What is a temperature sensor?

Have you ever left your smartphone in your car on a hot day? If so, your screen might have displayed an image of a thermometer and a warning that your phone has overheated. That is because there is a tiny embedded temperature sensor that measures the interior temperature of your phone. Once the inside of the phone reaches a certain temperature (iPhones shut down at approximately 113 degrees Fahrenheit, for example), the temperature sensor sends an electronic signal to an embedded computer. This, in turn, restricts users from accessing any applications or features until the phone has cooled back down, as running programs would only further damage the phone’s interior components.  

A temperature sensor is an electronic device that measures the temperature of its environment and converts the input data into electronic data to record, monitor, or signal temperature changes. There are many different types of temperature sensors. Some temperature sensors require direct contact with the physical object that is being monitored (contact temperature sensors), while others indirectly measure the temperature of an object (non-contact temperature sensors).  

Diagram of a thermocouple.
Diagram of a thermocouple.  Source: Bright Hub Engineering

Non-contact temperature sensors are usually infrared (IR) sensors. They remotely detect the IR energy emitted by an object and send a signal to a calibrated electronic circuit that determines the object's temperature.

Among the contact temperature sensors are thermocouples and thermistors. A thermocouple is comprised of two conductors, each made of a different type of metal, that are joined at an end to form a junction. When the junction is exposed to heat, a voltage is generated that directly corresponds to the temperature input. This happens on account of the phenomena called the thermoelectric effect. Thermocouples are generally inexpensive, as their design and materials are simple. The other type of contact temperature sensor is called a thermistor. In thermistors, resistance decreases as temperature increases. There are two main types of thermistors: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). Thermistors are more precise than thermocouples (capable of measuring within 0.05-1.5 degrees Celsius), and they are made of ceramics or polymers. Resistance Temperature Detectors (RTD) are essentially the metal counterpart of thermistors, and they are the most precise and expensive type of temperature sensors.

Temperature sensors are used in automobiles, medical devices, computers, cooking appliances, and other types of machinery.