Loop detector

An induction loop is an electromagnetic communication or detection system which uses a moving magnet to induce an electrical current in a nearby wire. Induction loops are used for transmission and reception of communication signals, or for detection of metal objects in metal detectors or vehicle presence indicators. ... The aerial system of an induction loop installation can consist of one or more loops of a conductive element. In industrial applications this might be a large single- or multi-turn loop or a complex multi-lobed phase coincident sub-loop design, most effectively mounted above the required reception area in industrial applications. An audio induction loop inside of a hearing aid might have one or more loops sometimes with a phase shift between them near the area in which a hearing aid user would be present. Many different configurations can be used depending on the application. Such an induction loop receiver is classically a very small iron-cored inductor (telecoil), although rediffusion demonstrated a prototype Hall-Effect system in its PLL FM system. The system commonly uses an analogue power amplifier matched to the low impedance of the transmission loop. The transmission is normally direct rather than superimposed or modulated upon a carrier, though multi-channel systems have been implemented using modulation. [Induction loop. Wikipedia]