There are many types of sensors. They are used to measure and/or detect a huge variety of conditions including temperature, pressure, level, humidity, speed, motion, distance, light, or the presence/absence of an object and many other types.

Sensors in some cases react to the environment in which they are placed and this reaction is used to measure the property being sensed. For example, a common temperature detector is known as an RTD (resistance temperature detector) and this contains a platinum wire. The electrical resistance of the wire changes with temperature so how the resistance changes can be used to measure the temperature.

Many sensors use this type of principle where the variation of an electrical property of a sensing element is a measure of a property being sensed.

Other types of sensors emit a signal and either measure how the area reacts to the emission or measure the reflection of the signal bouncing off an object in front. Inductive proximity sensors are one of the commonest sensors in use today. They emit a small electromagnetic field and use this to sense the properties of the area in front of the sensor.