Home Security System

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ere is a low-cost, invisible laser circuit to protect your house from thieves or trespassers. A
laser pointer torch, which is easily avail-
able in the market, can be used to oper- ate this device.
The block diagram of the unit shown
in Fig. 1 depicts the overall arrange- ment for providing security to a house. A laser torch powered by 3V power- supply is used for generating a laser beam. A combination of plain mirrors

MALAY BANERJEE



M1 through M6 is used to direct the laser beam around the house to form a net. The laser beam is directed to fi- nally fall on an LDR that forms part of the receiver unit as shown in Fig. 2. Any interruption of the beam by a thief/ trespasser will result into energisation of the alarm. The 3V power-supply cir- cuit is a conventional full-wave recti- fier-filter circuit. Any alarm unit that operates on 230V AC can be connected at the output.

6V relays (RL1 and RL2), an LDR, a transistor, and a few other passive com- ponents. When switches S1 and S2 are activated, transformer X1, followed by a full-wave rectifier and smoothing capaci- tor C1, drives relay RL1 through the laser switch.
The laser beam should be aimed con- tinuously on LDR. As long as the laser beam falls on LDR, transistor T1 re- mains forward biased and relay RL1 is thus in de-energised condition. When a person crosses the line of laser beam, relay RL1 turns on and transformer X2