Small Temperature Controlled Relay Circuit?

I'd like to build a small temperature controlled relay circuit. I need to sense water temperature and send +12vdc if temp is <95F and -12vdc if >104F. It's driving a linear actuator so it should advance the rod (+12v) at low (<95F) temperature, hold position (0v) between 95 and 104, and retract the rod (-12v) at high temperature. (>104F) Any circuit suggestions? Thanks.

legionlabs8 years ago
An MCU that has an integrated digital to analog converter would indeed be the easiest way. This projects here may be of interest: http://diyistheway.blogspot.com/2009/03/thermosmart.html

However, painful and convoluted analog options also exist! I thought of one for your amusement:

An dual-rail opamp with fixed gain, with a thermistor set as a voltage divider such that the opamp is comparing a voltage that varies with temperature to a fixed reference voltage, could work. The output would need to be biased, and passed through Schmidt-trigger NOT gates of different hysteresis voltages to account for each triggering condition, which would in turn use transistors to provide power to your linear actuators.
dmcgraw7 (author)  legionlabs8 years ago
Thanks for the link. I started by modifying an op amp comparator circuit, but the gate hysteresis is non-trivial. I'll punt and start over with an Arduino.
psymansays8 years ago
I would say a simple thermostat would work, but I don't know what kind of input your linear actuator requires. Maybe a thermostat rigged to an MCU would be best.