A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by controlling the flow of heat energy into or out of the system. That is, the thermostat switches heating or cooling devices on or off as needed to maintain the correct temperature.
A thermostat may be a control unit for a heating or cooling system or a component part of a heater or air conditioner. Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature. The output of the sensor then controls the heating or cooling apparatus.
Common sensors include:
Bi-metallic mechanical or electrical sensors
Expanding wax pellets
Electronic thermistors and semiconductor devices
Electrical thermocouples
A Honeywell electronic thermostat in a retail store
These may then control the heating or cooling apparatus using:
Direct mechanical control
Electrical signals
Pneumatic signals
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