sensor

In computing, a device designed to detect a physical state or measure a physical quantity, and produce an input signal for a computer. For example, a sensor may detect the fact that a printer has run out of paper or may measure the temperature in a kiln.

In electronics, a sensor is triggered by changes outside an electronic system. There are many different types of sensors including heat sensors (thermistors, thermocouples), light sensors (LDR, infrared), sound sensors (microphones, ultrasound), moisture sensors, and many more. Most sensors change their internal resistance as conditions change. This allows them to let more or less electricity to pass through, depending on the condition. Although it is possible to fit some sensors directly into a circuit, they are usually arranged with a resistor as part of a potential divider subsystem. This increases their sensitivity. The subsystem can then be used to activate a transistor, logic gate, or an operational amplifier.