Solar-driven Moisture Detector

Using its two electrodes, formed from two stiff pieces of bare copper wire, the circuit can be stuck into the pot of any plant you want to monitor. As long as the plant isn't thirsty, i.e. the soil in the pot is moist enough, it will just sit there and do nothing at all. But when the soil dries out below a certain threshold (which you can adjust to suit the soil used and the plant being monitored), it starts 'squealing' to tell you it is time to give the poor plant a drink.