At Maker Faire Austin, Evil Mad Scientist Labs was demonstrating some really simple and useful circuits powered by solar cells. Today, they’ve posted the details on their site.
Interruption-resistant direct drive: The “direct drive” circuits work well for their design function, but are rather basic. They provide no energy storage, and so are quite vulnerable to blinking out when a bird or cloud passes overhead. For some applications, like running a small fan or pump, that may be perfectly acceptable. For other cases, like powering a microcontroller or other computer, a brief power interruption can be disruptive. Our next circuit design adds a supercapacitor as a “flywheel” to provide continued power during brief interruptions.
Adding a microcontroller: Our last circuit examples extend the previous designs by adding a small AVR microcontroller. We use the voltage output from the solar panel again to perform darkness detection, but instead take it to an analog input of the microcontroller. The microcontroller is potentially a very low current, efficient device that lets you save power by not running the LED all the time, but (for example) waiting until an hour or two after darkness and/or fading the LEDs on or off, or even intermittently blinking for very low average power consumption.