http://www.futureelectronics.com/en/Pages/index.aspx - A proximity sensor is an electronic sensor capable of detecting the presence of nearby objects without physical contact. It emits an electromagnetic field or a beam of electromagnetic radiation and then looks for changes in the return signal or field. Depending on the proximity sensor targets, different sensors may be used. For example, an inductive proximity sensor requires a metallic target, whereas a capacitive photoelectric sensor may be suitable for a plastic target.