This project describes all the revisions and design decisions for my solar harvesting blinking gadget. I started selling with revision 9 and consider this as a milestone.
I have poured the PCBA with this epoxy into a silicone piramid and heart shape. The pouring is a delicate process. You need to work very clean, have everything prepared before actually starting. I have cleaned the silicone mold with water and soap and used a can of compressed air to blow out the water, then I have put the mold in the oven at 100 degrees celsius to dry further. On the table I have put couple sheets of protective baking paper. And put everything on it, the bottles of 2 parts epoxy, the bottle of UV stabilizer, a couple disposable cups, a couple wooden stirring sticks, a roll of kitchen towel, a kitchen weighing scale. And then I started mixing the 2 parts epoxy, 50g of part A, 30 grams of part B, 1 gram of UV stabiliser. To avoid bubbles stir slowly and in 1 direction, after 2 minutes pour the stirred mixture slowly in another cup, and mix again, this makes sure that the two parts are well mixed. Then to reduce amount of bubbles I slowly poured the mold 3 quarters full with epoxy and put the electronics slowly in. Then I filled the mold full. Using the stirring stick I tried to remove the bubbles. The curing is slow, you have about 15 minutes to finish, after a few hours it feels hard and after 24 hours curing is done.
It was quite easy to remove the epoxy from the silicone molds.
renamed to AEMLBLINK R1 because that better fits its function
added 0.1" pitch test pads so the board can be tested after assembly and before super capacitor and solar panel are soldered. Also added one for 1.8V. A user may connect a BLE module or something there.
removed copper from the left side of the board because BLE module may need copper clearance
set solder mask oversize to 0.05mm, previously it was 0.00mm
added clear LED polarity indicators on silkscreen
Below is the schematic design
I had 10 boards assembled at Elecrow
They did a great job with the PCB and soldering
I have created a simple pen shaped PCBA test tool. It is to test the PCBA after assembly and before the solar panel and supercapacitors are soldered. It's just a small perf board with four 0.1" pitch male headers, a 1.5V alkaline battery to simulate a solar panel and a 220uF capacitor to simulate the supercapacitor. When the LEDs on the AEMBLINK board start to blink I know that the AEM10941 and the TPL5110 circuits work correctly. It also has a LED on the 1.8V output from the board. I
The super capacitor is full in 5 minutes in full sun.
Blinking duration is at least 8 hours on a full super capacitor.
I have tested blink duration with with 20, 27, 33 and 43 ohm LED series resistance. All 10 assembled boards blinked for at least 8 hours with a 33R series resistance.
So far what I've learned
yeah I like the PCB design
I need to change LED series resistor to 33R on all boards
next step is selling on Tindie. Some boards I will coat and sell as earrings or necklace, some I will pot with epoxy in a heart shape and sell as necklace, and some I will pot into a silicone piramid shape and sell as desktop gadget
I only made a small change because I received a discount code from Eurocircuits and I thought let's try that out. I only moved the TPL5110 circuit to the side to make space for any circuit that an user may want to add. I did not even assemble the PCB.
What I've learned
Eurocircuits is fast, they deliver in 3 business days
PCB thickness any other than 1.6mm don't fit in the pooling batch and are very expensive and so the board is 1.6mm in stead of the desired 0.6mm.
1.6mm is too thick for my application. It add unneeded weight. I need 0.6mm PCBs and they are cheap with the Chinese PCB manufacturers
In the previous versions I didn't like the LED blink interval (5s) and the ON duration (80ms) was fixed. I have read somewhere (sorry for no link) that the human eye/brain perceives a 10ms long pulse as full brightness and that flashes with an interval between 0.5s-5s strongly pulls attention. And so I can reduce average consumption while drawing more attention. I found that the TPL5110 "Ultra Low Power Timer with MOSFET driver" that is also for sale on Adafruit and Sparkfun was a great choice to make a bright and low power flash that draws a lot of attention. This is a timer IC ideal for low power applications. Normally it is used to power gate applications and power it ON at a programmable interval. In shutdown it consumes only 35nA. I can choose the blink interval between 100ms and 7200s by choosing a resistor value. This IC is intended to be connected to a microcontroller that sends a DONE signal to the chip when it is done performing its task, for example sending a sensor value wireless, and then it is completely powered off until the timer expires. In my application I don't have a MCU but a simple LED that is powered on. I wanted to feed back the power signal through a RC low pass filter to the DONE pin
Design changes in revision 7
Instead of using the status2 pin from the AEM10941 to blink the LED I have used the TPL5110
I made the PCB rectangular (30x30mm) and put the solar cell in the middle of the board
disabled the AEM10941 LV output (1.2V) because it saves some power and one 10uF capacitor
make place for an adhesive pin so it can be used as a badge
put 2 LEDs on opposite side of the board
changed back to 1 mosfet for 2 LEDs
remove copper pour around the LEDs so that reflected light can transmit through the PCB
See the schematic below.
On bottom right you see the TPL5110 circuit. The LED is power gated through a P-channel mosfet. The LED current flows through a 20R resistor and two LEDs in parallel. The power gated signal is fed back through a RC low pass filter to the DONE pin. The DONE pin is considered high when the voltage exceeds 0.7*VDD. With a 1M/22nF the RC time (63%) is about 22ms and close to the desired 10ms. A 6.8kohm resistor sets the interval to ~2seconds. I have added a 100k pull down to the feedback signal to make sure the RC filter capacitor is discharged before it is re-enabled after 2s.
The PCB wihtout any components assembled and a solar panel for reference
I have used the to measure actual LED pulse duration and current from the supercapacitor.
And it shows actual LED was ON for ~14ms. I have experimented a it with the RC values and found shorter pulses were actually perceived as less bright and taking into account tolerances on the capacitor value so I sticked with 14ms.
The average current with the 47 ohms LED series resistor was 6.8uA acording to the Qoitech Otii Arc. I calculated that with a full 1F supercapacitor and 0.5V voltage drop the LED would blink for 20 hours which is far more than the desired 8 hours.
I have changed the series resistor to 20 ohms which gave 1.6mA peak current and 9.6uA average current. Then the LED flashes were nice and bright and the blinking would last 14 hours which is great.
Then I built 4 devices to test actual charge time in full sun and blink duration in the dark.
I found at 800W/m2 it takes only 5 minutes to fully charge the super capacitor and they kept blinking in the dark for ~10 hours! This is less than the expected 14 hours. So I need to check what is going on.
What I've learned and can improve:
I really like the circuit with the TPL5110 because flashes...
In previous AEMLOVE designs I used the STATUS2 output from the AEM10941 to flash a LED which is high for ~80ms every 5 seconds. I think 80ms is actually pretty long and it can be shorter while keeping brightness perception. I learned about 1.5V powered LED flash circuits that have very low average current (~10uA) and wanted to make a similar circuit.
Normally the 1.5V is not enough to light the LED because red led forward voltage is at least 1.7V. So how does it work? First the capacitor is charged to 1.5V through the two resistors. On the left it is 1.5V and on the right 0V. When the mosfet is closed the left side suddenly changes from 1.5V to 0V the right side remains 1.5V lower than than left, and so it is -1.5V Then shortly there is 3V over the LED and it flashes until the capacitor has discharged.
And since the LED can be powered from a very low voltage that is basically doubled it was possible to try out LEDs with higher forward voltage, like a white LED. Since the AEM10941 has two regulated output voltages 1.8V and 1.2V I could design the circuit twice and experiment with it.
I read here that the eye perceives 10ms flashes at full brightness and after that it decays to 50% in 20ms. This means for a 10ms flash you actually perceive 30ms. This could really reduce average current
In revision 5 I made following design changes
Implement new low power LED flash circuit
try out white LED
Move USB connector to lower side of the PCB
Use a smaller dimensioned 1F 2.7V supercapacitor Nichicon JUWT1105MCD this has only 6.3mm diameter while previously diameter was 8mm.
Reduce the hole for the supercapacitor even more
Use surface mount solder pads for the supercapacitor
Reduce board size even more
Below is the circuit diagram
What I've learned from Revision 5
the LED flash is actually not so bright
the supercapacitor discharged in a few hours while I expected much more
the hole in the PCB was designed so it could also fit another supercapacitor a 0.5F cheap chinese supercapactors. But I really like this Nichicon super capacitor so the hole can be reduced.
I also like the SMD solder pads for the supercapacitor
the USB connector on bottom is definately better than on top
I didn't like the USB connector anymore, the PCB has to be 2.0mm thick and then it needs the right solder thickness for the USB connector. And still it didn't feel snug. I thought such a connector could actually destroys people laptops.
I was disappointed by the LED brightness and life time. So I made a simulation in LTSpice. Not perfect but it gives me an idea.
And I found that peak current is actually really high and the duration very low, something like 5ms.
That basically explains why it was not perceived as very bright. In addition it was not low power. So I decided this is not the circuit for me.
I skipped revision 3 because I ordered the board and very shortly after that I ordered revision 4. In revision 4 I made following changes
reduce PCB dimensions further
smaller holes for the necklace clips
put solar panel on opposite side of the SMD components
put all SMD components one one side of the board for easier assembly
smaller PCB hole for the supercapacitor
select a new LED that can be reverse mounted on the PCB
I selected the KPTL-3216SURCK-01 LED from Kingbright. According to the datasheet it has 550 mcd at 20mA, so more than the Wurth 150060RS75000 which has 250 mcd.
What I've learned from this redesign
LED viewing angle is actually very important for visibility. I found the new Kingbright KPTL-3216SURCK-01 LED brightness disappointing. It has 70 degrees viewing angle while the Wurth 150060RS75000 has 140 degrees. That is perceived much brighter from different angles.
The reverse LED hole in the PCB actually limit the viewing angle and the visibility
the necklace actually interferes with the USB connector
the smaller PCB hole for the supercapacitor is good for the epoxy resin, it is now well glued to the board
I dont like so much the through holes for the supercapacitor because the leads penetrated to other side of the PCB and touch the solar panel
use cheap ($0.50) 1F 2.7V supercapacitor from Aliexpress or ebay
make a slot in the PCB to embed the supercapacitor
use a cheap ($0.40) solar panel form Aliexpress, make two solder holes in the PCB
few holes for hanging or mounting the board
different PCB color
PCB with rounded edges
simplify schematic, remove the configuration resistors from revision 1 and use only the 1.8V output, use only one LED
remove the LED series capacitor
What I've learned from revision 2 PCB
the USB connector has sharp corners
the 2x 0.5V/44mA Ixys solar cells are cool but very expensive and low power when compared to the 1V/80mA solar panel from Aliexpress
the cheap 1F/2.7V supercapacitors from Aliexpress work just fine
the cheap chinese solar panel is just $0.40 in qty 10 and works fine
the large slot for the super capacitor is still too large
the PCB is still large and heavy
I also wanted to experiment with waterproofing because it must be wearable. The boards were poured over with super clear epoxy resin.
The epoxy was pretty thin. In the beginning it made a nice layer but after 6 hours most of the epoxy dripped off the board. It does not have same thickness everywhere. The epoxy does not fill the holes around the supercapacitor because it is too thin. The epoxy layer on the surface mount solar cells is actually ugly
It's December 2018 when I wanted to design a solar harvesting gadget.
Specifications:
cheap
easy to assemble
uses the E-peas AEM10941 solar harvesting IC
the IC STATUS2 pin controls the LED ON time (80ms high every 5 seconds)
LEDs connected through a simple resistor
on board 0.5V/44mA solar cell KXOB22-12X1L or an external solar panel
stores it's energy in a through-hole supercapacitor
can als be charged from USB
additional LEDs connected through a series capacitor for very short flashes
Since I have 2 children and no time I designed a quick and dirty PCB and ordered from Elecrow. Below is revision 1 schematic.
Every 5 seconds the status2 pin goes high when the IC does MPPT evaluation (check solar panel open circuit voltage), I have no idea what applications would need this information but I can use it toe blink a LED with low duty cycle. The LEDs are switched through a DMG1012UW N channel mosfet.
I designed LED on the 1.2V and the 1.8V output of the IC. It was silly to design it on the 1.2V becaise there is no LED that works at such low voltage.
The battery can also be charged from the USB 5V. I used a voltage divider to make a 2.7V voltage for the supercapacitor and a diode to prevent reverse current.
In January 2019 I received the PCB
On the left is the supercpacator A 1F 2,7V through hole supercapacitor from Aliexpress (CXHP2R7105).
On bottom is the solar cell.
On left side is the AEM10941 IC and it's large boost inductor.
The other two large though hole capacitors I did some experimentation with that I won't explain here.
The PCB has 2.0mm thickness. That is not enough for the USB connector so I had to manually add more solder paste.
As you can see she solar cell is surface mount solderable.
What I've learned from revision 1 PCB
even with 2.0mm PCB thickness and a big blop of solder paste on the USB connector it does not fit snug into the USB port. It feels not right.
the 0603 size LED from Wurth (150060RS75000) is actually super bright while the programmed current is about 2mA
after 5 mins charging on USB the LEDs started blinking, but it initially requires some light on the solar panel other wise the IC regulated outputs don't work
after about 10 mins of USB charging the supercapacitor reaches the programmed voltage of 2.7V
the LEDs blinks about 14 hours on a full supercapacitor. This is long enough for a wearable.
the voltage divider on the 5V gets untouchable hot
a single 0.5V/44mA solar panel does not charge the 1F super capacitor quickly, it took about 10-20 mins in outdoor light before the LED started blinking
the series capacitor make the LED blink only once, so that does not work.
PCB revision 2 you will find in the next project log.
Jasper Sikken
FrazzledBadger
DailyDIY
Jan--Henrik
I used a silicone shape from Alirexpress and super clear two compound epoxy resin