Security alarm device. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Security alarm device. 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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This device, developed for the protection of a country house many years ago, has proven itself over the years of "security service". The peculiarity of its design is that it determines unwanted approaches to the protected object at a distance, and it does not use a loop break or a mechanical switch, as in conventional widespread security devices, but a capacitive relay.

When a person approaches the door at a distance closer than 1 meter, the device generates sound and light signals. It is clear that such a device can be used not only to protect the dacha, but also other objects, for example, an apartment or a garage.

The schematic diagram of the device is shown in the figure.

(click to enlarge)

A low-power generator with a generation frequency of 1 kHz is assembled on the transistor VT90. The input circuit VT2 is tuned to the same frequency. Approaching the grid sensor (0,5 x 0,5 m in size), we change the capacitance, as a result of which the generation frequency changes. The circuit L2, C7 turns out to be upset, and a blocking voltage appears on the base of VT2 (before that, VT2 is open). As a result, a voltage close to the supply voltage appears on the VT2 collector (in the presence of a signal based on VT2, it was close to zero). This voltage, having exceeded the stabilization threshold of the zener diode VD3, enters the base of the transistor VT3 and opens it. Through the relay P1, included in the collector circuit VT3, current flows, the relay is activated and closes the executive circuit with its contacts, the load of which is a bell and a light bulb.

The use of the VD3 zener diode in the base circuit of the VT3 transistor sharply increases the sensitivity of the capacitive relay, and also almost completely eliminates such a phenomenon as hysteresis - the relay actuation and release voltages differ by less than 0,26 V. A metal plate (copper, duralumin, iron) is used as a sensor ). It is installed on the door from the inside, it can be disguised as a decorative upholstery. Can also be used as a sensor doorknob, lock, deadbolt, metal window grille.

Coils L1 and L2 are the same, wound with PEL-0,15 wire on unified frames from portable radios. 200 turns are evenly distributed over all three sections of the frame.

The inductance of the coils is 30 mH. T1 - any with an output voltage of 12V. Establishment consists in adjusting the circuit L1, C4 to a frequency of 90 kHz. The L2 circuit is tuned to the same frequency. Capacitor C1 regulates the sensitivity of the capacitive relay. Resistor R6 sets the voltage on the zener diodes, close to the stabilization threshold. The sensitivity of the sensor of the security device depends on the position of the slider of this resistor.

The wire connecting the grid sensor to the circuit must be of the minimum length.

The device can be self-powered from a 12 V battery. The bell and signal light must be rated for the same voltage.

In order to expand the operational capabilities, you can use self-locking and a time relay to turn it off. However, even in the form in which the device is presented in the diagram, it also operates flawlessly, i.e. able to reliably protect the entrance doors from intruders.

See other articles Section Security devices and object signaling.

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