This humidity detector circuit diagram is very simple and uses few components and can be used for activation of an electronic devices when the detector senses a certain humidity. The sensor is constructed from two pieces of copper placed at a short distance of each other. when the electrical resistance of copper conductors drops below a certain va
lue, Schmit trigger built using transistors T1 and T2 switch. Bistable multi-vibrator is flip through C1, so that at point B there is a low voltage (relay triggers).