taping over" memories from suspended projects. So, this is just a jumble of thoughts I'll update as they come until I start this thing back up.

    The Energizer Cylindrical Lithium Primary Battery Applications Manual has some great info about this unique battery chemistry, and discusses the two-stage discharge seen in the graphs, temperature performance, etc. The only thing I don't like about these batteries is the need to use spring contacts, which are notoriously unreliable. I would like to spot-weld tabs on them. A homebrew spot-welder may be in my future. I guess I should ask Energizer if they produce tabbed versions first.

    The previous lumens/watt calculations I did for the Luxeon Z LEDs use the datasheet-reported output at 500mA, so they're a valid comparison assuming the three LEDs share a similar efficiency curve, which is doubtful. I need to actually run the tests to compare these LEDs properly.

    I should compare the battery model to back-of-envelope calculations. I should also add a model for ultra-capacitors, which would be trivial. Of course, just running with a capacitor (if you have one) would be a quick test, since the energy density is low.

    The V1.0 design isn't protected against reverse battery insertion, which is easy to do with coin cells. This paper from Ti discusses adding a MOSFET to protect the circuit with minimal loss. Assuming I end up using a MOSFET for the LED driver, I might be able to find a dual version cheaper than two individual parts.

    I didn't think to compare the efficiencies of different LED colors - it turns out that green LEDs have traditionally been less efficient than other colors, although this information is probably dated. It would be interesting to survey the field and see where the most efficiency (electrical-to-eye-sensitivity) ones are. The luminosity curve probably dominates here, so even inefficient LEDs near the eye sensitivity peak win.

    It might be interesting to consider smaller batteries. The CR2032 has decent capacity and is widely available, but is a little bulky compared to the other components (at least until I start using larger LEDs).

    I need a way to make some simple absolute intensity measurements. All the data I have been able to collect so far for LED intensity has been relative. If I can measure the absolute output at even one current setting for an LED, I can calibrate the relative intensity measurements. This would allow comparison of the intensity of different LEDs. I think I'm going to build an integrating sphere with a 12" hollow styrofoam sphere and barium sulfate paint. It should be easy enough to get a reading for the total output at high current, which will be enough to calibrate the whole relative curve.

    Jim Williams wrote a few applications notes detailing his designs for very efficient fluorescent LCD backlight drivers. The technology is different, but the idea of measuring and optimizing electrical-to-optical efficiency is the same. There are probably some good clues in there.

  • Estimating Battery Lifetime

    Ted Yapo09/02/2016 at 22:16 0 comments

    I've created a model for common lithium primary cells to predict the run-time for very-low-drain devices. Here's the estimated intensity of the V2.0α circuit run from a CR2032 cell. The light lasts for over two years, with the brightness only dropping about 10% in the last six months.

    To estimate the performance, the first thing I did was make a better prototype, shown here consuming less than 13uA from a not-quite-fresh CR2032:

    This prototype uses an SMD 1mH inductor, which was glued to the top of a V1.0 PCB. In this case, the PCB just acts as a mount for the Luxeon Z LED, which has to be reflow soldered - no other components of the V1.0 circuit were populated. The circuit is the same as shown in the previous log, using a PIC12LF1571 and a ZTX618 transistor as switch. No RC snubber network is used. This breadboarded version uses almost 1uA less than the solderless-breadboard one. I haven't directly compared the brightness yet, but it seems similar. The difference may be due to the new SMD inductor vs the previous through-hole one.

    LiMnO2 Battery Model

    I've been using a simple model for LiMnO2 batteries (the "CRx" series like CR2032's or CR123A's) for some time. The model seems to work pretty well under the following assumptions:

    1. current drain is averaged, by capacitance if required, to near-DC: no strong current pulses
    2. average drain is less than 0.0005C. This is around 100uA or less for a CR2032 or 800uA for a CR123A.
    3. temperature is assumed to be constant around 20C
    4. battery shelf-life is not exceeded. The model considers the battery dead after the shelf-life has elapsed, regardless of the predicted remaining capacity. Shelf-life for LiMnO2 batteries is commonly considered to be 10 years, while that of LiFeS2 cells (lithium AA and AAA) is 20 years.

    I based the model on the following curve from the Maxell CR2032 datasheet, which shows the cell voltage vs used capacity for a 1M resistor (solid line; ignore the lower pulsed discharge curve):

    I digitized this curve and wrote some python code to model the battery from this data. The battery object is initialized with the nominal cell capacity (in mAh), so that different sized cells can be modeled. Three functions complete the battery model:

    • battery.dead(): reports true once either the remaining capacity drops to zero or the shelf life is exceeded
    • battery.voltage(): returns the cell voltage based on the remaining capacity using interpolated digitized data from the above curve
    • battery.use_amp_seconds(amps, seconds): decreases the remaining capacity of the cell by amps * seconds

    The model is intended to be used in an integration loop, as shown in the following code for simulating constant-resistance discharge. At each time-step (chosen to be one hour here), the battery voltage is estimated based on the remaining cell capacity. The current draw of the device (in this case a resistor) is evaluated given the source voltage, and this current is used to decrease the remaining cell capacity. The loop continues until the battery reports dead.

    #!/usr/bin/env python
    import sys
    import Li_battery
    # constant resistance discharge
    R = float(sys.argv[1])
    # integration time step in seconds  
    timestep = 3600.
    battery = Li_battery.LiMnO2(220.)
    t = 0
    while not battery.dead():
      voltage = battery.voltage()
      print t/3600., voltage
      current = voltage / R
      battery.use_amp_seconds(current, timestep)
      t += timestep

    Using this program, I was able to re-create the following plot from the datasheet for constant-resistance discharges:

    The curves match very well, and the model is able to re-produce the expected run-times and voltages with two exceptions. First, the predicted voltage for the 15k curve is too high - but, this resistance results in a nominal drain of 200uA, or 0.0009C, which exceeds the valid range of the model (violates assumption 2 above). For high-drain devices, which deplete the battery relatively quickly, you might be better off just testing the real device than trying to model...

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  • Version 2.α

    Ted Yapo08/30/2016 at 18:33 0 comments

    I breadboarded a first cut at a more efficient circuit this past weekend. Consensus is that it appears a little brighter than the 1.0 circuit (not measured with the camera yet), and a back-of-the-envelope estimate says it will run for two years on a CR2032 battery (a better analysis is coming in the next log). It consumes 49% less current than V1.0, and can be made to blink bright flashes instead of a constant glow while maintaining the same current draw. The circuit incorporates some good ideas suggested by Jaromir and Tony. Here's the mess of a prototype:

    Pulse Circuit

    The design started with an analysis of the efficiency of sawtooth current drive waveforms for the Luxeon LED, as I did for a 5MM LED earlier:

    With sawtooth waveforms like those produced by the inductor-LED circuit I had simulated before, the best LED efficiency is seen at around 13 mA peak current. After a little experimentation with PIC clock speeds and inductor sizes, I came up with this circuit that generates peaks of around 15 mA (roughly estimated from peak LED forward voltage and the measured I/V curve):

    The design is based on discussions I've had with Tony about single-shot boost converters, and it works pretty well, as long as the LED doesn't have a reverse-protection diode inside (this one does not). A 1 mH inductor is huge for a switching power supply, but all of the four different through-hole styles I had in this value worked in the circuit to some degree. One was superior, though - I will need to find an SMD version with similar properties. As discussed earlier, pulses from the PIC turn on the transistor to magnetize the inductor, then the inductor "discharges" through the LED to create the light pulse. For these tests, I used a modern PIC as suggested by Jaromir. So far I've only used a PIC12LF1571 I had on-hand; I am also going to try a PIC12LF1552 which is supposed to have a slightly lower sleep current.

    The ZTX618 is an awesome transistor (at a premium price), with super-high beta and super-low Vce(sat) - evidence of this can be seen in the waveforms below, where the collector of Q1 drops to 38 mV during the pulse. The high beta allows a high value for R1, limiting the wasted base current. D2 is critical to achieving high efficiency with this circuit - without it, Q1 turns off slowly, wasting power at the critical peak inductor current. Here are the waveforms observed at the input, driven by a single PIC output pin (channel 2, cyan), and at Q1's collector (channel 1, yellow). The supply voltage of 3.04 V (channel 3, magenta) is shown at the same offset as channel 1.

    Two things are immediately apparent. First, you can see the trick to lighting LEDs with these small current pulses - use multiple pulses every time the PIC wakes from sleep. Here, the PIC is waking every 16 ms to output two pulses. Waking every 8 ms to output a single pulse is less efficient because of the overhead involved in starting the PIC clock each time. Waking less often to generate more pulses is even more efficient, but the blinking becomes noticeable. Push this far enough, though, and you get a very brightly flashing beacon at the same power. Pulsing the LED 128 times every 1 second produces a nice attention-grabbing display using around the same average current.

    The second thing evident is that the circuit rings like a bell after each pulse. The delay between the pulses is actually chosen to coincide with the ringing, so that the next pulse starts on the down-swing of the oscillation. Fine tuning this delay seems to add a few percentage points of efficiency. You can see the spikes in the spectrum at 145 kHz and the second harmonic at 290 kHz here:

    A PCB made for this circuit probably wouldn't make an efficient radiator for these LF frequencies, but since one of my hobbies is hunting LF navigation beacons, I wanted to clean this up. I had previously dealt with this kind of ringing in another circuit following the advice in the Ti Applicaton Report SLPA010 "Reducing...

    Read more »

  • Calculating Scotopic Luminous Flux

    Ted Yapo08/27/2016 at 13:17 0 comments

    I'm working on a new circuit (really!), but did these calculations last night, and thought the results were interesting.

    LED datasheets often quote the luminous flux in lumens, based on the daytime visual sensitivity curve (photopic vision). This is calculated by integrating the LEDs spectral radiant flux with the photopic luminosity function, and reflects how bright the LED appears to daytime-adapted eyes. For nighttime marker use, we are interested in "scotopic lumens" which would more accurately measure how bright the LEDs look to dark-adapted eyes. I scraped the LED spectral curves from the datasheet and with some numeric manipulation in an octave script, calculated the scotopic luminous flux for the three Luxeon Z LEDs I have been looking at:

    Color Photopic Lumens (datasheet) Scotopic Lumens (calculated) lm/W (photopic) lm/W (scotopic) Electrical Power (W)
    Cyan (LXZ1-PE01) 68 782
    46 528
    Green (LXZ1-PM01) 108 618
    71 407
    Lime (LXZ1-PX01) 199 642
    142 459

    The datasheet lists the output at 500 mA, so I multiplied by the forward voltage (from datasheet curves) for each of the LEDs to get the input power.

    The lm/W (electrical watts) columns in the table are the most interesting. For true dark-adapted vision (like in a cave or really good astronomy site), the cyan LED wins at 528 lm/W - but, this LED is poor for daytime vision. During the day, the lime LED is far superior to either of the others at 142 lm/W - and it takes a close second place behind the cyan at night. If you're making a glow marker and want it to be generally useful in all lighting conditions, the lime LED seems like the best choice. The only downside: it costs $4.20 vs $2.76 for the others. The marketing department over at Lumileds has their act together.


    Here's how I calculated the values in the table. Math and/or photometry nerds may want to check my approach; others may feel the desire to skip this entirely :-)

    The datasheet provides the photopic luminous flux (in lumens) and a normalized spectrum for each LED. I digitized the spectra with Engauge Digitizer, shown replotted on the same graph here:

    The datasheet lists the photopic luminous flux, , for each LED in lm. This is calculated by weighting the spectral flux, , by the dimensionless photopic luminosity function, :

    To calculate an analogous scotopic luminous flux, ,we can perform the same integration, this time weighting by the scotopic luminosity function, :

    This would be straightforward, except that the spectral flux isn't given directly in the datasheet. However, we can calculate the spectral flux from the given luminous flux and the dimensionless (normalized) LED output spectrum, which I'll call . The spectral flux is proportional to the normalized spectrum:

    Substituting this, we can solve for the constant C:

    and then evaluate the scotopic luminous flux:

    The raw data files and the octave script I used to perform these calculations can be downloaded here.

    Update: writing this log revealed a scaling error in the original octave code; I've re-run the numbers and updated the code and table.

View all 24 project logs

Enjoy this project?



bulrush15 wrote 3 days ago point

Ted, would you consider selling a kit we can solder together? I don't know how much shipping is now for Digikey but it's normally outrageous for companies like that. 

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Ted Yapo wrote 3 days ago point

Digikey will ship packages under 8 oz for $3.39 by USPS first class mail, and 1 lb by USPS priority mail for $7.50.  I've used both, and you get a *lot* of SMT components per pound.  You have to order the OSH Park PCBs in multiples of three ($6.15/3), so let's say you order enough parts to build three of them.  That costs maybe $26 with shipping from DigiKey.  So, you might pay $33 for all the parts for 3 of them - $11 each, including shipping.  Rough numbers.

The rule-of-thumb for pricing kits and assembled units is to mark up costs by 2.5x.  If I buy the parts in 100-quantities and order cheap Chinese-made PCBs, I could probably drive my cost down to $4.50 for a kit (guess) - not including any labor, packaging, etc.  A mark up of 2.5x puts my selling price right at $11, *plus* shipping, minimum, assuming my time costs nothing.  You might buy from me for convenience, or because I'm a nice guy with a winning personality and a killer smile, but you wouldn't save any money.

I don't know how many will sell, so I'm not going to buy 1k units to get the next price break (approx $4k outlay), so I can't really do better than that on parts.

Then, next April when tax time rolls around, I've got a bunch of accounting to do - and accounting for manufacturing in the US is absolutely fucking ridiculous. I'm willing to bet that nobody does it correctly at the small scale (including tracking inventory - counting all those $0.01 0603 resistors you haven't used yet).  Manufacturing in the US is so impossibly over-regulated that to do it on a small scale, you are basically forced to break the rules (IRS, FCC, EPA, state nonsense...), or spend a crazy amount of time and money on overhead costs.  A younger me might not have cared; at this point in my life, it doesn't seem worthwhile.

Having said that, the design in GitHub is released under an MIT license.  If someone else wants to sell kits, or PICs pre-programmed with the firmware, or assembled units, they are free to do so, subject to the license.  I recently had someone ask if it was OK to do this, which was genuinely a nice gesture, but totally unnecessary.  I've had my rant about why it doesn't make sense for me - but what do I know - this might turn out to be a nice side business for someone else. 

Edit - I forgot about the LEDs - $7.90/10.  So, adjust the above numbers accordingly - same basic outcome.

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bulrush15 wrote 6 days ago point

Have you considered these low-power circuits for powering an LED? From Don Kipstein, the guru of lighting. I thought it might provoke a brain storm.

Another interesting link, ultra-low power circuits:

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Ted Yapo wrote 6 days ago point

There might be something to the first link.  It's interesting stuff, but not knowing how bright he's trying to go for, I'm not sure how to evaluate it.  The basic idea of using PWM at more efficient LED currents is the same, just achieved by different means. Other than that, a few points come to mind:

1. It's a higher voltage source 9V in some cases, 5 in others.  I'm not a big fan of 9V batteries.  They're huge, and even though they're available in LiFeS2 chemistry, the energy density is still lousy.  

2.  The "excess" voltage seems to be dropped across a resistor.  This jumps out as inefficient, but maybe not that bad.  This also won't work when the battery voltage dips below the Vf of the LED.

3. Because of the 9V supply vs 3V, you need to multiply the quoted currents by a factor of 3 to compare with operation from coin cells.  The 45 uA current at 9V is equivalent to 135 uA at 3V from a power standpoint.  This is more than 10x the power of the standard V2.2 TritiLED.  Again, I don't know how much light the two put out compared to each other - maybe I need to build one of the circuits from that link.  On the other hand, I think the TritiLED output is probably sufficient for the intended purpose as-is.

4. Just as a point of reference, the circuits described on that page consume around 40uA from a 9V source.  A 9V LiFeS2 battery has about 750mAh of capacity, so would run such a circuit for about 2.2 years.  This is about 6 months shorter than a V2.2 TritiLED runs off a CR2032.  Again, no idea about the brightness comparison, but a 9V battery is huge compared to a CR2032.  The TritiLED would theoretically run for 47 years on 2x lithium AA cells (about the size of a 9V) - but the battery only has a shelf-life of 20 years, so let's say 20+ years.

The second link has some interesting circuits.

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crjeder wrote 07/11/2017 at 14:27 point

Just read about your Project with great interest. Thank You for posting this online!
I can't get one thing out of my mind, though: wouldn't an electroluminescence device work better for efficient dim lighting than an LED?

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Ted Yapo wrote 07/11/2017 at 14:49 point

I haven't really explored EL devices for this application, but my understanding was that modern LEDs had an efficiency advantage - and they're constantly improving, driven by the LED lighting industry.  I can't find solid efficiency data though.  My understanding was that LEDs were now the efficiency king, having surpassed fluorescent tubes some time ago.

The short half-life of EL devices (quoted as 3k hours here: ) would be a concern. I've seen this happen with plug-in EL night-lights; they don't last very long. The shortest-lived markers I'm considering these days run for 2 years (18k hours), which is (6)  3k hour half-lives - I'd expect the EL to be completely burned out by the end.  You might get longer life at lower current, but the same is true for LEDs, which get a 50-70k half life estimate at nominal currents.  On the other end, I have some devices intended to run 10 years, and am playing with ideas about 20-25.  I suspect that LEDs have a huge advantage here.

There's the necessity of high-voltage to drive them - this complicates the board, and might create safety concerns.

Finally, I don't see small EL "dots" being widely available.  LEDs are easy to source - especially for the amateur/maker/etc.

I'm open to the idea, but have doubts about all of these issues.  Maybe I'm ill-informed: what are your thoughts?

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crjeder wrote 07/11/2017 at 15:13 point

I was asking because you are have definitely more knowledge than me. What I understood is that EL has a higher theoritic limit on efficiency and is long lifed. An other advantage might be that it can be of any shape (just cut the sheet as needed). I was thinking that LEDs are efficient only on the high end (high lumen). But I may be wrong. Have to dig out the efficiency claims of EL...

The driver (DC to AC converter and high voltage) was the main reason I belived that the efficiency for the whole device in the battery operated case is to bad. But again: I don't know and was just asking because you might know..

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Ted Yapo wrote 07/11/2017 at 15:37 point

I don't have too much experience with EL - so there may be something to this.

A quick scan reveals that even the EL manufacturers quote about 10x the lifetime for LEDs than EL.  Maybe at super-low power, they both last long enough anyway?

As for the efficiency, this page claims 6 lumens/watt for EL panels:

while this Luxeon-C LED claims 76 lumens/watt:

Maybe there are better numbers for EL panels somewhere?  I have seen it mentioned that they are more efficient than LEDs for low brightness applications, but this whole project came about because using LEDs naively for low-brightness is very inefficient.  You are correct - with DC drive, LEDs *are* only efficient at the high brightness end.  By driving them at higher currents near peak efficiency with a low-duty-cycle PWM, though, you can regain much of this efficiency.  They're bright and efficient for 0.05% of the time, resulting in an efficient "glow".  I wonder how this changes the comparison?

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crjeder wrote 07/12/2017 at 08:59 point

All "high efficiency & long life" claims I've found for EL are from the last milenium. Back then LEDs have been completely different form what they are now (performance wise). So that may explain those claims.

Efficiency values for dim / low power LEDs is also not so great, definitely in the single digit (lm/W) range.  e. g. which has (aprox) 1,75 [edit, was 0.5] lm/W at 2 mA * 2 V (converted candela to lumen with

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Ted Yapo wrote 07/12/2017 at 12:23 point

@crjeder Yes, that's why there has been some confusion about why I am using "3W" LEDs for a marker consuming tens of microwatts!  LED efficiency is a strong function of areal current density - too little current, and they're horribly inefficient; too much, and they drop off by maybe a factor of 2 (called "droop").  So, I use large-area LEDs which are very efficient with mA-range current pulses, and reduce the duty cycle to bring down the power consumption.  The rest of the circuit is just trying to generate these ideal pulses efficiently.

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Michael Stoiko wrote 07/07/2017 at 13:29 point

As an additional though idea (probably more for instrument faces) I recently acquired and disassembled one of the old 1st gen night vision scopes that runs it's image intensifier solely on the squeeze of a piezo element. I wonder if a dial or marker could be similar activated with phosphor and a button/motion generating the HV needed to excite it.

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bulrush15 wrote 07/07/2017 at 09:15 point

Since we're talking about saving power, I thought using tiny amounts of power and boosting it would be related. I found this interesting voltage booster which takes input as low as 250mV and it seems to create enough voltage to light an LED.

Interesting device if the specs are accurate. I'm interested in voltage boosters like this for using thermoelectric generators to charge a battery.

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Ted Yapo wrote 07/07/2017 at 14:13 point

I've seen those converters around.  They seem to excel at taking a low-voltage relatively higher-current source and boosting the voltage to something more useful - thermoelectric being the prime example.

The converters aren't necessarily ultra-low-power, though.  The LTC3105 consumes 10uA in shutdown and 24uA when active, according to the datasheet - just to run the converter.  That might be at very low voltage input, so maybe the power consumption is also low, but to put this current in perspective, the lowest-power LED circuit I have been using consumes around 3.7 uA.

But, then again, if you have a source of heat that you can exploit, and it generates more than the converter consumes, it may not matter what the converter eats.

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Michael Stoiko wrote 07/06/2017 at 15:03 point

So I have actually been noodling with something similar, but using BPW34 photodiodes as photocells

It's been purely in the "messing around in Eagle" phase, but upon mention of optimizing this using a dedicated analog circuit, i just stumbled upon this chip while browsing.

Obviously, the "done" feedback would have to be addressed, which could mess with the pulse width, but I'm shooting from the hip here.

But a 35 na supply current would be a significant savings. Would have to drive that output to a JFET or something, to feed the inductor.

I jumped back into BEAM recently, jumpstarted by
and intrigued by ultra tiny, low energy solar circuits. I think this would be a great candidate for that sort of energy harvesting application.

Theres also the possibility (but this would ditch low cost) of using something like an LTC3588 and a piezo element. If it goes dead for some reason in the field shake it to reactivate. But some of the double layer supercaps are getting to the point where you can stuff a 4 farad, 5.5V capacitance into a 3/4" diameter coin. At these power outputs, that's nothing to sniff at. You ring your little board with 10 BPW34, you are talking decent recharge.

Use the solar voltage to hold a Mosfet like the one above open until it's truly dark, and carefully position it so the glow doesn't switch it back off.

Stick the whole thing on a round PCB in front of your coin double layer cap and go.

I'm not as adept at the piezo generation calculations, but the idea that you could shake to get a glow is also appealing, even if you have to deal with rectifying and voltage spikes and whatnot.

EDIT: Some additional information on coin cells that seems particularly relevant to your design

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Michael Stoiko wrote 07/06/2017 at 15:16 point

If my back of napkin calcs are right, and they probably are not, you could ideally charge from 1.2V to 3.2V in about 98 hours at 1000 lx, but could also run for 328 or so hours on such a charge (just using the 40 ua, not recalcing for everything). That's not a terrible ratio, 1 hour of light to 3 and a third hours of glow.

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Ted Yapo wrote 07/06/2017 at 18:20 point

What solar cell area are you assuming?  How many BPW34s?

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Michael Stoiko wrote 07/06/2017 at 19:19 point

I was looking at 8-10 of them, as my previous goal was similarly glow markers that could be glued to an outdoor surface and could last for a decade or so, at low cost, for things like road markers.

I've just moved, and finally have a workshop after waiting for 5 years to get back to electronics. So I've been mostly amassing parts and modelling (as my new bride prefers we finish setting up our living space before I vanish most nights into my cave). Short circuit current is about 70 ua and voltage is .365v in 1000lux, i've also got some nifty tesla photodiodes that comparatively huge, but hit 500 mv at 6ma in full sun. The BPW34s seem to hit about 475 millivolts (open circuit) @ 1.86mA (short circuit) max in full sun.

The tesla ones are like a dollar apiece though, and the BPWs, if you scrounge, can be had for .31 apiece or so.

I'll look into the TS3005, I see some for sale on Ebay.

I need to trim my exuberance, though, I think I have parts now for every fleeting circuit Idea I've had in the past 2 months, and still have boxes of unsorted components to organize consuming all my workspace!

The other [form factor] that would be interesting would be something tiny, like an Attiny4, with a LED bead stacked on top, optimized to fit on the end of an AA or a 1.5F radial supercapacitor, whatever one's option. As long as the diffused LED has a sufficient viewing angle, obviously. In my {solar} case, using the BPWs to ring the radial cap in 4-5 rings of 4, so it's always getting light from 1 side. Stack them close, you have something akin to a lanyard bead with a wide angle glow on the end.

There's just a lot to explore here with form factors. I'm really happy I found this, because it's exactly what I've been fiddling with, from the UV LED + Instrument Phosphor powder, to the same questions, just from a solar angle.

It wasn't ever really about being practical, in my fashion, though. It was about engineering the hell out of something innocuous for the fun of it. You just took it farther than I had the capability to.

Edit: My newness in actually commenting here is showing; I see below none of the chips or info I suggested are news to you, and this project is probably reaching end of lifecycle for your interest/attention!

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Ted Yapo wrote 07/07/2017 at 01:29 point

Interesting.  Now, I'm wondering if some small area of solar cell around 7-segment LED displays could be used to boost the brightness when the ambient light is high; otherwise, the display can fall-back to a default battery-only dimness appropriate for dark conditions.  This might be interesting for #Micro-Power 7-Segment LED Display.

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Ted Yapo wrote 07/06/2017 at 18:19 point

I looked at the TPL5010 - I think I have some in a bin somewhere.  The problem is that the minimum timeout is 100ms, which is a 10Hz blink rate and easily visible.  I don't know if you can run the part at faster rates than the datasheet specifies, but it might be possible.  And, yes, tickling the "done" pin is an issue.  I really like the low-power consumption, though - maybe this is a good way to make low-power blinking markers.

I also looked at the TS3005 It used 1.35 uA, and had timeouts down to 1.7ms, so seemed perfect.  I have a few of those around, too, which is good and bad because they're now discontinued :(

I've played with capacitors (I got some of those 4F caps you mention), and it works pretty well.  The 5.5V rating is wasted, though, if you are going to charge them to only around 3V.  I'd like to find some 3.3V caps in the same capacitance range - smaller and cheaper.

You could definitely use solar to recharge the supply, if you knew you were going to get enough light periodically.

I read those articles by [Jack Ganssle] a few years ago.  He was primarily looking at higher current drains, but it's good info.  The oversized 10uF capacitor I use is based partially on the second article.  Thanks for posting the links here; I really should start a references log to put these and other interesting links.

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Michael Stoiko wrote 07/12/2017 at 18:21 point

Not to blow you up about various chips, but just ran across this one over lunch. Again looks intriguing.

At least for stability and ruggedness sake, it looks very solid.
Miniscule, too!

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bulrush15 wrote 07/06/2017 at 09:17 point

I think I'm missing something basic. Is this a special circuit to power a normal LED? Or does the circuit power a special LED? I'd like to try out a couple boards of v2.x if they are not too expensive.

I'm very interested in this continuing. I'd like to see more of this, but focused on a low-cost version. I mean an LED, as a low brightness marker, that can run a year non-stop is a step up to me.

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Simon Merrett wrote 07/06/2017 at 12:54 point

The LED is slightly special in that it is at one of the best frequencies to match the spectral sensitivity of the eye in low light environments. But you could use any colour. The LED is "sized"  to the circuit or the circuit is "tuned" to the LED die size. This is all component selection rather than adjusting parts. 

The driving circuit is special in that it is optimised to produce adequate light in the most efficient way. It pulses so that it is on only part of the time (but the eye doesn't notice) which saves energy. When it is on, it glows at an efficient brightness (best photon for your coulomb rate!). 

I'm sure you could "value engineer the #TritiLED but it would all depend on your scale I suppose. Without putting words in @Ted Yapo 's mouth, I'm pretty sure you'd have to do the legwork, although I can say from first hand experience that he's very supportive if you show effort. 

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Ted Yapo wrote 07/06/2017 at 18:34 point

@Simon Merrett got it right.  I've been playing with special circuits to drive special LEDs, but you can use these ideas to drive normal LEDs, too.  It just might not be optimally bright or efficient.  [Elliot Williams] did this in #ATtiny45 EverLED where he used the same idea with a different microprocessor and a variety of different LEDs.  Just about any of them will work, some better than others.

I don't know how much cheaper you could make them - I also don't know what the exact cost comes out too now, but they're interesting questions.

I'm re-doing the design in KiCAD now - Eagle finally forced my hand with the new licensing structure.  It's my first KiCAD design, so it may take a little while.  I'm thinking about cost, too.

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Alphas wrote 07/06/2017 at 05:51 point

Power loss of the resistor is not calculated correctly.

resistor P = I^2R = 1.44x10^-8W

Assuming 3V on led. P=(12^10-6)(3)= 5.22x10^-4 W

% = 0.052%

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Ted Yapo wrote 07/06/2017 at 18:44 point

I think you made a mistake somewhere - I still get 0.04% .

Your calculation yielding 5.22e-4 is incorrect, it should be:  12e-6A * 3V = 3.6e-5W

My original method was to reason that the current through the resistor and the rest of the circuit is the same, so the ratio of powers (efficiency loss) is:

eta = (Vr * I) / (Vc * I)

for the voltage drop across the resistor Vr, and the drop across the circuit Vc.  The I's cancel, and you can just calculate the efficiency by the ratio of voltage drops. I should have been more explicit in the log.

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Galane wrote 07/05/2017 at 21:27 point

Zippo made something like this. 6 in 1 Constant Light where the white LED was always on at a very low level, and returns to that after 10 minutes in any other mode.

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Ted Yapo wrote 07/06/2017 at 00:39 point

Thanks for the link! That's very interesting, I hadn't seen those before.  They use 2x coin cells for a 6V supply, but don't mention the battery life.

So, the fact that Zippo made one makes me wonder if there is a market.  The fact that they no longer make them also makes me wonder.

Oh, and I can't find any used ones on ebay :-(

I would like to see what's inside.

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Dylan Brophy wrote 07/05/2017 at 20:43 point

Congrats - you made it on the HaD Newsletter!

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Ted Yapo wrote 07/06/2017 at 00:33 point

Thanks!  It's funny; I knew something had happened from the stream of like/follow notification emails I started getting.

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Dylan Brophy wrote 07/06/2017 at 00:40 point


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freefuel wrote 07/05/2017 at 19:03 point
pkobla have you taken a look at the green arrays multi core chip?

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Adam wrote 06/22/2017 at 13:32 point

Please Please don't let this project be dead. It has been 7 months since the last update and the files on OSH are (I believe) 2.0 not 2.2. Simple instructions, a BOM, the gerbers (or the boards shared on OSH) and the hex are all that is needed for this project to be great and completed, don't let it just fizzle out like this. Also think about the hackaday prize for assistive tech coming up! surely these would be great as markers for those with partial sight loss.

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Ted Yapo wrote 06/22/2017 at 16:11 point

To be honest, I didn't see the level of interest that would motivate "completing" this project to produce a final design.  It's more of an idea, and in that sense has been picked up, modified, and used at least three times that I know of, which I consider a success.  Maybe my perspective is different - I'm not sure I've ever built someone else's project as-is.  I look at the schematic, idea, or interesting technique, and maybe get inspired to try something on my own.  Once it gets reduced to "insert capacitor C2 into the holes marked 'C2'", I lose interest very quickly: I have a box of randomly-purchased never-opened kits as evidence.  My approach on has been to target like-minded people as an audience.

I did see some interest in buying completed units, but I don't want make these things for a living.  Any one of seven billion other people could do so.

Thanks for pointing out where I hadn't posted everything you might need
to re-create this project exactly.  I have it on my mental list to
re-work the board to remove the reverse-battery-protection MOSFET and
replace it with a resistor, which provides some additional protection, as discussed in #Decade Flashlight.  Meanwhile, the other MOSFET now has a 2,000 piece minimum order on Digikey, so I'll have to find a substitute.  The code also needs cleaning up.  I wouldn't release them in their current state, but I will accept your criticism and work (maybe slowly) toward posting more complete and usable info.

Oh, The Prize.  I was selected in two rounds of the 2016 prize, and paid with a few months being over-caffeinated, under-rested, and stressed out trying to make quick progress on projects I thought would compete well: I neglected my health and some other responsibilities to find time to work on them. In the end, I think I ended up spending more on parts and boards and miscellaneous stuff than I got in prize money (after taxes) - at best, it was a break-even, although I do have a few big bags of assorted parts I may never use.  This is obviously all my fault, but I learned something about myself in the process. I'm sure many others have had different/better experiences, maybe because they approach projects differently than I do.  The Prize is great, and motivates a lot of people to make awesome stuff, and I'm not trying to disparage it in any way: I've just decided that it's not for me.  This hobby is supposed to relax me - and than means working on what I want, when I want, how I want, and taking seven month (or longer) breaks when necessary :-)  So, I've decided that when a contest comes along, *if* I have a suitable project that is already in a submittable state, I might submit it.

That said, I am not alone on this project.  If one of my collaborators wanted to submit this project to a contest, I wouldn't stand in their way.

I'll just be frank about this: simple instructions aren't going to happen - if you want to build one of these you'll need to know how to assemble  an SMD board and program a PIC, which you can find many other places described better than I would.  I chose the new LEDs based on the fact that they can be hand-soldered, which should help.  You'll also need a PIC programmer and probably an SOIC-8 test clip, which seems to be the easiest/cheapest way to go.



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Jarrett wrote 06/22/2017 at 21:58 point

I feel you on a lot of this, Ted - As soon as "the hard stuff" in a project is complete, I rapidly lose interest. Packaging everything up as a kit so that someone else can blindly follow along is what I do for a dayjob, not something I want to do for fun.

I do have one issue with this, though! You say, "The code also needs cleaning up.  I wouldn't release them in their current state, but I will accept your criticism and work (maybe slowly) toward posting more complete and usable info."

There are about three projects I have on my Github in exactly that state. I promised myself that I would come back and clean up the code, but it hasn't happened yet. New, shiny projects come along, etc.

What I've found is that a working but "messy" codebase sitting on my harddrive doesn't do anyone any good. If I release something as-is, with dire warnings about the code's defficiencies, it at least gets someone a head start if they want to pick it up.

Please do reconsider!

But either way, I have enjoyed this project :)

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Ted Yapo wrote 06/22/2017 at 23:18 point

@Jarrett Yes, you have a good point - if the originators of open-source (software) projects waited until it was "done" to release the source, we'd never get anywhere.  My comment was more about the hw side - I think as the 2.x design stands, there's a chance of catastrophic failure if the PIC locks up with the MOSFET conducting, or the MOSFET fails closed.  I have no idea how to assess the probability of these events, but the effect of either possibility can be mitigated with some design changes.  This means a board re-spin, buying parts, testing, then publishing.  This is exactly the kind of thing that makes me reconsider releasing "completed" designs - if someone who needs a completed kit-like design builds one, I feel I incur more liability in case there's a problem.  If I just present an idea, then someone implements their own version of the idea and happens to grow a second head as a consequence, any competent attorney would argue that the growth of the extra head was entirely due to their incompetence.

The sw is easier to just release.  It's actually mostly there in the original log from 8/30/2016:

It's just not cleaned up and githubified.  I think I hadn't used github at that point for anything - which is probably why this stuff never got in there.

Then there's the Eagle licensing stuff.  I own a 6.x license, but have been doing recent designs in a free 7.x version.  I don't know what to think about the subscription version, except that I don't feel like paying every month for something I only occasionally use, and then only so I can release stuff with a permissive/commercial-use-OK license. It's a mess, and I should really switch to KiCAD, but I might as well start writing with my left hand while I'm at it.  I guess this turned into another rant :-)

Anyway, yes, I can disclaimer all these problems away and release *something* that someone might find useful.  It's probably the right thing to do.  Thanks for the comment.

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Ted Yapo wrote 06/24/2017 at 19:22 point

So, just in case anyone else is looking for all these things - github repo, all design files and code releases, easy instructions, etc, they actually exist for a version of this design in another one of my projects.  It's been there for six months , has 276 views, 1 follower, and zero likes.  I don't know why nobody notices that one.  The only significant  difference is that it doesn't have a CR2032 battery holder integrated on the board.

The main project is at:

The circuit and PCB design and theory, along with a detailed BOM are at:

The code is described in:

A 3D-printed case for that particular use (a flashlight instead of a marker) is presented:

Detailed assembly instructions are given:

All the design files are in GitHub:

And the boards are shared on OSH Park:

Until I have a new version of the TritiLED project ready to release, that other one is available, if anyone wants to build one. The only thing you may want to change is the number of pulses per cycle, currently set in the code to 6, which results in a 40uA current drain - about 7 months on a CR2032 (10 years on a pair of LiFeS2 AA's).  You can modify that number to change the brightness & run-time.

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pkobla wrote 05/18/2017 at 20:49 point

Hi Ted,

regarding choice of MCU. I've been programming PIC and MSP430 for low, really low power applications, but have now ended up with a Cortex M0+, the NXP (Freescale) mkl03z8vfg4.  Less $ than the msp430. Farnell (Denmark) lists them at $0.49/100s.  8kB flash, 2kB RAM, ADC etc. You  don't need to go assembler but can stay with C. 

Regarding the issues that have been discussed earlier about power use and time used during startup or wakeup. The mkl03 has a mode, VLPR, where wakeup latency from sleep, using an internal clock timer interrupt, is 7-8us and consumes about 2uA while sleeping. All inclusive. You can go lower, down in the  nA ranges,  but at a cost in latency, etc. Code may be placed and executed from RAM. When executing from RAM the "cost" per 1MHz is about 90uA. Instructions execute in 1 cycle except branches. HW multiplication in 1 cycle. The core clock can be scaled from around 25kHz up to 48MHz.  Power consumption scales accordingly. The internal clocks are pretty accurate and stable over temperature. It'll run from 3.3V down to 1.8V or is it 1.4V? All numbers are from memory, but read the datasheet. I've been around most features and the datasheet numbers are conservative compared to what you measure in real life. One drawback is the packaging. It's QFN with a 0.5mm pitch. Another of course is the learning curve which is steep coming from the msp430. Took 3-4 weeks for me to feel comfortable. IDE: I use IAR. Most people use CodeWarrior I guess. As for debugger I am using, or re-using,  the one that comes with the msp432 Launchpad which has an CMSIS-DAP mode which is supported by IAR and it works fine. IAR also supports the ST-Link-V2. A $2 Chinese ST-Link-V2 debugger has worked well too. With CodeWarrior you have to use something else than these 2. IAR does not cost anything if code is < something. I think it's 5k.  All-in-all, pretty amazing that you can get an ARM dev env up and going for a 1 digit dollar amount. Even more amazing that with ARM Cortex you can mix and match different IDEs, Cortex MCUs from different vendors, and debuggers.

As for a smaller battery holder: On aliexpress search for "CR2032 shrapnel" <sic>

Hope you and other readers may find this useful,

Peter K

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Simon Merrett wrote 06/06/2017 at 19:42 point

Peter, thanks - I found that useful as an insight into what's on offer in ARM and what the cost of entry looks like.

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Ted Yapo wrote 06/07/2017 at 19:43 point

Hmm ... I didn't seem to get a notification about your comment; only saw it when Simon replied.

I have been looking at the Cortex M0(+) for some time, but haven't overcome the inertia of experience and comfort with the PIC to try it out.  It's probably overkill for this particular project - even the tiniest PICs really are, but they seem to be the cheapest and easiest solution at this scale.  I think if you designed an (analog) ASIC for driving LEDs like this, you could improve the efficiency quite a bit.  There's no reason for having a whole microcontroller to blink an LED periodically (which is all this is really doing).

For some more complex projects your suggestion sounds really good.  I'm going to have to dive in and try one out - thanks for the pointers!

As for the "CR2032 shrapnel", I see the parts on aliexpress - small and cheap, very nice.  I hope the use of the word "shrapnel" in their description is a mis-translation of the Chinese "sheet metal" rather than an indication of what people are buying these things for.

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Enrico wrote 01/14/2017 at 13:48 point

Hi, I am quite ignorant on spectrometer instruments: what is the one you've used to see the wavelenght? My best choice up to now was a broken CD/DVD... 

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Ted Yapo wrote 01/14/2017 at 15:58 point

I don't know that much about it, either.  The spectrometer I used was one of these:

purchased a long time ago for less money.  It's more of a student demonstration than a serious instrument.  You could probably build one that gives similar results with materials you already have around - including the DVD grating.

There are some good spectrometer projects around here.  For more refined designs, see @David H Haffner Sr's projects.

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[this comment has been deleted]

Ted Yapo wrote 11/30/2016 at 13:58 point

Yes, yes, I do need an SOIC8 test clip!

I hadn't seen them before, either.  I bought an SOIC socket when I started this project so I could program parts before soldering them, but the test clip is much better.

It would be nice to save the real estate I'm using for pads now :-)

How well does it work on less-than-perfectly-mounted chips?  When I hand-solder SOICs, sometimes it's a bit blobby.

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Dan Julio wrote 11/21/2016 at 00:56 point

This is a neat project, Ted.  I thought of it this week as I walked up to my house in the early evening winter dark and surveyed all the dead solar LED lights that used to light the way...  How is the light output of V2.1/2.2 compared with a "typical" solar garden light (with, I assume, the right kind of white LED)?   It would be nice to have a set of lights that don't fail after a year...  Which would seem to be the case pairing your board with a couple of lithium primary cells.  I wonder if one could get enough light out of an energy harvesting setup using existing solar light solar cells, a super-cap and your board.  One could even add a sense input the PIC to turn the light off during the day.

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Ted Yapo wrote 11/21/2016 at 01:54 point

I'm running them pretty dim compared to typical garden lights, I would assume.  I haven't compared them head-to-head, but the typical garden light is probably a hundred times brighter or more - it's the difference between a light being visible as a marker and being able to illuminate other objects (like a walkway).  I might have an old garden light in the garage I can take a look at.

But, there's no reason not to apply the same principles to a garden light.  If you take the 3W LEDs I've been using, they have an efficiency peak when run at around 10-15 mA (each color is a bit different), and they're pretty bright at that current.  If the garden lights aren't running near this current, they're wasting energy.  So, you could make a smart garden light controller to run them at peak efficiency.  Even so, I think you need the capacity of rechargeable batteries - supercaps still don't have the energy density required.

I did make a 10-year flashlight with which I am able to read and find my way around in the dark.  It's definitely not as bright as a garden light, though. You could crank up the brightness to garden-light level, but that would kill the battery lifetime.

If you are willing to go to d-size cells, the energizers claim a 10-year shelf life and 18Ah capacity.  You might get a garden-light to go for a few years on that.  They're lousy in the cold, though.

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Dan Julio wrote 11/21/2016 at 02:02 point

I figured that, but since you support multiple brightness levels, thought I'd ask.  One interesting thing is that garden lights really only need to last a few hours each night (since most of us don't get home at 3:00 AM every night...).  I am looking forward to playing around with your design when you release the V2.x files.

I don't know a lot about the garden lights but I did look at one once.  The IC in there uses some sort of boost topology with an inductor to take the 1.2V out of the cell and power the 5 mm White LED.  I have no idea if it is approaching the peak efficiency of the LED.

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Ted Yapo wrote 11/21/2016 at 02:35 point

If they're using 5mm LEDs they're almost certainly driving the LED above the point of maximum efficiency (probably around 1 mA).  If they're using the usual 20 mA, they might be throwing away half the energy.  The fix is buying an LED with a larger die area, which adds cost.  I think you could probably make a better light, but marketing it would be difficult (inferior but inexpensive sells).  But that's the joy of DIY - you can make one "the right way" for yourself.

I still have a few tests on the V2.2 to do, and a bunch of measurements, but so far it's looking pretty good.

I am seriously considering a version of the board for solder-on CR2450 or CR2477 batteries.  They're 620 mAh or 1000 mAh, respectively, and you could run the 2.2 circuit for 10 years or more.  But I want to get the 2.2 design out first.

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Dan Saito wrote 09/07/2016 at 09:58 point

I wondered if energy harvesting could be a possibility here, so I read some app notes and did some enveloped arithmetic. Solar would work, with the limitations that entails, if that counts as energy harvesting, but RF would not. That’s not a surprising result, but it is disappointing.

Now, if they’re being embedded somewhere, the equation can shift.

One of the uses I have in mind for TritiLEDs is lighting the dials of mechanical clocks. In that application, there’s enough area to work with that harvesting RF may be feasible. Alternatively, energy could be sapped from the clock’s drivetrain. Of course, these would mainly be for the cool factor, because if you have enough space to do something like that, you could just hide batteries large enough to power the LED for decades.

If you don’t care about it working immediately if left on a shelf for a long while, sufficient energy could be gathered from motion, if left in a bag that’s carried around or a vehicle that’s driven.

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Ted Yapo wrote 09/08/2016 at 19:05 point

Powering these things via energy harvesting is a neat idea.  Besides finding an energy source, storage options may also be a problem for long-term use.  I don't know that I'd trust electrolytic caps for 20 years, and ceramics are bulky and expensive for larger values.  A mechanical harvester and/or storage mechanism sounds interesting.

As far as RF powering, it depends where you are :-)  I'm 18 km from the 50,000W transmitter of WGY at 810 kHz, and I can run a version 2 from a crystal radio and voltage multiplier:

It's not fully-powered, but it lights up.  I might also be able to get it to light from microwave oven leakage or near a WiFi antenna - I used to do that with red LEDs/UHF diodes/dipole antennas before I knew they weren't very efficient at low-current DC. Probably not practical, though.

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Dan Saito wrote 09/09/2016 at 04:41 point

I’m < 2 km from a 25 kW FM transmitter. I’ll have to try that out. It’s a good excuse to build an FM crystal radio.

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Ripper121 wrote 08/29/2016 at 15:46 point

Do you found good alternatives for the Luxeon Z LED?

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Ted Yapo wrote 08/29/2016 at 16:13 point

I have about 15 different LEDs to test, but haven't had time to run the curves yet - the analyzer over on #Automated LED/Laser Diode Analysis and Modeling is still in a very early prototype stage.  The only real problem with the Luxeon Z is that it can't be hand-soldered - this will keep some people from building them.  It would be nice to find a cheaper part, too, but I'm not sure I'd trade performance for cost here.

You can see a partial list of the candidates under the boards at the end of this log:

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Ripper121 wrote 09/05/2016 at 12:12 point

Thank you. Yes i agree you. Performance is the Aim :)

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SarahC wrote 08/28/2016 at 06:31 point

I would LOVE to buy a completed one or two of these... are you selling them anywhere? I had a look online and couldn't find anything. sarah AHT untamed dot co dot uk

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Ted Yapo wrote 08/28/2016 at 06:55 point

Unfortunately, I'm not selling them; it would complicate my taxes too much.  The design is all open-source, so if some entrepreneurial spirit out there wanted to start making and selling them, they could do so (hint, hint - not necessarily directed at you).

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jaromir.sukuba wrote 08/01/2016 at 07:19 point

Regarding the PIC12F508 - is there any specific reason to use this one? It is build on older manufacturing technology, causing its higher current consumption. In this case, a few microamps at 3V for the complete sleep+WDT current. 

For newer PIC12LF1552, for example, you can achieve less than half a microamp consumption for sleep+LPWDT current.

On the other hand, its current handling capabilities of IO pins are different, though better suited for 3V than 12F508, designed for 5V mainly (replacement for PIC12C508).

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Ted Yapo wrote 08/01/2016 at 14:07 point

Thanks for the note!  That's a very good observation, and one I've done some thinking and experimenting on.

I originally used the 12F508 because I had some left over from another project (I actually still have 12C508/09's left from an era before that, too).  It worked well enough for a first iteration, so I stuck with it.

I have tried the newer XLP PIC parts, and have had mixed/bad results.  I haven't written it up yet because I wanted to be sure of my findings before speaking poorly about the newer PICs, but you can read essentially the same thing in Ti's white paper here:

Basically, with the high-speed internal oscillator, it takes a *long* time for the PLL in the PICs to stabilize (~1ms), so very low on-time applications don't really achieve good power savings. If you want an accurate clock (which I need to generate accurate pulses), you have to wait for the PLL lock and waste power.  It's discussed in TI's document. See the image on page 8 - they're using a PIC24F, but I see the same thing with PIC12's and 16's.  The PIC12F508, in contrast, has an "instant-on" oscillator, like the MSP430.

I'm still experimenting with newer PICs, so I haven't ruled them out, but I also have a MSP430 LaunchPad board sitting on my desk (and a few sample MSP430 parts), waiting for me to test Ti's claims.

I also am going to test the new ultra-low-power timer chips in this application.  I have some TS3004's (just waiting for PCB's from the fab): Documents/TechnicalDocs/TS3004.pdf

These are 1.9uA, pulse fast enough to get above the flicker fusion rate, have a PWM control you can use to tune the pulse width.  Achieving an average 1.9uA even with the microcontroller would be decent, because running bursts at MHz rates brings up the average.

I am also looking at the Ti TPL501x parts:

these run at 35nA. Unfortunately, the frequency only goes as high at 10Hz, so they aren't good for "constant" LEDs, but could probably be used for visibly blinking versions.

A lot of my focus has been on improving the efficiency, too.  39% is pretty bad. I got fooled by the relative inefficiency of the LED at very low currents before - I think I can double the runtime with a new architecture (single-shot inductor converters).

About the power handling of the PIC pins - I'm adding mosfets to reduce the voltage drop here and handle the higher current.  The I^2R losses using GPIO pins are killing efficiency.

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Ted Yapo wrote 08/01/2016 at 14:25 point

This link shows the problem exactly for PIC8 XLP parts:

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jaromir.sukuba wrote 08/02/2016 at 11:19 point

Yes, there are deviations from ideal state, but I believe the overall current consumption will be lower than with the 12F508. This is interesting topic, anyway - I use PIC MCUs all the time, utilizing its power saving modes often. I still haven't met with the oscillator startup time, as the wakeup from sleep takes place only during user intervention - once you turn it off, it stays in sleep mode until you wake it up. Microseconds doesn't play any role in here. On the other hand, waking up every 18ms for a few microseconds doesn't matter much too (the total duty cycle of "high power" time fragments makes its contribution to total power budget quite low anyway), IMHO.

But as usual, devil is in details, so it is not simple to estimate everything with no actual circuit. You design is simple and will try to breadboard it and return back with results.

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Ted Yapo wrote 08/02/2016 at 14:44 point

I originally tried it with a PIC16F723A, which I think has the same XLP/Nanowatt features as the newer PICs ( just because I had some of these parts around).  I have some PIC16LF723A's now (just because I have some breakout-type boards for this part).  I think the only difference is the exclusion of the 3.6V internal LDO on the LF version, but I have a test setup that I just need to mount the part for to test it out.

Anyway, I'll re-create my original measurements with the LF version and post the results.  I have some PIC12F1571 (and '72) around I can use, too.  I suspect the oscillator start-up time will also be an issue with these parts.

Taking the '1552, for instance, check out section here on page 38:

when coming out of sleep, I'd want to wait for the HFIOFR (hf osc. ready) bit and the HFIOFS (hf osc. stable) bits of the OSCSTAT register to be set before generating the LED pulses.  These bits indicate that the HS osc has started running and is within 0.5% of its nominal rate.  Before those bits are set, the oscillator speed is AFAIK undefined - not something I would want to rely on to generate a current pulse.  When I tested how long it took to set these bits on the '723A, it was at least hundreds of microseconds of just waiting for those bits to set, negating much of the savings.  I could dig through my notes, but it's probably easier to try it again and post the results.

Yes, I definitely encourage you to try it out.  I could certainly be wrong about these new parts; I didn't look too hard after my initial experiments (and was rather disappointed, because I've been using PICs since the early PIC16F84 days). I'd like to see how it turns out :-)  Very interesting.

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Ted Yapo wrote 08/02/2016 at 14:48 point

But actually, now that I think of it, the tests with the newer PICs were with a slightly different driver circuit, so I don't have a good apples-to-apples comparison.  I'll have to make one.

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jaromir.sukuba wrote 08/02/2016 at 20:19 point

My first PIC was PIC16F84 too ;-)

For the PIC12LF1572 (and I believe other PIC12LFxxxx are similar, but probably not older PIC12(L)Fxxx) the datasheet states 5us typical, 15us maximal start-up time for HFINTOSC, while consuming approximately 200us at 4MHz. The consumption during power-up transient may be different, but this is subject of experimentation. The good thing is that it's pin compatible to venerable 12F508, so testing it should be somehow easier than complete rework. 

Note that 12F unlike 12LF version contains internal LDO allowing VDD up to 5,5, though at the expense of higher current consumption. It is low delta current, but it gets very obvious when you need to conserve the power (sleep mode).

16F723 is generation older than PIC12LFxxxx and honestly, I never really liked the 16F72x family, it has a few quirks and gotchas I wouldn't expect in 2008 or what was its release date. The newer PIC16(L)Fxxxx (and 8-pin versions 12(L)Fxxxx) do have much better peripherals, better specs, I think even lower price and more consistent scale-up/down migration path, while being reasonable compatible with older models.

No matter what, your tritiLED project piqued my interest and will test it minimally at breadboard and we will see.

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