Our industry has developed several ways of sensing temperature, from mercury filled glass bulbs and bi-metals, to RTD’s (resistive thermal devices) that vary resistance based upon temperature. When we marry the function of a temperature sensor with a controller that makes a decision and sends an output based upon information it receives from that sensor, we’ve created a thermostat. Thermostats are available in many different configurations, dependent upon available power supply, required control signal to an end device or devices, and aesthetics.