This humidity controlled switch circuit turns on and off an electrical load (for example a heater) depending on the moisture content of the surrounding air. The moisture is “sensed” with the help of a plate capacitor C2. This capacitor is similar to the old air dielectric plate capacitors in your vintage AM radio. It uses 2 ICs: LM358 and 4001.
How does the humidity controlled switch works
At a certain humidity level, the circuit switches on the load. The moisture level is converted to a voltage through R3 in the first part of the circuit.
The first opamp A1 functions as a high impedance. The second opamp A2 functions as a comparator with a hysteresis of about 15%.
The variable scale of the potentiometer P1 produces voltages from 0.6V to 3 volts and proportional to moisture levels from 20 to 100%. The P1 therefore sets the moisture level where the circuit activates. Once the moisture level reaches above the set level, the triac TR1 triggers and the attached load (heater) turns on.
The current consumption of the circuit is around 13 mA while activated. If you use it to control a heater, you can plug in two 100 watt bulbs. If you use light bulbs as heating element, enclose them in a metal container.
Calibration of the moisture control switch
Dissolve some salt in a glass of water and put it inside a closed room. Soon, the moisture level will climb to about 75%. Then set C2 to get a voltage of 2.25 volts at R3. After that, set P1 to a level that triggers the triac. This concludes the calibration. Later, when you use the circuit, it will trigger at around 80% air humidity.
Article sent by Mihai Sorbevici, LV