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TritiLED

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 3.0 of the design is complete and released under an MIT license.

Concept

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.

Hazards

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 3.0

Version 3.0 uses a new architecture with the PIC12LF1571's synchronized 16-bit PWM outputs controling a 74LVC1G123 monostable producing the current pulses.  This allows tuning the peak pulse current to optimize LED efficiency, while separately tuning the overall brightness (and associated battery life).

This version allows the use of three different LEDs, chosen for their unique characteristics.  Each runs for a configurable 1-10 years (or longer) from a CR2032 cell. At minimum brightness, the circuit is estimated to run from 17.6 to 20.2 years on a CR2032.

The V3.0 PCB with battery fits inside inexpensive "5g" plastic jars for weatherproofing.  The jars can be permanently sealed with silicone sealant or epoxy if you don't mind breaking a $0.16 jar when you need to replace the battery in a few years.

Details of the 3.0 version can be found in the build log.

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...

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tritiled.lbr

Eagle library for components used in TritiLED designs

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

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

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Li_Battery_Model.tgz

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

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luminosity_calculation.tgz

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

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efficiency_cal.zip

initial octave code for LED current drive waveform efficiency optimization

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  • V3.x Calibration Procedure

    Ted Yapo10 hours ago 0 comments

    I added some code in GitHub for calibrating the V3.x boards.  Since the inductor has a +/-30% tolerance, the brightness and runtime can vary significantly from board to board.  These differences are easily calibrated out by running a little python script I wrote that calculates a calibration table.

    There are just a few steps required to calibrate a newly assembled V3.x board:

    1. Run The Calibrator Image

    Download the calibrator.hex image.  This image runs the LED at three different brightness levels, 16 seconds each.  When the image is running, record the currents (in uA) consumed at each brightness.  In one recent test run, I recorded the following:

    10.3
    19.4
    37.7

    These correspond to hard-coded blink frequencies of 100, 200, and 400 Hz.  Save these three current measurements in a file called currents.dat.  The C-code source, calibrator.c, is in GitHub.

    2. Run the Calibration Script

    The python script, calibration.py, reads the currents.dat file you just created, fits a line to current vs frequency, and calculates the frequencies for 1-10 year run times.  The script pops up a window with the above plot to check the calibration.  All three points should be on the line.  Once you close the plot window, the script generates a header file called modes.h:

    #define N_MODES 10
    struct run_time period_table[N_MODES] = {
      {1, (int)(31000./262.124721 + 0.5)},
      {2, (int)(31000./124.768381 + 0.5)},
      {3, (int)(31000./78.982934 + 0.5)},
      {4, (int)(31000./56.090211 + 0.5)},
      {5, (int)(31000./42.354577 + 0.5)},
      {6, (int)(31000./33.197487 + 0.5)},
      {7, (int)(31000./26.656709 + 0.5)},
      {8, (int)(31000./21.751126 + 0.5)},
      {9, (int)(31000./17.935672 + 0.5)},
      {10, (int)(31000./14.883309 + 0.5)},
    };
    

     3. Compile the Calibrated Hex File

    The tritiled_v30_selectable_runtime.c code automatically includes the generated modes.h file, so you just have to recompile to create a calibrated hex file for the board.  Once you recompile, just program the hex image to the board, and it's done.

    If you want, you can verify the current drain for the various modes.

    That's it.  It shouldn't take more than a few minutes to calibrate your board.

    I originally had a script that would do this all automatically, but my PICkit3 has proven to be a little unreliable.  I often have to reset it before it will program correctly.  It could be that my particular programmer is flaky, some incompatibility with linux, or something else, but it was just too frustrating.  The new calibration process is simple, quick, and gives good results, so I'm going to leave it at that.

  • V3.1: Configurable Brightness

    Ted Yapo5 days ago 4 comments

    I think this is the last version I'm going to make.  I added a tiny (3x2mm) switch to the V3 board to allow you to select the brightness/run-time at startup.  You can see the switch in the upper right, just above the incorrect "V3.0" silkscreen text (I fixed this since these boards were made).  It basically fits in a 1206 footprint! The Cree XPE2 is on the left, with the OSLON Signal Verde LED on the right. I didn't make a V3.1 PCB for the Luxeon C LED.  The Eagle files and gerbers are in GitHub.

    The switch was added to go with the new C-code.  The code allows you to tune the brightness when you first insert the battery.  Pressing the switch cycles between 1-10 year burn rates.  You get a preview of the brightness for 4 seconds, then the LED blinks out the run-time in years (e.g. four blinks = four years).  Once you let it cycle uninterrupted three times, the brightness gets locked in until the battery is removed.

    The code also has some additional enhancements (some are enforced in the makefile).  Most importantly, the code fits into the lower half of flash memory, which is write-protected in the configuration bits.  This prevents accidental over-write of the code by an errant program.  The code has to write to program flash memory to store the brightness mode, since there is no EEPROM on this part.  To make things safer, the brightness mode is stored in the upper half of flash (read/write), with the code itself safely in the lower half.  Microchip probably could have provided a finer granularity on their write-protection (all/none/half), but the part is what it is.

    BOM

    The V3.1 is identical to the V3.0 except for the switch.  This particular switch is available in three different activation forces (100g, 160g, 240g).  I have samples of the 160g and 240g, and am not sure which I like best.  They both seem to work OK:

    (160g) KXT 321 LHS switch, DigiKey part #CKN10778CT-ND, $0.25 each

    or

    (240g) KXT 331 LHS switch, DigiKey part #CKN10779CT-ND, $0.25 each

    Next Up

    I'm still tweaking the automatic brightness tuning script.  I'll be posting that soon.

    I'm also going to run a head-to-head experiment to answer the question: "why don't you just use an LED and resistor".

  • Stupid Case Tricks

    Ted Yapo01/08/2018 at 19:36 2 comments

    Here's a quick rundown on what I have learned from playing with the case:

    TritiLEDs Sink

    I had hoped they would float, but I guess that would make the case larger.  This one stayed under water for a day with no leakage.  It was sealed with a 25x1mm o-ring.  The fit isn't very good, though, and I'm going to try to find some better ones.

    Spare Battery

    You can fit a spare battery in the bottom of the case:

    The "5g" jars are a little too deep, and there's just enough space for a spare CR2032.  For 10-year run-times, a spare cell probably isn't too interesting, but for a bright 1- or 2-year marker, having a spare cell right in the case could be handy.

    Magnets!

    Like lasers, magnets make everything more interesting.  Here, I glued a cheap 18mm ceramic magnet to the bottom of a case with a dab of epoxy:

    The ceramic magnet isn't very strong, but holds the light OK.  A strong neodymium magnet will hold it without glue, just from being attracted to the coin cell through the case.  The best solution is probably glue+neodymium, but then you might have trouble extracting the PCB to replace the battery.

  • Determining maximum runtime: 17.6 to 20.2 years (CR2032)

    Ted Yapo01/06/2018 at 21:34 24 comments

    OK, so for the coin cell contest, we want to know the maximum runtime.  I have considered estimating this three different ways.  As an example, I'll take the minimum pulse rate of the V3.0, 0.5 Hz (the light blinks once every 2 seconds).  I measured the current drain to be around 1.15uA at this rate. 

    How long will this run on a CR2032?

    First, you need to decide on the cell's capacity.  I'll use 225mAh, which is on the Panasonic CR2032 Datasheet, since I'm using Panasonic cells.  Other manufacturers quote other numbers.

    Capacity / current

    In this method, which you see most commonly used, you divide the capacity of the cell (in Ah) by the current drain of the device (in A), to yield a lifetime in hours. To get years, we divide by 24 hours/day, and 365 days/year (if you want to get technical, you can use 365.25).  In this case, we get:

    0.225 / 1.15e-6 / 24 / 365 = 22.3 years

    Nice, but this exceeds the "shelf life" quoted as 10 years. 

    WTF is shelf life, anyway?  It certainly doesn't mean the battery is dead at that point if left on the shelf.  Instead, it's a measure of self-discharge.  Elsewhere in their Lithium Handbook, Panasonic quotes a 1% self discharge rate per year.  So, at the end of the 10 year shelf life, the cell should retain more than 90% of its initial capacity (depending on 1% of what: see below).  Considering this, the 22.3 years calculated above doesn't seem correct anymore - at that point, the battery would have lost some of its initial capacity sitting on a shelf.  It would be nice to take this into account.

    Considering Self-Discharge

    To include the effects of self-discharge, I first assume the 1% per year means 1% of the remaining capacity.  This means the capacity obeys the differential equation:

    where s is the self-discharge rate, in this case 0.01/year.  The solution of this equation is an exponential decay describing the capacity of a battery on the proverbial shelf, which is interesting, but doesn't answer the question.  To figure the runtime, we also need to include the circuit's current drain.  For the moment, assume the drain is a constant current of d amps.  The equation now looks like:

    In 2018, there's no need to be intimidated by (simple) differential equations like this: just throw them into Maxima.

    I got maxima to solve the ODE, selected a specific solution by giving the initial conditions (fresh battery at t=0), and finally solved the result for the time when C=0.  The result: 20.2 years.  Two years shorter than the earlier estimate. (Note: earlier, I posted 19.4 years here due to a typo in the maxima code).

    Linear Self-Discharge

    It's also possible that 1% discharge per year means 1% of the initial capacity per year.  This would mean a linear decay of capacity obeying:

    The solution to this one is trivial: we just add (0.225 Ah * 0.01 / 365 / 24) = 257nA to the device current.  For the lifetime, we get 0.225 / (1.2e-6 + 257e-9) / 24 / 365 = 17.6 years.  Two and a half years shorter still.

    So, there you have it: the TritiLED V3.0 can run for somewhere between 17.6 and 20.2 years on a CR2032.

    There's a subtle problem with each of these estimates.  When the cell voltage falls over time, the circuit current also falls.  This would extend the run-times, since the current is continually dropping.  Ignoring this effect would tend to under-estimate the run-time, so the above estimates may be slightly conservative....

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  • Selecting brightness/battery life at runtime

    Ted Yapo01/06/2018 at 19:16 2 comments

    The simple assembly code I wrote to get V3.0 up and running works fine.  You can configure the brightness in code, then download a new image to the PIC to change the battery lifetime.  This is OK if you are building these devices for your own use, but inconvenient for people who just want to buy them.  So, it would be nice to have a simple user interface on the board to select the runtime.

    I was initially concerned that the runtime might get accidentally changed over the years, then @jaromir.sukuba suggested a configuration mode that was only accessible briefly after the battery was inserted.  After that period, the brightness (and battery life) are locked-in until you remove the battery.

    I wrote some C code that does this yesterday - you can see it in the GitHub repo.  Well, it's mostly C-code.  There are two helper functions I had to write in assembly to access the high-endurance flash (HEF) portion of program memory to store the current mode setting.

    The user interface is simple.  After inserting the battery, the LED blinks out the runtime in years: 5 blinks = 5 years, etc.  To increment the runtime, you short pin 4 to pin 8 (RA3 to GND).  This "switch" is properly debounced in software.  I found some 2.6x1.6 mm switches, and I think I'm going to spin a set of V3.1 boards with them.  The code blinks out the runtime 10 times, then enters the running mode, and can't be re-configured without removing the battery.

    The only downside to this approach is that you sometimes have to wait a long time for the capacitors to discharge.  At 10-year run-times (or more), the mere 11uF of capacitance on board can power the LED for many seconds!

    There are a few configurable parameters in the code controlling the interface:

    // number of cyles of blinks to show before exiting config mode
    #define N_CYCLES 10
    
    // number of pulses per "blink" in config mode, higher = brighter blinks
    #define N_PULSES 10
    
    // year run-times and corresponding PWM periods
    #define N_MODES 10
    struct run_time
    {
      uint8_t years;       // number of years of run-time or mode number
      uint16_t pwm_period; // PWM period (in units of 1/31000 s)
    };
    
    // Note: frequencies converted to pwm periods
    const struct run_time period_table[N_MODES] =
      {{1, (int)(31000./326. + 0.5)},
       {2, (int)(31000./155. + 0.5)},
       {3, (int)(31000./98. + 0.5)},
       {4, (int)(31000./69. + 0.5)},
       {5, (int)(31000./52. + 0.5)},
       {6, (int)(31000./41. + 0.5)},
       {7, (int)(31000./33. + 0.5)},
       {8, (int)(31000./27. + 0.5)},
       {9, (int)(31000./22. + 0.5)},
       {10, (int)(31000./15. + 0.5)}
      };
    
    // default mode index 
    #define DEFAULT_MODE_IDX 1
     

    The run-times and associated PWM frequencies are stored in a table in the flash.  In this case, you can select from 1 to 10 year lifetime in 1-year increments. These values can be manually determined, or you can use a script and some hardware to find them automatically (I'll be publishing this script soon, I'm just cleaning it up now).  The table shown here is not quite right: there is no need for the frequencies to be integers. Frequencies with greater precision (e.g. 21.7 vs 22), will result in more accurate PWM period counts (e.g. 1428 vs 1409).  The differences can be significant for multi-year run times.

    I guess I just promised at least two more logs: the auto-tuning script, and V3.1 PCBs with a tiny switch.  It might feel like work if I didn't love it :-)

  • V3.0 Release

    Ted Yapo01/03/2018 at 15:02 0 comments

    I've finished the design and tuning for the OSLON Signal Verde LED flavor of V3.0.  The design is documented here; as I tune the other LED flavors, I'll add to this log.  Key features of V3.0 are the ability to finely tune the brightness/lifetime trade-off, build-time configuration for different optimal LED drive currents, and using a "5g" plastic jar as a case.

    The PCB uses a through-hole CR2032 holder (smaller than the SMT version), allowing it to fit in the jar.

    Circuit

    The V3.0 circuit uses a 74LVC1G123 monostable to produce short, accurate pulses for ramping up current through the inductor.  The inductor size and monostable pulse width are selected to optimize the drive current to the LED, based on measured LED efficiency vs current.  A PIC12LF1571 uses two synchronized PWM outputs to control the pulses.  One PWM gates power to the monostable (it consumes too much quiescent power), while the second PWM triggers the pulse.  The 16-bit PWMs on the PIC allow very fine tuning of pulse frequency and hence battery lifetime vs LED brightness.  Combined with inductor and monostable resistor selection, the circuit lets you tune for very good electrical and LED efficiency while allowing accurately tunable runtime.

    Note: a different value of R3 is required for each type of LED.

    Bill of Materials

    The following components are required for any flavor of V3.0:

    Depending on the LED you choose, you will need a different PCB, resistor, and possibly inductor.  Choose from one of the LEDs in the following sections:

    OSLON Signal Verde (505nm)

    This LED has the best optical match to dark-adapted eyes.  If you are just going to build one version, use this LED, and forget about the $3.49 price tag. It requires a 1mH inductor and 51k resistor for optimum efficiency.

    Luxeon C Cyan

    This LED is my second choice for dark-adapted eyes.  It's cheaper than the OSLON at $2.74, but just a little too blue for optimum visibility.  The higher optimal current also makes it flash more visibly at extended run-times (see below).

    Read more »

  • First V3.0 PCB

    Ted Yapo01/03/2018 at 03:44 5 comments

    I populated the first V3.0 PCB today.  It's actually been through a few iterations, but I didn't even bother filling any of the earlier boards.  This one is finally worth building.  The board is smaller now (23mm diameter), so it fits inside cheap, widely available "5g" polystyrene jars, but still uses an inexpensive replaceable CR2032 for power.

    This first PCB uses an OSLON Signal Verde LED.  I tuned this particular PCB to a 2-year run-time (12.5 uA) on a CR2032, and it blinks at 157 Hz.  The breadboarded version needed to be tuned to 123Hz to achieve the same run time (probably due to the 30% inductor tolerance).  The ability to fine-tune the current draw was one of the design goals of the V3 (as was fitting inside these jars).

    I also have PCBs for Luxeon C and Cree XPE / Cree XPE2 LEDs.  They are identical to this one except for the LED pads.

    Here's a visual comparison with the previous two generations for reference:

    One of the key differences is the use of a through-hole CR2032 holder, which is actually smaller in diameter than the surface-mount version (since the through hole doesn't need flat mounting tabs).  This was the final change required to fit the PCB inside the jar.

    Tuned to the same 2-year run time as the V2.0, the V3.0 is noticeably brighter.  It's roughly as bright as the V1.0 (which only runs 1 year).  But, you can tune the V3.0's brightness to last from less than 1 to more than 10 years on a CR2032 cell.

    I'll put together a more detailed post for those wanting to build some of this new version, including a BOM and placement diagram.  The PCB designs are already in GitHub, I just need to polish the software a little and put it up there.

    EDIT: I discovered that the clear jar makes a nice dim ring of light when placed upside down.  I usually keep a few of these lights on my nightstand, but the light can actually be enough to bother me.  Being able to attenuate the output somewhat by turning them upside down is an added bonus, especially since it looks so cool.

  • Programming PICs with Python

    Ted Yapo12/31/2017 at 04:09 8 comments

    No, not *in* python; they're still coded in assembly.  I mean using python to drive the assembler and PICkit3 programmer.  It turns out that MPLABX tools have a decent command-line interface.  I wrote some simple python classes to take the pain out of the command-line switches and enable automatic tuning of V3.x TritiLEDs.

    One of the problems with the V2.x design was the inability to fine-tune performance for variations due to part tolerances (the inductor, for example is +/-30%).  Since the pulse frequency of the V3.x design can be very accurately tuned, I would like to be able to "tune out" the component tolerances once the devices are assembled.  This calibration process means programming the part, measuring the current drain, adjusting the pulse frequency, recompiling, and re-programming.  Then, maybe looping through the whole process a few times.  This is not something I want to do manually for each unit.

    So, that's where the python comes in.  The fundamental code is shown below.

    Scripting the MPASMX assembler is trivial.  In order to tune the frequency parameter, I simply write a one-line include file from python including an "equ" directive.  This file is included by the main assembly file.

    Scripting the PICkit3 programmer is much more interesting.  You control the programmer through a script called "ipecmd.sh" (check for the name on different OS's), and it's documented in a file called "Readme for IPECMD.htm" in the mplabx/docs directory The programmer has provisions for powering the target device directly, and allows you to adjust the Vdd voltage.  Coupling this with a serial-enabled DMM makes a very nice little test kit: you can program different code on the device, then measure the current drain over a range of supply voltages.  There are probably a lot of interesting uses.

    I still have to integrate the DMM code and write the iterative tuning algorithm, but that's probably of less general interest than programatically driving the PICkit3.

    One thing I've discovered so far is that the output voltage of the PICkit3 doesn't seem very accurate.  I may add another serial-DMM to the test setup to measure the actual voltage instead of relying on what I tell the programmer to supply.

    #!/usr/bin/env python
    
    import subprocess
    
    mplabx_path = '/opt/microchip/mplabx/'
    
    class MPASM:
      def __init__(self, path):
        self.path = path + 'mpasmx/mpasmx'
      def assemble(self, filename):
        subprocess.call([self.path,
                         filename])
    
    
    class PICKIT3:
      def __init__(self, path, part):
        self.path = path + '/mplab_ipe/ipecmd.sh'
        self.part = part
      def program(self, hexfile_name, voltage):
        subprocess.call([self. path, 
                         '-TPPK3',
                         '-M',
                         '-F' + hexfile_name,
                         '-P' + self.part,
                         '-W' + str(voltage),
                         '-OL'])
      def setVoltage(self, voltage):
        subprocess.call([self. path, 
                         '-TPPK3',
                         '-P' + self.part,
                         '-W' + str(voltage)])
    
    class Frequency:
      def __init__(self, filename):
        self.filename = filename
      def set(self, freq):
        f = open(self.filename, 'w')
        f.write('FREQ    equ    .' + str(freq))
        f.close
    
    voltage = 3.2;
    PK3 = PICKIT3(mplabx_path, '12LF1571')
    ASM = MPASM(mplabx_path)
    freq = Frequency("frequency.asm")
    
    
    freq.set(123)
    ASM.assemble('tritiled_v30.asm')
    PK3.program('tritiled_v30.HEX', voltage)
    #PK3.setVoltage(1.8)
    

  • V3.0 Prototype Tests: 1-10 year run times

    Ted Yapo12/28/2017 at 03:41 0 comments

    I prototyped the V3.0 circuit, re-using one of the V2.x breadboards.  A few #Ugly SMD Adapters  hold the 74LVC1G123 monostable, OSLON Signal Verde LED and SOT23 MOSFETs.

    I tested the circuit at various pulse frequencies, and fit a line to the current drain.  The resulting equation is:

    where f is the pulse frequency.  Since there is a fixed cost of 1.15uA, the circuit is more efficient for higher frequencies (larger current drains and brighter LED). This equation is close enough to get you in the ballpark when tuning for a specific current drain (or lifetime), but then you may need to tweak the frequency by a Hz or two to fine-tune.

    Using the above equation and some manual fine-tuning, I came up with the following pulse frequencies for various run-times with a CR2032 cell.  The cell capacity is assumed to be 220 mAh, and the run-time calculated as:

    Frequency (Hz)Current (uA)Runtime (years)Notes
    26225.01.0
    12312.42.0
    788.153.1
    556.124.1
    424.965.1just flickering
    324.066.2 noticeable flicker
    263.537.1noticeable flicker
    213.078.2noticeable flicker
    172.719.3noticeable flicker
    142.4310.3noticeable flicker
    11.2420.24exceeds shelf life

    I threw in that last point (1Hz pulse rate) just because it's the limit of the code as currently written.  Since the shelf life of CR-series cells is commonly taken to be 10 years, I don't think run-times beyond that point make sense to predict.

    Of course, the frequencies shown here are just examples chosen to correspond roughly to integer year run-times.  You are free to choose any frequency in between or exceeding these.  As a test, I turned the frequency up to 5kHz (it measured 4.65 kHz on the scope), and the circuit consumed 405uA.  The LED was very bright.

    The brightness at 1- and 2-year run-rates seems equivalent to the V2.0 circuit with the single LED I've tested so far.

    Here's the PIC code.  It's not in GitHub yet.  The code just sets up the PWMs and then goes to sleep.  The WDT is set for a 256-second timeout, at which point it wakes and the PIC resets to re-initialize the state just in case some bits rot over the years.

    ;;;
    ;;; tritiled_v30.asm:
    ;;;    PIC12LF1571 code for CR2032-powered LED glow marker
    ;;;
    ;;;  20171227 TCY
    
        LIST        P=12LF1571
     #include    <p12lf1571.inc>
    
    ;;;
    ;;; adjustable pulse frequency parameter
    ;;; 
    FREQ            equ .123
    
    
    ;;; calculated and fixed parameters
    PHASE1          equ .0
    PHASE2          equ .0
    DUTY_CYCLE1     equ .2
    DUTY_CYCLE2     equ .1
    OFFSET_COUNT    equ .1
    PERIOD_COUNT    equ (.31000 / FREQ)
      
      ERRORLEVEL -302
      ERRORLEVEL -305
      ERRORLEVEL -207
    
      __CONFIG  _CONFIG1, _FOSC_INTOSC & _WDTE_ON & _PWRTE_OFF & _MCLRE_OFF & _CP_OFF & _BOREN_OFF & _CLKOUTEN_OFF
      __CONFIG _CONFIG2, _WRT_OFF & _PLLEN_OFF & _STVREN_OFF & _BORV_HI & _LPBOREN_OFF & _LVP_ON
    
    
      ORG     0
    RESET_VEC:
      nop
      nop
      nop
      nop
    INTERRUPT_VEC:
      BANKSEL   ANSELA
      movlw     b'00000000'     ; all digital I/O
      movwf     ANSELA
    
      BANKSEL   LATA
      clrf      LATA
    
      BANKSEL   TRISA
      clrf      TRISA           ; set all lines as outputs
    
      BANKSEL   WDTCON
      movlw     b'00100101'     ; WDT 256s timeout
      movwf     WDTCON
    
      BANKSEL   APFCON
      movlw     b'00000011'
      movwf     APFCON
    
      BANKSEL   PWM1CON
      movlw     b'00100001'    ; PWM2 triggered with PWM1
      movwf     PWM2OFCON
    
      movlw     HIGH(PHASE1)
      movwf     PWM1PHH
      movlw     LOW(PHASE1)
      movwf     PWM1PHL
    
      movlw     HIGH(PHASE2)
      movwf     PWM2PHH
      movlw     LOW(PHASE2)
      movwf     PWM2PHL
    
      movlw     HIGH(DUTY_CYCLE1)
      movwf     PWM1DCH
      movlw     LOW(DUTY_CYCLE1)
      movwf     PWM1DCL
    
      movlw     HIGH(DUTY_CYCLE2)
      movwf     PWM2DCH
      movlw     LOW(DUTY_CYCLE2)
      movwf     PWM2DCL
    
      movlw     HIGH(PERIOD_COUNT)
      movwf     PWM1PRH
      movlw     LOW(PERIOD_COUNT)
      movwf     PWM1PRL
    
      movlw     HIGH(PERIOD_COUNT)
      movwf     PWM2PRH
      movlw     LOW(PERIOD_COUNT)
      movwf     PWM2PRL
    
      movlw     HIGH(OFFSET_COUNT)
      movwf     PWM1OFH
      movlw     LOW(OFFSET_COUNT)
      movwf     PWM1OFL
    
      movlw     b'00000011'
      movwf     PWMLD
    
      movlw     b'00000010'
      movwf     PWM2CLKCON
      movlw     b'00000010'
      movwf     PWM1CLKCON
    
      movlw     b'00000000'
      movwf     PWM1OFCON
    
      movlw     b'11010000'
      movwf     PWM1CON
     movlw b'11000000'
    ...
    Read more »

  • Tuning V3.0

    Ted Yapo12/27/2017 at 19:14 0 comments

    The V3.0 circuit allows you to tune the pulse width and repetition rate independently.  Throwing in inductor size and peak current, and you have a four dimensional space in which to choose an operating point.  To simplify the design process, I condensed three of the variables (pulse width,  inductor size, and peak current) into a nomograph showing the efficiency of each pulse:

    The y-axis shows the efficiency of each LED pulse, just considering the per-pulse overhead of the PIC and monostable.  The overall electrical efficiency will be somewhat less than this value due to an approximately 1.1uA baseline current for the PIC, plus any losses in the inductor and MOSFET.  This chart also assumes a 100pF capacitor for the monostable; the resistor is chosen based on the desired pulse width from the formula:

    I experimentally determined k = 1.28 for C = 100pF at 3V (the datasheet charts don't include this value).

    How To Use

    Don't panic.

    The chart is easy - you use it to choose an inductor size and pulse width based on the peak LED current you want.  The vertical axis reads the pulse efficiency, which is to say, how much energy goes into the LED vs how much is wasted in the PIC and monostable during a pulse.  You want to maximize this efficiency.  The horizontal scale shows the monostable pulse width.

    There are two sets of lines on the chart.  The cyan lines, labeled on the left, represent different inductor values.  For example, following the 470uH line to a 3us pulse width shows that around 84% of the energy used during a pulse can get to the LED; the remaining 16% being wasted by the PIC and monostable.

    The magenta lines (labeled on either side) represent the peak current through the LED.  Using the example above of 470uH and 3us, we see that this point is intersected by the 20mA line, so the peak current through the LED for this combination is 20mA.

    In practice, you typically know the desired LED peak current, and want to choose the inductor and pulse width.  So, here's another example: you know your LED is most efficient with peak currents of 50mA.  To get the most efficient drive, you examine the 50mA magenta line, and  evaluate each possible inductor value.  If you have a 1mH inductor available, you can get 50mA pulses at about 17us with 98% efficiency.  On the other hand, if you choose a 100uH inductor, you'll end up with around 1.7us pulses and 88% efficiency.  That's it.

    Complications

    So, the answer seems easy: choose the 1mH inductor, which is more efficient.  In reality, you may want to choose a smaller inductor since it will allow you to run at lower currents without visible flashing.  For a given current, using a larger inductor means more energy per pulse, so for a specified current drain, you will need to choose a slower flash rate, which may become visible.

    A second complication is that the optimum LED current may not turn out to be the best choice considering the monostable pulse overhead.  This is especially true for small LED currents.  As an example, the OSLON Signal Verde LED I have been testing has a current around 4mA for peak efficiency.  Unfortunately, driving this LED at 4mA with a reasonable 1mH inductor results in around 40% pulse efficiency (60% of the energy used by the PIC and monostable).  So, you can do better by choosing a larger current: you trade electrical efficiency for LED efficiency to get a better overall efficiency.  You can see this tradeoff directly in a plot of the three efficiencies (electrical, LED, and overall) vs pulse width for a 1mH inductor:

    Although the LED would perform best with a pulse of approximately 1.5us, and the rest of the circuit at 10 or more, the combination performs best at around 6.5us.  Here, the electrical efficiency is over 90%, while the LED efficiency is 86%.  Multiplying the two reveals that the circuit is emitting...

    Read more »

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Mike Harrison wrote 01/05/2018 at 14:49 point

That inductor looks way oversized for the currents involved - you might get a tiny advantage from lower DCR but should be able to use something a lot smalller with negligible performance loss.

Also you may find that a white LED ( with high colour temp) will give more light at low currents.

  Are you sure? yes | no

Ted Yapo wrote 01/05/2018 at 15:28 point

Yes, I agree, the inductor looks way oversized. Except that there are only (25) 1mH inductors stocked by DigiKey that are shorter.  I have horizontal board space (dictated by the battery), but am short on height, so to speak.

Of the 25 shorter ones I can find:

https://www.digikey.com/products/en/inductors-coils-chokes/fixed-inductors/71?k=inductor&k=&pkeyword=inductor&pv7=3&pv7=2&FV=1140003%2C1f140000%2Cmu1mH%7C2087%2Cffe00047&mnonly=0&ColumnSort=1500&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=50

The next best one for DCR is 6.5-ohm (but is the same footprint and only 0.2mm shorter) and costs $1.23 vs  4.5-ohm and $0.74.  The shortest ones they have are 2.0mm high (vs the current 3mm), and the best of those is 11.5-ohm DCR for $1.79.

If I'm willing to go to a 31.5-ohm DCR, I can get a 1007-smt package one, but I think that would really take a toll on the efficiency: a quick simulation shows 30% loss vs 5%.

1mH is a large inductance, and you just don't find great ones, but I think this particular one is in a sweet spot for height, DCR, and cost.

If you could generate much shorter pulses (efficiently) to drive the MOSFET switch, you could use smaller-valued inductors to get the currents required, but that has proved elusive so far.

I've tested a bunch of white LEDs, and the phosphors don't seem to work well at low currents.

The bigger problem is that light away from the eye's response peaks (505 or 555nm depending on dark adaptation) is wasted in this application.  White LEDs are useful for seeing objects in color, but where you'd care to put a glow marker, your eyes won't see color anyway, since they'll be dark-adapted.

  Are you sure? yes | no

Simon Merrett wrote 01/05/2018 at 22:50 point

Hi @Ted Yapo, I'm probably way out of my depth here but I'm using the 2.2mH part from the Bourns SRF3216 series, which is apparently available on digikey in the US (https://www.digikey.com/products/en?keywords=SRF3216). Could you make use of these? They appear to have inductance in the range you are considering and low DCR. They're tiny too. 

  Are you sure? yes | no

Ted Yapo wrote 01/06/2018 at 02:35 point

Hi @Simon Merrett , that's an interesting part you have there.  It's clearly intended as a common-mode choke to filter noise, more akin to a transmission-line transformer than a power inductor, but maybe you found a hidden gem.

My suspicion is that the core is lossy, which is what you want for noise suppression, but I could be surprised.

Which specific part number are you using?  The datasheet lists impedance vs frequency, but doesn't list inductance values - that makes me think a good portion of that impedance is real, as in resistive, as opposed to imaginary, as in inductive (reactance).

They're cheap enough that I can throw a few in my DigiKey cart and test them out.  The size is certainly very nice :-)

  Are you sure? yes | no

Simon Merrett wrote 01/06/2018 at 08:08 point

@Ted Yapo here's the link to the part where I bought it (they might have mis-categorised it) https://uk.rs-online.com/web/p/wire-wound-surface-mount-inductors/8118777/

BTW, if I was selecting for core lossiness (?), is there anything apart from the physical size of the core that I could look for in the specs to give me a clue whether it is high or low loss? 

Would be great to see you characterise this part as it would help me understand what's going on in the current and next planned version of #Yapolamp.

  Are you sure? yes | no

Ted Yapo wrote 01/06/2018 at 14:19 point

@Simon Merrett Thanks for the link.  I found them at DigiKey and added 10 to my cart.

Loss is a function of core size (for saturation issues) and wire size (DCR), but also of core material, which isn't often listed on inductor datasheets.  Saying the core is "ferrite" gives you about as much information as saying it's "wood" vs "metal" (e.g. is it balsa or oak?) There many ferrites with very different properties - some more suited to power inductors, others useful at RF, etc.

  Are you sure? yes | no

crjeder wrote 11/30/2017 at 07:20 point

Have you considered entering the Coin Cell Challenge? 

https://hackaday.com/2017/11/29/coin-cell-challenge-use-coin-cell-win-prizes/

  Are you sure? yes | no

Simon Merrett wrote 11/30/2017 at 13:29 point

+1

  Are you sure? yes | no

Ted Yapo wrote 12/04/2017 at 01:59 point

Thanks for the heads-up on the challenge.  It's tempting, and maybe I'll enter some variation of the idea.  There's a version I've been meaning to make that's much larger and more expensive, but also more efficient and longer-running.  It's not practical, really, except maybe for a contest...

  Are you sure? yes | no

crjeder wrote 12/04/2017 at 14:23 point

Go for it! If this were a public vote contest you would have mine!

  Are you sure? yes | no

KSUdoubleE wrote 08/18/2017 at 17:04 point

Nice project!  I made it last week, etched my own board so I had to arrange things somewhat differently to reduce vias since I don't do through hole plating.  I also used a 12LF1501 instead for the processor because I had some around.  Took a little tweaking in the code as it doesn't have a OSCCAL register, but it is working well.  Good job matching the glow/color.

  Are you sure? yes | no

Ted Yapo wrote 09/04/2017 at 16:44 point

I'm glad you found it interesting/useful.  Post a picture if you get a chance - I really like to see these things in the wild.

Having OSCTUNE doesn't give you that much control - just a fine tuning, really.

  Are you sure? yes | no

KSUdoubleE wrote 09/08/2017 at 19:30 point

  Are you sure? yes | no

Ted Yapo wrote 09/08/2017 at 23:47 point

Awesome!  I think that's my favorite build yet.  Do you mind if I post the images in a build log?  I'd like to make a gallery of builds done by others.

  Are you sure? yes | no

KSUdoubleE wrote 09/11/2017 at 13:27 point

Sure that is fine.  Thanks again for sharing your project!

  Are you sure? yes | no

bulrush15 wrote 07/17/2017 at 12:50 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. 

  Are you sure? yes | no

Ted Yapo wrote 07/17/2017 at 14:11 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.

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.

  Are you sure? yes | no

bulrush15 wrote 07/14/2017 at 20:23 point

Have you considered these low-power circuits for powering an LED? From Don Klipstein, the guru of lighting. http://donklipstein.com/ledlocup.html. I thought it might provoke a brain storm.

Another interesting link, ultra-low power circuits: http://www.discovercircuits.com/M/micropow.htm

  Are you sure? yes | no

Ted Yapo wrote 07/14/2017 at 22:23 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.

  Are you sure? yes | no

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?

  Are you sure? yes | no

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: https://focuslcds.com/journals/el-backlights-vs-led-backlights-legacy-display-technology-vs-cutting-edge/ ) 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?

  Are you sure? yes | no

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..

  Are you sure? yes | no

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:

http://www.edisontechcenter.org/electroluminescent.html

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

https://www.digikey.com/product-detail/en/lumileds/L1C1-CYN1000000000/1416-1917-1-ND/5872100

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?

  Are you sure? yes | no

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. https://www.digikey.com/product-detail/en/vishay-semiconductor-opto-division/VLMP20D2G1-GS08/VLMP20D2G1-GS08TR-ND/4074306 which has (aprox) 1,75 [edit, was 0.5] lm/W at 2 mA * 2 V (converted candela to lumen with http://led.linear1.org/lumen.wiz)

  Are you sure? yes | no

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.

  Are you sure? yes | no

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.

  Are you sure? yes | no

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. http://www.ebay.com/itm/LTC3105-Energy-Harvester-demo-module-/332136264440?&_trksid=p2056016.m2516.l5255

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

  Are you sure? yes | no

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.

  Are you sure? yes | no

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.

http://www.ti.com/lit/ds/symlink/tpl5010.pdf

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 https://www.fetchmodus.org/projects/beam/photopopper/
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

http://www.embedded.com/electronics-blogs/break-points/4429960/How-much-energy-can-you-really-get-from-a-coin-cell-

http://www.embedded.com/electronics-blogs/break-points/4430050/Using-a-capacitor-to-sustain-battery-life

  Are you sure? yes | no

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.

  Are you sure? yes | no

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

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

  Are you sure? yes | no

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!

  Are you sure? yes | no

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 https://www.silabs.com/documents/public/data-sheets/TS3005.pdf. 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. 

https://www.sitime.com/products/datasheets/sit1534/SiT1534-datasheet.pdf

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. http://www.zippo-windproof-lighter.de/6in1/flashlight.html

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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

lol

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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? http://www.greenarraychips.com/home/products/

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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 hackaday.io 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.

EOR

(end-of-rant)

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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:

https://hackaday.io/project/11864-tritiled/log/44864-version-2

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: https://hackaday.io/project/19113-decade-flashlight

The circuit and PCB design and theory, along with a detailed BOM are at: https://hackaday.io/project/19113-decade-flashlight/log/50821-circuit-design-led-driver-board

The code is described in: https://hackaday.io/project/19113-decade-flashlight/log/50822-pic12lf1571-code

A 3D-printed case for that particular use (a flashlight instead of a marker) is presented: https://hackaday.io/project/19113-decade-flashlight/log/50823-printed-case

Detailed assembly instructions are given: https://hackaday.io/project/19113-decade-flashlight/log/50824-assembly-instructions

All the design files are in GitHub: https://github.com/tedyapo/ten-year-flashlight

And the boards are shared on OSH Park: https://www.oshpark.com/shared_projects/0BWPH1JP

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:

https://www.amazon.com/Science-First-Spectrometer-Project-Star/dp/B008ELSDVA

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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[deleted]

[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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