An infrared (IR) sensor is a piece of electronic equipment that can find and measure infrared light in its surroundings.

Infrared light, also called infrared waves, is a type of electromagnetic radiation that can't be seen but can be felt as heat.

All objects in the universe give off some amount of IR radiation, but the amount depends on how hot or cold they are. The more IR radiation something gives off, the hotter it is.

Infrared sensors can be either active or passive. Infrared sensors that are active both send out and pick up on infrared radiation. Active IR sensors have a transmitter and a receiver, which are two separate parts. When an object gets close enough to the sensor, the infrared light from the transmitter bounces off the object and is picked up by the receiver. Since an active IR sensor sends out infrared light, an object doesn't have to be very hot for it to be picked up.

Passive IR sensors, on the other hand, only have a receiver, which means that an object with a low temperature can't be found.