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I'm curious if anyone has experience with getting a simple board like the Adafruit Trinket or DigiSpark or any of the other ATTiny based clone boards running on solar power. E.g. with a solar panel and a lipoly or other rechargeable battery.

There are a combination of things that must be accomplished for that to work "well" (e.g. with the smallest possible battery and panel) which are not trivial.

  1. The Trinket has to sleep in low power as much as possible. That probably requires cutting off the power indicator LED, and then using some tricky code to actually put the chip into low power modes.

  2. The Solar Panel has to be controlled so it doesn't discharge the batteries at night, and doesn't over charge the batteries during the day time.

  3. The battery needs minding as well, depending on the type.

When you add all that up... well... I'm hoping someone else has done it? Or part of it?

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  • \$\begingroup\$ You could probably deal with possible overcharging by simply running the CPU more to waste the excess power. \$\endgroup\$ – Dave Tweed Jan 5 '15 at 22:31
  • \$\begingroup\$ Welcome to my (other) nightmare :-) \$\endgroup\$ – Russell McMahon Jan 5 '15 at 23:55
  • \$\begingroup\$ There are many LiIon/LiPo charger ICs - the one Dave cites is about as good as any. The ADAFruit charger is surprisingly dear at $17.50 and does little more than put a $2.15/1 IC - alas in QFN - on a PCB (plus a few lights. [$20 buys you the uChip demo kit from Digikey]9http://www.digikey.com/product-detail/en/MCP73871DM-VPCC/MCP73871DM-VPCC-ND/2125386) - datasheet](ww1.microchip.com/downloads/en/DeviceDoc/51812b.pdf) - .... \$\endgroup\$ – Russell McMahon Jan 6 '15 at 0:06
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    \$\begingroup\$ LiIon/LiPo / (LiFePO4) are by far the easiest chemistries to manage. | In a SOT23-5 pkg in stock $0.61/1 Digikey you can get MCP73811T - datasheet that will charge the battery properly. | To stop dark-discharging a Schottky diode is fine - 1N5817 or equiv. Panel Voc is > Vbattery and small loss in diode has minimal effect on available power. LiIon/LiPo needs >= 4.2V + what ever headroom charger IC wants. 10 cells 5.5Voc probably OK. \$\endgroup\$ – Russell McMahon Jan 6 '15 at 0:18
  • \$\begingroup\$ Can't upvote, but thanks for the replies. Still hoping someone has actually done it. \$\endgroup\$ – James Newton Jan 6 '15 at 19:19
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Is size a concern? Are you planning on making a custom board?

If you just want to slap a few modules together, this is easy enough.

Along with the Trinket, get the USB/DC/Solar charger from Adafruit - http://www.adafruit.com/product/390

The IC on the board will prevent any over-charging.

Sleep code is easy enough for the 85 - here is a more info: https://learn.sparkfun.com/tutorials/h2ohno/low-power-attiny

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  • \$\begingroup\$ Can't upvote or anything else here (yet) but thanks for your answer. I was hoping someone had done it already. The Adafruit board is very overpriced (as Russell noted), and seems to have little to no competition... "Someone" should change that. \$\endgroup\$ – James Newton Jan 6 '15 at 19:21

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