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  • 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

    Moisture Controlled Switch Circuit Schematic

    humidity controlled switch circuit schematic

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    8 Responses to "Humidity Control Switch Circuit"

    1. sharad mahana says: on February 18, 2011 at 3:35 pm

      have u test the circuit?

    2. sharad mahana says: on February 18, 2011 at 3:51 pm

      please load how the circuit diagram works.
      we use this circuit, when we applied 230v,ac on the terminal as shown in figure, than the resistor 390/1w i.e. connected in series with power supply are burned out? what is the problem occur in circuit please write it?

    3. menard says: on May 12, 2011 at 1:21 am

      Hello, could you please elaborate more on how to obtain this air dielectric capacitor. I would like to build a customized egg incubator. I need more information regarding optional or specification of this capacitor when i check this in electronic shop. thank you.
      Can you provide an presettable alarm circuit to monitor a particular level temperature, Ph, and humidity and trigger a switch when these 2 falls below or above the preset level?
      I would like to have this for my backyard aquaponic system and egg incubator.
      I don’t know about PIC programming so I will just opt for this bulky circuit.

    4. maarten says: on July 20, 2013 at 7:37 pm

      The anode of the zener diode D3 should be connected at the joint between C9 and D4.

      • Where is the ground(GND), anyway? C7, C8, D2 have one common
        terminal. Of course, the current through D2 is ZERO. Unless the
        anode of D2 is connected to GND(negative rail). But where is this?

        NOR gates (U2, U4) look weird; there’s no DC bias at their inputs.
        (U3, U5 too)

        Because A2 is wired as comparator, the feedback should be positive(hysteresis), so R14 should be connected to pin 5(non-inverting input).

        A discharge resistor in parallel with C9 is a good practice.

    5. please send the IEEE format paper

    6. how could it be inverted so that the circuit turns on a mister when the humidity gets low…. please help thanks

    7. Is there anyone to answar above questions?

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