detector, sensor, sensing element

any device that receives a signal or stimulus (as heat or pressure or light or motion etc.) and responds to it

• metal detector — detector that gives a signal when it detects the presence of metal; used to detect the presence of stray bits of metal in food products or to find buried metal
• mine detector — detector consisting of an electromagnetic device; used to locate explosive mines
• photoelectric cell, photoconductive cell, photocell, electric eye, magic eye — a transducer used to detect and measure light and other radiations

• device — an instrumentality invented for a particular purpose