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suppose I want to keep an Arduino powered during the night, and recharge it during the day with a solar cell. What kind of circuit do I have to make, how many and which type of batteries, and what size of solar panel do I need to keep it going?

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SolArduino v1

Board

PCB

Solar Piston v1

Schematic

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I'd start by measuring the power consumption of the Arduino and figure out how to put it to sleep when possible. From what I've read, the Arduino Pro Mini consumes something like 5 mA at 3.3 V, which is around 15 mW. With a solar panel, your capacity factor is likely to be around 10 percent, meaning that it will produce an average power of around 10% of its peak power. This suggests that you need a panel that can produce 150 mW, but I'd probably go for a 1 W panel unless size is an issue.

For batteries, you want to get as low a voltage as you can, while still being above 3.3 V. I might try a 6 V lantern battery.

For wiring, put all three devices (Arduino, battery, and solar panel) in parallel. Example (minus the battery) here.

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  • \$\begingroup\$ why parallel? I've seen a slightly more complex diagram for wiring, involving one electrolytic and one diode. Does it make any difference ? \$\endgroup\$ – Stefano Borini Nov 1 '10 at 17:04
  • \$\begingroup\$ The diode is a good idea-- it's to prevent the battery from pushing current back through the solar panel when it's dark. I don't think the cap is necessary. If there were no battery, you'd want the cap as a mini-battery to smooth out fluctuations in the panel voltage, but the battery itself will act like a capacitor. (I'm assuming you're using a relatively large battery, not a coin cell.) \$\endgroup\$ – pingswept Nov 1 '10 at 18:05
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    \$\begingroup\$ If you run the Arduino at 3.3V (8MHz), you can use a 1W solar panel, 3.7V Li-Poly battery, and Li-Poly charging circuit. A 3.3V DC-DC converter circuit allows you to keep the 3.3V even with 1-3V (1-2 AA batteries). \$\endgroup\$ – OIO Nov 1 '10 at 18:54

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