A thermostat is a control device that is used to manipulate other devices based on a temperature setting. This temperature setting is known as the setpoint. When the temperature changes in relation to the setpoint, a device can be switched on or off.

For example, let's imagine a system where a simple thermostat is set to switch an electric heater on when the temperature drops below 25 degrees Celsius.

Within our thermostat, we have a temperature-sensing device such as a thermistor that returns a temperature reading every few seconds. When the thermistor reads a temperature below the setpoint (25 degrees Celsius), the thermostat will switch a relay on, completing the circuit between the wall plug and our electric heater and providing it with power. Thus, we can see that a simple electronic thermostat can be used to switch on a variety of devices.

Warren S. Johnson, a college professor in Wisconsin, is credited with inventing the electric room thermostat in the 1880s...