Ordinary table salt can readily extract moisture from and return it to the atmosphere. The electrical resistance of salt varies over a large range. If we adjust resistance in sensor circuit to turn ON/OFF relay we have our humidity controlled switch.
In circuit SPST switch is to show on and off in simulator circuit.
Variable resistor at top is where our salt resistor goes. Lamp under lid is to help evaporate moisture from salt when power circuit relay is ON. Salt absorbs moisture more easily than it returns moisture to the air. Lamp at right is load that is being powered.
The salt resistor is made from shallow lid off plastic container that has a slight rim able to allow free access to surrounding air. Push two metal pins such as banana plugs through plastic lid about 1 inch from other pin to connect circuit wires.
To test sensor circuit: pour a small amount of salt of the free pouring variety in the lid sensor using just enough random sprinkling of salt to make good contact with both electrodes in the lid. Add or adjust second adjustable resistance in sensor circuit so when you breathe on salt as you would to clean eyeglasses the circuit comes on.
