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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Hidden wiring finder based on a digital multimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur [an error occurred while processing this directive] If you have a digital multimeter with an audible indication, you can easily turn it into a hidden network or radio wiring finder. But for this you need the proposed prefix. Currently, pocket digital multimeters are becoming more common due to their relatively low cost and good electrical parameters. Many of them are equipped with various service functions, in particular "sound dialing". This is a mode in which the instrument emits an audible signal if the resistance of the circuit being tested does not exceed some fixed value. So, in the M832 and M838 multimeters, an audible signal will sound if the circuit resistance is 1 kOhm or less. Using this mode of operation of the device, you can create various household devices, for example, the proposed hidden wiring finder. It was developed on the basis of the idea presented in the article by V. Ognev "A simple hidden wiring finder" ("Radio", 1991, No. 8, p. 85). It lies in the fact that a field-effect transistor is used as a sensor, the channel resistance of which can change under the influence of alternating voltage interference on the gate circuit. The implementation of such a device is facilitated by the fact that at the input jacks of the multimeter in the continuity mode there is a voltage of about 3 V, which can be applied to the transistor. The scheme of the attachment to the digital multimeter for searching for hidden electrical or radio wiring is shown in fig. 1. Its basis is a field-effect transistor VT1 with an insulated gate. It is connected with a shielded wire to the input sockets of the device, and the X2 plug must be connected to a common socket. Through this wire, a constant voltage will begin to flow to the transistor from the multimeter. In this case, the multimeter will monitor the resistance of the transistor channel.
To be able to adjust the sensitivity of the seeker, the transistor is selected with a low initial drain current. In this case, the initial resistance of the channel can be set by applying an opening voltage to its gate from the trimmer resistor R2. To protect the transistor gate from powerful pickups and charges of static electricity, diodes VD1, VD2 are installed. The device works like this. After connecting the set-top box to the multimeter, by moving the slider of the resistor R2 from the left according to the output circuit, a single-tone sound signal is achieved. Then smoothly move it in the opposite direction until the signal disappears - in this position the device will have maximum sensitivity. If we now lead the finder along the wall with hidden wiring, then at its location in the WA1 antenna, an alternating voltage will be induced, the amplitude of which will be enough for the transistor to start opening. The resistance of the transistor channel will change in time with the alternating voltage. When it becomes equal to or less than 1 kOhm, a sound signal will be heard in the multimeter, but not monophonic, but in the form of a low-frequency (with the mains frequency) "trill". By changing the sensitivity of the finder and its distance to the wall, the route of the hidden wiring is determined. Similarly, you can find the route of the radio transmission line. To find the place of a wire break in a bundle or, for example, in a New Year's garland, all wires, including a broken one, must be grounded, and the other end of the broken wire must be connected to the phase wire of the network through a resistor with a resistance of 0,5 ... 1 MΩ. By moving the finder along the wire, starting from the resistor, they determine the place where the sound signal disappears - here is a break. The design of the prefix can be arbitrary. On fig. 2 shows the author's version.
It will take a little time to make it. A plastic container from under the medicine with a diameter of 15 ... 25 mm was used as the body of the attachment. A tuning resistor R2 is fixed on the container lid, on the terminals of which a transistor, diodes and resistor R1 are mounted by surface mounting. The shielded wire is brought out through a slot in the case. The role of the antenna is performed by a round metal plate - it is glued to the bottom and connected to the parts with a piece of wire. The appearance of the structure is shown in Fig. 3.
In the device, it is permissible to use, in addition to those indicated in the diagram, the field effect transistor KP305A, KP305B, KP313A, diodes KD102A, KD102B, KD104A. Variable resistor - SPO, SP4, constant - MLT, S2-33 (it can be made up of several smaller resistors connected in series). The seeker does not need to be adjusted, but if its sensitivity turns out to be too high, you need to select a resistor R1 of lower resistance. Author: I. Nechaev, Kursk
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