I have been searching in vain for a circuit to make a humidistat using a Honeywell HCH 1000 Series
Capacitive Humidity Sensor. which is cheap and easily available, or any similar sensor
It is intended for use in a greenhouse to turn a mist system on and off so requires relay output and to
have a variable setpoint between 20% to 80 % humidity
Any help would be greatly appreciated
Many Thanks
Whym

Start with a capacitance to voltage converter. There are several such circuits posted on this site. Then, once you know the relationship between the capacitance and the humidity, now converted to a voltage, you can establish what voltage you want to switch on your mist system. A voltage comparator with some hysteresis can be used for this simple on/off decision. There are lots of voltage comparator circuits posted here too.

If your into avr's this circuit could be used.

http://sites.google.com/site/angophora/projects/temperature-humidity-sensor