Solar Light Controller

This circuit charges a single NiMH or NiCd cell from a solar panel, while only allowing the battery to discharge when the light input is low. It's intended to control a Joule Thief or similar to run some kind of LED lighting, very similar in theory to solar garden lights.

Fundamentally, this is a PNP transistor controlled by a resistor divider sensing the solar cell input. While the solar cell voltage is above a certain point, the divider keeps VBE below the threshold voltage and the transistor remains off. But when the light level gets low enough, current flows through R3, turning on the transistor, and flowing battery current into Vout. The diode prevents the battery from backfeeding into the solar cell in this case. That plus the 100Ω resistor provides some head room for the solar cell and ensures the divider keeps the base voltage well above the turn-on point. The NiMH/NiCd cell will also not go above 1.5-1.6V, keeping the emitter clamped.

Theoretically, this could probably still work at higher battery voltages, provided the solar cell puts out enough voltage. I wouldn't use this directly with a lithium ion cell, though, as they will not self-regulate a trickle charge the way nickel cells will. But if you put in a suitable charge control chip, it would probably work fine.