You enter the room, and suddenly light flashes, “alive” is a radio or a tape recorder. To make such a “miracle” the strength of the capacitive relay (the diagram in the figure).
The generator on the transistor T1 produces a frequency of 90 kHz. To the same frequency tuned and L2С5 circuit, the output of which contains the detector D1. When you approach the antenna, the frequency of generation varies, and therefore, the output signal of the detector decreases, causing a decrease in current through the transistor T2. Relay R1 is deactivated and its contacts completing the electrical circuit of the actuator.
Now for the details. Antenna is a piece of a metal mesh with a size of 0,5X0,5 m. It is necessary to “disguise”: to place under the rug, linoleum or Wallpaper. T1 — high frequency transistor П401, T2 — transistor MP20.
As a P1 you can use the solenoid РЭС6 (winding resistance DC 200 Ohms).
Coils L1 and L2 to 30 mH (200 turns of wire of PEL 0,15).
D1 — the diode D2, D9; D2 — Zener Д814Г, Д814Д.
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