A hygrometer is a device for measuring, or giving an output signal proportional to, ambient humidity. It is, therefore, very suitable for switching on a ventilator or dehumidifier in spaces such as a bathroom or kitchen where the humidity at certain times can reach uncomfortable or unacceptable levels. Normally, hygrometers use a hygristor as sensor, but the present circuit uses a capacitor whose capacitance is dependent on the degree of humidity. With values as specified in the diagram, the frequency of the output signal of oscillator IC2 varies from 30 kHz in dry conditions to 25 kHz when the ambient humidity is 100%. The output of the oscillator is applied to retriggerable monostable multivibrator (MMV) IC1b, whose Q output remains high as long as the oscillator frequency is high. When the humidity rises, the oscillator frequency drops and short pulses appear at the Q output of IC1b. These trigger MMV IC1a, whose output then goes high, whereupon the electro-optical relay trips. Resistor R4 provides some hysteresis to prevent relay clatter. The humidity at which the relay should trip is set with P1. The desired hysteresis is set with P2. The mono time of IC1a is set to 30 ms, which is more than ample to switch on T1. The electro-optical relay is a Type S201S02 from Sharp, which can switch loads up to 1 A. The circuit draws a current of about 25 mA.


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