Hidden wiring finders on transistors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Indicators, detectors
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A relatively simple device made on three transistors will help determine the location of the hidden electrical wiring in the walls of the room. A multivibrator is assembled on two bipolar transistors (VT1, VT3), and an electronic key is assembled on a field transistor (VT2).
The principle of operation of the finder is based on the fact that an electric field is formed around the electric wire - it is caught by the finder.
If the switch button SB1 is pressed, but there is no electric field in the zone of the antenna probe WA1 or the finder is far from the mains wires, the transistor VT2 is open, the multivibrator does not work, the HL1 LED is off
It is enough to bring the antenna probe connected to the gate circuit of the field-effect transistor to a current-carrying conductor or just to the mains wire, the transistor VT2 will close, the shunting of the base circuit of the transistor VT3 will stop and the multivibrator will start working. The LED will start flashing. By moving the antenna probe near the wall, it is easy to follow the laying of network wires in it.
The field-effect transistor can be any other of the series indicated on the diagram, and bipolar ones can be any of the KT312, KT315 series. All resistors - MLT-0,125, oxide capacitors - K50-16 or other small-sized ones, LED - any of the AL307 series, power source - Corundum battery or 6 ... 9 V battery, push-button switch SB1 - KM-1 or similar.
Some parts of the device are mounted on a board made of one-sided foil fiberglass. The body of the finder can be a plastic case for storing school counting sticks. The board is fixed in its upper compartment, the battery is placed in the lower one. A switch and an LED are attached to the side wall of the upper compartment, and an antenna probe is attached to the upper wall. It is a conical plastic cap with a threaded metal rod inside. The rod is attached to the body with nuts, from the inside of the body a metal lobe is put on the rod, which is connected by a flexible mounting conductor to the resistor R1 on the board.
The antenna probe may be of a different design, for example, in the form of a loop from a piece of thick (5 mm) high-voltage wire used in a TV. The length of the segment is 80 ... 100 mm, its ends are passed through the holes in the upper compartment of the case and soldered to the corresponding point on the board.
The desired oscillation frequency of the multivibrator, and therefore the frequency of LED flashes, can be set by selecting resistors R3, R5 or capacitors C1, C2. To do this, temporarily disconnect the source output of the field-effect transistor from resistors R3 and R4 and close the switch contacts.
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