Alarm device when approaching an object. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Alarm device when approaching an object. 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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Such devices are usually triggered when a person or other object in the protected area comes into contact with a sensitive antenna or approaches it.

The figure shows a diagram of such a device. It works on the principle of changing the capacitive load introduced through the antenna into a high-frequency generator operating at the edge of excitation breakdown. The antenna is part of the oscillatory circuit of this generator. As a result, with an increase in the capacitance between the antenna and the common wire, additional losses are introduced into the oscillatory circuit, its quality factor deteriorates, and generation is disrupted. This is used to turn on the alarm (sound, light and other kind) through the executive relay.

(click to enlarge)

The transistor VT1 with the elements connected to it and the oscillatory circuit L1C2C3 form a generator according to the capacitive three-point circuit. The generator is set to the excitation threshold with a variable resistor R11. The antenna is connected to the base of the transistor VT1 through the capacitor C1. The generator operates at a frequency of about 300 kHz and has a low output impedance due to the use of an emitter follower on a VT2 transistor. The supply voltage of the cascades on transistors VT1 and VT2 is stabilized by the elements R5.VD1.

The signal from the output of the emitter follower is rectified by the VD2VD3R6C7 rectifier and fed to the base of the transistor VT3 through the resistor R7. On transistors VT3 and VT4, a DC amplifier is made. The load of the second stage is the executive relay K1.

In the generation mode, the transistor VT3 is open, the voltage on its collector is close to zero. In this state, transistor VT4 is closed and relay K1 does not work.

When you touch the antenna, the generation breaks down and the voltage at the rectifier output drops to zero. Transistor VT3 closes, and a positive voltage is supplied to the base VT4 through resistor R9 from the collector VT3. Transistor VT4 goes into saturation mode, and relay K1 is activated. At the same time, the HL1 LED lights up, signaling the operation of the device.

The proposed version of the device does not have a standby alert system after the first operation, but returns to its original state after the influence on the antenna is eliminated. In order not to miss the moment the system is triggered, it is enough to turn on one of the groups with normally open contacts of the same relay K4 between the collector and emitter of the transistor VT1. When triggered, this group of contacts keeps the relay energized until the device is forcibly turned off by the operator.

The device is adjusted with the antenna connected, which can be used as an ordinary wire connected at one end to the capacitor C1. The second one remains free or is connected to some metal part of the protected object (door, gate, etc.). First, touching the antenna, move the variable resistor slider from the lower (according to the circuit) output and achieve the excitation of the generator - the indicator is the operation of relay K1 and the LED lights up. Then the engine is returned back until the relay is turned off. Then you can experiment with the antenna, its location and find the position of the variable resistor engine at which the sensitivity of the device is maximum - when you touch the antenna or approach it, the device fires, and when removed, it returns to its original state.

An instance of the device with an antenna in the form of a metal plate 200x120 mm in size and 0,2 mm thick worked when approaching the hand plate at a distance of 30...50 mm.

The device can use transistors of the KT3102 (VT1 -VT3) and KT615 (VT4) series. Coil L1 on C6-23-17a, the winding must have an inductance of 1 mH. Relay of any type with actuation current 10...100 mA at voltage 9..12V.

Publication: N. Bolshakov, rf.atnn.ru

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