Thermostats are devices which control a heating or cooling system. Typically, a thermostat is programmed to what is known as a set point which is triggered when the temperature of a designated location reaches a particular degree. This is often utilized in applications such as ambient air control.
Thermostats are generally categorized as either mechanical, electrical, and programmable. As the name suggests, mechanical thermostats can sense the temperature through mechanical means and are designed with different configurations. Mechanical thermostats are not used much today as they were commonly manufactured with mercury. Another disadvantage of mechanical thermostats is they not able to alternated between heating and cooling automatically.
Electronic/digital models have a LCD display which allows the user to create the set point. Most types of digital thermostats are not necessarily programmable thermostats and are designed for specifically single stage or multiple stage heating/cooling. Digital thermostats have no moving components and use thermistors or semiconductor systems. Programmable thermostats are equipped with the most number of features and utilizes a built in clock as well as memory to process all of the possible functions.
One design of thermostat is known as bimetallic switching thermostat which has the ability to determine air temperature by using differential expansion of two different metals components. These systems will automatically switch on when the temperature drops below the set point and then will switch off when the temperature rises above a determined value.