| 摘要 |
This intrusion alarm apparatus has a housing suspended from a metal doorknob such that the housing is tilted when the doorknob is turned. A 100 kHz. oscillator is synchronized with a single shot which produces narrower pulses than the oscillator. An exclusive OR circuit subtracts the single shot pulses from the oscillator pulses. When the door is of wood or other electrically non-conductive material, the body capacitance of a person touching the doorknob increases the width of the pulses produced by the single shot and this change is reflected through the exclusive OR circuit, an integrator, a level detector and a flip-flop to turn on an audible alarm device. When the door itself is metallic, a selector switch can disable the alarm device from responding to the body capacitance of a person touching the doorknob, in which case the tilt switch in the housing will turn on the alarm device when the doorknob is turned. The alarm apparatus also contains a thermostatic switch and a smoke detector for turning on the alarm device in case of fire.
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