Infrared detector, infrared motion detector, infrared detector circuit

 An infrared detector is a transducer of radiant energy that converts radiant energy in the infrared into a measurable form. An infrared detector is a detector which reacts to infrared radiation. The two main types of IR detectors are thermal detectors and photodetectors.

https://www.futureelectronics.com/c/semiconductors/optoelectronics--photodetectors-infrared-detectors