A cheaper solar harvesting blinky - FAIL

A project log for Designing a solar harvesting blinking gadget

Based on earlier solar harvesting projects I wanted to design a solar harvesting gadget

jasper-sikkenJasper Sikken 06/07/2020 at 12:170 Comments

Since solar panel open circuit voltage remains relatively constant under different light levels this time I designed a solar harvesting blinky circuit WITHOUT an expensive solar harvesting IC. 

I thought the open circuit voltage of a 3V solar panel under low light would still be high enough for a blinky circuit that needs minimal 1.8V so I designed following circuit. Solar energy goes through the diode into the 2.7V supercapacitor. The TPL5110 blinks the led for 15ms at 2s interval. The MCP102 is a voltage supervisor that disables the TPL5110 below 1.9V.

I learned that the LEDs are super bright when supercapacitor voltage is more than the required 1.8V LED forward voltage.  

After a full charge in sunlight the LED stopped blinking early in the night while my previous design with the AEM10941 harvesting IC continued blinking all through the night. To extend battery lifetime I have increased LED resistance from 33 ohm to 330 ohm. 

Then I left the solar blinkies in a relatively dark cabinet in my living room for a couple days to understand how they compare. I noticed the old blinkes, with the AEM10941 solar harvesting, restarted blinking in the morning while the new design was not. When I measured the solar panel voltage I found only 1.15V which means it would never charge the mimimal required 1.8V. Then I tested a bare solar panel (wihtout blinky circuit) I also noticed voltage was less than 1.5V. So apparently this solar panel does not have high enough open circuit voltage in low light conditions, while the older blinkies, with the AEM1094 solar harvesting IC were actually harvesting energy. So my idea that the open circuit voltage of a solar panel is high enough under low light conditions failed.