cap humidity sensor circuit

This document describes how to read a capacitive humidity sensor directly from a micro controller using 1 resistor, 1 diode the sensor and two IO lines. It does this without an ADC by measuring the amount of time it requires to charge the capacitor through a resistor until the charge level is sufficient to register as logic 1. As the humidity chan
cap humidity sensor circuit - schematic

ges the capacitance changes which changes the time required to charge the circuit. We measure the time in a tight processor loop and average the readings to reduce hysteresis. The readings are calibrated to two known humidity points using aqueous salt solutions. The calibration allows simple conversion to percentages which can then be used internally or shipped out through a digital network such as RS422. Most humidity sensors change consistently at a given capacitance change per 1% humidity change so the humidity can be calculated as a direct correlation to capacitance changes. While this circuit was designed specifically to support humidity sensors it should work the same with any circuit which uses a variable capacitor that predictably changes across it`s set of input such as stress gauges. It can be extended with lookup tables to support devices which do not deliver a linear ramp across it`s operating range. I ran out of ADC pins and didn`t want to install a multiplexer or external ADC for my 3 humidity sensors. I was tired of paying $15 to $35 each for humidity sensors like the vout HM1500 or the digital Sensirion SHT75. In addition I found that many of the vout and freq out units had drifts exceeding 10% after 1 year in our application. My other issue is that the more sophisticated vout sensors keep getting model changes which requires design changes for us while the base level cap sensors remained the same and...

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