A simple laser burglar alarm system. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple laser burglar alarm system. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling

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The burglar alarm circuit shown in the figure uses a laser beam or other light source to detect intruders. For the manufacture of the device, it is necessary to have only an integral timer of the 555 type, an operational amplifier, a photocell, as well as several passive and active components.

(click to enlarge)

The circuit contains four stages: detection, amplification, switching, generation and sound output. When a beam of a helium-neon laser or other more trivial light source strikes the photocell D1, the diode opens and a small DC voltage pulse is applied to the amplifier U1. In turn, the output voltage of this amplifier opens transistor Q1. In this case, the silicon controlled rectifier Q2 closes and there is no alarm signal.

In the event of an interruption in the light beam, transistor Q1 is turned off, and a high voltage on its collector opens the ECU Q2. The latter, in turn, includes a self-oscillating generator U2, the frequency of which is determined by the expression f=1/1,1RC. The generator continues to run until the ECU closes.

Briefly pressing the normally closed switch S1 breaks the circuits and turns off the alarm.

Author: Harry Gibbens, Linwood, pc. California; Publication: N. Bolshakov, rf.atnn.ru

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