Designing a solar harvesting blinking gadget

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

In Februari 2019 I designed revision 2.

I changed following specifications

• 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.