humidity sensor circuit II

When the 14B-12B pair opens, 12B is connected to Al's summing point via 13B. The sensor now discharges into the summing point through the 1 µ capacitor. Since the charge voltage is fixed, the average current into the summing point is determined by the sensor's humidity related value. The 1 µ value ac couples the sensor to the charge-discharge path, maintaining the required zero average voltage across the device. The 22M resistor prevents accumulation of charge, which would stop current flow.
humidity sensor circuit II - schematic

The average current into Al's summing point is balanced by packets of charge delivered by the switched-capacitor gives Al an integrator-like response, and its output is dc.

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