cap humidity sensor circuit

Posted on Feb 7, 2014

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
Click here to download the full size of the above Circuit.

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...

Leave Comment

characters left:

Related Circuits

  • New Circuits



    Popular Circuits

    Video DVM circuit
    PC thermometer schematic
    Solar to 6V Batt to LED emergency light circuit
    Wireless Piezo Beeper
    moving message display
    Microphone Amp
    metal detector using cs209a
    Micro Inverter by 2N6121
    Electronic Head or Tail Circuit
    start up aid for pcs
    Amazing All-Band Receiver
    Crystal Radio Circuits
    An AVR microcontroller based Ethernet device
    Darlington phototransistor type light-triggered circuit diagram of a switching application