This is a simple design circuit that can be used to switch ON or OFF a device when the dew present in the surrounding atmosphere crosses a set value. This circuit uses a dew sensitive resistive element and a comparator based on LM 358 to perform the above said operation. This is the figure of the circuit.
In operation of the circuit, at normal condition the resistance of dew sensor element will be low and so the voltage drop across it. So the voltage at the non inverting pin of LM358 (IC1) will be less than the voltage at the inverting input of the LM358.So the output of the op amp will be low. This keeps the opto coupler (MCT2E) deactivated. When the dew increases the resistance of the element increases and so do the voltage across it. Now the voltage at the non inverting pin of LM358 (IC1) will be higher than the voltage at the inverting input of the LM358.So the output of the op amp will be switched to high. This in turn activates the opto coupler. The LED glows to indicate it. As a result we get an opto coupler activated and de activated according to the amount of dew in the atmosphere. The output pins of opto coupler pin (5&4) can be used to control the external device.
Diode D1, resistors R6&R3 and capacitor C1 is employed here to derive the power for the circuit directly from mains. The dew sensor is hard to find in market. But it can be easily obtained from a old VCR. Also the type no of the sensor is not so important here. Try with any thing you get. I used one from a old Hitachi VCR. LM 358 is a dual op amp. Here only one op amp inside it is used.
