Sunday, July 17, 2016

Power IoT devices with solar energy, the $1.50 cheap way!

Had to find an easy solution to power some of my low-power device, like the one on the right (it is a minimalist 433MHz receiver).

A long time ago I bought a bunch of solar flashlight keyrings specifically for scavenging the parts.

They proved to be better than my expectations, with a very convenient 37×22×1.1mm solar cell which gives 7V unloaded, and up to 8mA for tested loads of 1 and 100 ohms (it "rains fire" as we often say here in summer: specs are only 5.2mA~6.3mA at 3.4V~4.5V loaded).

Admittedly, it is not much, but it is enough for a great lot of IoT devices, and given the size, it is just GREAT.

I tried to source the cells directly. They are referenced as ''SC-3722-9'' and they can be found at the manufacturer. They offer variants of these interesting "strong light type amorphous silicon solar cell" (including "poor light" cells). But no way I could find a reseller which is cheaper than what I bought in the first place, namely these dirt-cheap solar keyrings:

These are just super cheap and super useful ... for the parts!
(less than $1.50 each on ebay or on bangood)
Interestingly, Aliexpress listed the solar cells at $30+ for a pack of 10, but no way: not only the solar keyrings above can be found at $1.50 each and include a useful LIR 3023 rechargeable lithium cell behind a schottky diode. So it really is a bargain as it solves globally the problem of powering non-hungry IoT devices at once!

Disclaimer 1: there is no regulation! The provided cell may charge up to 4V so an LDO regulator (or a zener diode) may be required according to the circuit. Without the cell, it may be a bigger issue though for my circuit above! A zener diode would make sense to cap the highest voltages given the small needs of the circuit (check the comments related to my question).
Now, for a 165mA 9V rechargeable battery, the trickle charge would be 165mA / 20, i.e. approx 8mA: perfect match to keep the battery up when it is not used too often.

Disclaimer 2: out 30 items purchased, 5 of them where seemingly DOA (defective on arrival). No biggie: only some bad solder job (a battery lead did not make contact with the respective PCB pad). Eventually, all of them proved to be OK. May be this rate explains why the original seller stopped selling them, but there are still many resellers of these cheap keyrings on ebay.

And even if the battery was dead or fak, the mini solar panel is still a deal :)

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