Measuring humidity with an Arduino
I have salvaged a couple of capacitive humidity sensors which I intend to use for measuring humidity(!) with an Arduino.
Problem now is: how to read capacitance values using an Arduino?
The Arduino website suggests this:
But I am pretty sure this works only with large capacitors. The Vishay is only 122 pF. In the above circuit I might need a resistor of 10 M or more.
After checking application manuals of this type of thing I figured I need a 555 timer and build an astable circuit like this:
C would be the humidity sensor. C, R1 and R2 together define the frequency of the output signal on pin 3.
By connecting pin 3 to the Arduino, I'll be able to read out the frequency the 555 is generating. Now I need to figure out the optimal values of the resistors. I found others did this too: Check out the Cricket of Blueroomelectronics. They are using the HS1101 humidity sensor, which is slightly different from my Vishay.
To be continued...