Simple but effective good-old-555 based circuit that detects condensation and drives a fan in a bathroom. The original design uses a capacitive voltage divider for the supply voltage of the control circuit, but we didn't want to take any risk with a (resistive) moisture sensor that is not isolated from mains voltage! That's why we added a small 350mVA transformer to the design, not the most elegant and space saving solution, but safety first!