To help prevent mould growth in a shed, I installed a 12v computer case fan powered by a small solar system. It had been running for a while on a timer relay that would turn it on for an hour or so each day, but this wasn't an ideal setup as it would turn on during heavy rain, or stay off during full sunlight, it had no way of knowing when was a good time to run the fans.
This circuit was designed to solve that issue, along with acting as an expandable weather station. At 1 minute intervals, it reads a number of internal and external sensors, writes the readings to a .csv file on a microsd card, and decides based on these whether to start, stop, or leave the fan as it is. The current program is based on 2 variables, battery voltage and brightness. If both of these are above preset start values, the fan starts. If either are below preset stop values, the fan stops, otherwise there is no change.