| 摘要 |
<p>The relative movement type alarm system allows opening or closing movement to be detected, circuit integrity to be checked, and incorporates safeguards against attempts to disable the circuits. The total circuit network is d.c. powered and requires only two or four continuous wires. A parallel arrangement of sealed reed relays (22, 23) is arranged so that one (22) is normally closed whilst the other (23) is normally held open by a fixed magnet (24). To or fro movement of an adjacent command magnet (29), fixed for example, to a door or window, causes a momentary shorting of two nodes (25, 26) as the former (23) relay opens and the latter (23) is forced closed. By incorporating various series and/or parallel resistors a bridge detection method may be used.</p> |
