Touch signaling device. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling
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The signaling device, the circuit of which is shown in the figure, is designed to transmit an audio signal at the moment of touching the sensor contact, which can be any conductive object electrically isolated from the ground.
The device is powered by the mains and practically does not consume current in standby mode. It can be used to give an audible signal at the entrance to the apartment or to protect any items at the exhibition stand.
(click to enlarge)
The signaling device contains an amplifier based on transistors VT1 ... VT3, a diode bridge VD2 ... VD5 and a bell BA1. When the touch contact E is touched, a small leakage current flows through the base circuit of the transistors, and the transistors open at negative half-cycles of the mains voltage. In this case, the BA1 bell beeps. Diode VD1 conducts positive half cycles of the leakage current.
When setting up the signaling device, it must be remembered that its elements are under mains voltage. To ensure the safety of working with the device, the power of the resistor must be at least 1 W, the resistance should be at least 2,2 MΩ. With such resistance, the leakage current passing through the human body is not felt at all. A selection of resistor R2 sets the required sensitivity of the device.
For reliable operation of the signaling device, the correct phasing of the power wires is necessary.
Author: B. Novash, Smolensk
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