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Small Temperature Controlled Relay Circuit? Answered

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.



Best Answer 9 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.


Answer 9 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.


9 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.