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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Small-sized hidden wiring finder. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur [an error occurred while processing this directive] The electromagnetic field indicator is a very useful instrument in the amateur radio laboratory. And although a lot of such devices have been described in the pages of the magazine, we offer a description of another one, which is distinguished by the originality of its development, thoughtful design and good results in work. Finding hidden electrical wiring, localizing a break or short circuit in cables and wires, non-contact phase indication, checking the performance of generators operating at audio frequencies, this is not a complete list of the device's capabilities. Descriptions of similar devices often appear on the pages of the magazine "Radio" [1-3]. Structural diagrams of hidden wiring seekers (ISE) generally look almost the same. They contain a sensor, a sensitivity control, an amplifier and an indicator. As sensors, pieces of wire, strips of foil or sheet conductive material of relatively large dimensions are used. Practice has shown that such sensors have a wide "directional pattern", and this greatly complicates the localization of the fault. In the proposed version, the sensor is a slightly modified electret microphone with a built-in field-effect transistor, in which the membrane and the front part have been removed (the manufacturing technology is described below). Experiments with this sensor showed its high sensitivity and narrow "beam pattern". In addition, with this solution, there is no need to search for scarce field-effect transistors, which is especially difficult for rural residents. The built-in field-effect transistor provides a high input impedance of the sensor. The disadvantages of the described sensor include a false alarm under mechanical influences (shocks), on the other hand, this can be turned into a virtue - by lightly tapping on the sensor, the device as a whole is checked for operability. If such a dignity seems doubtful, you just need to fill the internal cavity of the sensor with Hermesil automotive sealant ISE technical characteristics: supply voltage - 3 V; current consumption - 15...30 mA; dimensions - length 130 mm, diameter 18 mm; weight - 45 ... 50 g. Schematic diagram of the ISE is shown in fig. 1. The alternating electric field from the wiring by the BM1 sensor is converted into an alternating voltage, which is fed through the capacitor C1 to the sensitivity regulator - the tuning resistor R2 - and is amplified by the DA1 microcircuit. Resistors R2 and R3 set the gain DA1. If the sensitivity of the ISE is insufficient, the resistance of the resistor R3 should be increased, if it is excessive, it should be reduced. To outputs 5 and 8 of DA1, a connector for headphones (headphones) XS1 is connected, wired in such a way that when headphones are connected, the indication circuit assembled on the elements R4, VD1, VD2 is broken. Resistor R4 limits the current passing through diode VD1 and LED VD2. Diode VD1 is used to protect the LED VD2 from reverse voltage.
The advantage of the amplifier built on the EKR1436UN1 chip is the absence of isolation capacitors at the output, a minimum of external elements and the ability to reduce the supply voltage to 2 V, which significantly reduces the dimensions and weight of the device. Manufacturing of sensor 1 shown in fig. 2, start by removing the lavsan filter 6. Then, with a scalpel, a part of the aluminum case 5 with a diameter of 7 mm is removed along with the membrane 4. An ohmmeter checks the absence of short circuits between the case 1 and the plate located behind the membrane 2. It is this that is the sensitive part of the sensor . Further, a piece of wire with a diameter of 0,2-0,4 mm and a length of 4 ... 6 mm or a drop of solder in the form of an icicle of approximately the same size is soldered to the center of this plate 3 (trying not to overheat, since there is a very high probability of melting the gasket that insulates plate from the body). Again check the sensor for short circuits.
Next, the layout is assembled according to the scheme shown in Fig. 3, and by selecting the resistor R1, the voltage on the voltmeter is equal to half the supply voltage. The selected resistor R1 is further used in the manufacture of the ISE. By bringing the sensor to a source of an alternating magnetic field (for example, to the cord of an included soldering iron or a “phase” wire of a network wiring), a signal is observed. After detecting the signal, it is concluded that the sensor is ready and working. More detailed information about the microcircuit used and electret microphones can be obtained in [4]. A few words about the details. Electret microphone - with a built-in field-effect transistor from widely used in imported telephones and tape recorders (unfortunately, all the copies that came to me had not very clear markings). The EKR1436UN1 chip can be replaced with the KR1064UN2 or its imported analogue MC34119 (Motorola). Fixed resistors and capacitors - any, small-sized, for example, MLT-0,125, K10-176. Trimmer resistor R2 - SP 19, its nominal resistance can be in the range of 47-330 kOhm. LED VD2 can be replaced by AL336A, AL336B, KIPD14A1-K, KIPD35V-Kit. with an operating voltage of not more than 2,2 V. The VD1 diode can be replaced by KD521, KD522 with any letter index. XS1 is a standard headphone jack with contacts that disable the indication circuit. Headphones fit any, with a total internal resistance of 8-100 ohms. Button SB1 - any small-sized, for example, PKN-125. The body of the device can serve as an ordinary marker. The design of the ISE is clear from Fig. 4, on which the numbers indicate: 1 - sensor, 2 - negative battery contact plate with an insulating gasket and spring, 3 - SB1,4 button, 5 - battery compartment, 6 - positive contact plate, 1 - amplifier (DA1) and "mounted "elements (R1, C3, R7), 2,8 - trimmer resistor R1 - headphone jack (XS9), 2 - LED (VD1). Diodes VD2, VD4 and resistor R1 are soldered directly to socket XSXNUMX.
Before assembling the finder, it is desirable to fully configure the device on the breadboard. Setting up the ISE is reduced to the selection of resistors R1 and R3 according to the above method. It is desirable to carry out internal installation with MGTF wire. The assembly begins with the removal of excess plastic and cutting (drilling, turning) the necessary holes for SB1, R2, XS1. Then the negative output of the sensor is soldered to the negative plate of the battery with a spring (wire length 10 mm), and another wire (length 135 mm) to it. A wire 140 mm long is soldered to the positive terminal of the sensor (after marking it in some way), the sensor is lubricated along the edges with epoxy glue (or hot glue) and installed according to Fig. 4. Then glue the insulating gasket on both sides and put it on top of the sensor, put a minus plate with a spring on it (having previously released two wires), fix it in a vertical position and hold it until the glue has completely hardened (cooled down). Two wires (1 and 75 mm long) are soldered to the SB 120 button, the ends of the button are lubricated with epoxy glue (or hot glue) and, having passed the wires inside the battery compartment, the button is installed in a pre-prepared hole. Diodes VD1, VD1, resistor R2 and two wires 4 mm long are soldered to connector XS35. Two wires 2 mm long are soldered to the tuning resistor R35. Connector XS1 (complete with VD1, VD2, R4) and resistor R2 are installed in the cap in pre-prepared holes according to fig. 4. The voids are filled with epoxy putty (or hot glue) to the rear wall of the XS1 connector and left alone until the adhesive hardens (cools down). Before installing the positive plate, three wires are released: the “plus” of the sensor, the “minus” of the battery and one wire from the SB1 button to the compartment where the DA1 chip will be located, and the second wire from SB1 is soldered to the positive plate. The positive plate is fixed by fusing into the body of the marker three or four single-core copper wires with a diameter of 0,5-0,7 mm, soldered to the plate. Solder the remaining wires in accordance with the diagram. After checking the operability of the device as a whole, pour the rest of the cap with epoxy glue (hot glue) and install it in place. The ISE is ready for use after the adhesive has cured (cooled). The appearance of the ISE with the sensor removed is shown in the photograph (Fig. 5).
Such a detailed description of the microphone assembly technology is associated with certain difficulties that arose when friends repeated this design. Literature
Author: D. Makeev, Bryansk
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