The sensor is an RC oscillator in which, a water-absorbent plastic film is the insulator in the capacitive element. The capacitance of the film increases with the amount of water it absorbs from the air, and thus reduces the oscillation frequency. A frequency counter produces a digital output that represents the change inirequency and hence the change in relative humidity. The sensor can be used to measure humidity in the atmosphere, in the soil, and in industrial gases, for example.
Low-cost-humidity-sensor - schematic

A Schmitt-trigger-type IC is connected to the capacitor, which consists of a film of a commercially produced sulfonated fluorocarbon polymer, 2 in. (5.08 em) square, sandwiched between perforated metal plates. The oscillation frequency decreases almost linearly from about 100 to 16 kHz as the relative humidity increases from about 20 to 76%.

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