Metal Object Indicator

Encyclopedia of radio electronics and electrical engineering / Indicators, detectors, metal detectors

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When carrying out construction and repair work, information about the presence and location of various metal objects (nails, pipes, fittings) in the wall, floor, etc. will be useful. The device described below will help in this.

The principle of operation of the device is based on the property of metal objects to introduce attenuation into the frequency-setting LC circuit of the oscillator. The oscillator mode is set near the generation breakdown point, and the approach of metal objects (primarily ferromagnetic) to its contour significantly reduces the oscillation amplitude or leads to generation breakdown. If you indicate the presence or absence of generation, then you can determine the location of these items.

The scheme of the device is shown in Fig.1. It has sound and light indication of the detected object. On the transistor VT1, an RF self-oscillator with inductive coupling is assembled. The frequency setting circuit L1C1 determines the generation frequency (about 100 kHz), and the coupling coil L2 provides the necessary conditions for self-excitation. Resistors R1 (roughly) and R2 (smoothly) set the generator operating mode. A source follower is assembled on transistor VT2, a rectifier on diodes VD1, VD2, a current amplifier on transistors VT3, VT5, and a sound signaling device on transistor VT4 and piezoelectric BF1.

Fig.1. Schematic diagram of the detector

In the absence of generation, the current flowing through the resistor R4 opens the transistors VT3 and VT5, so the HL1 LED will shine, and the piezo emitter emits a tone at the resonant frequency of the piezo emitter (2 ... 3 kHz). If the RF oscillator works, then its signal from the output of the source follower is rectified and the negative voltage from the output of the rectifier will close transistors VT3, VT5. The LED will turn off and the alarm will stop sounding.

When the circuit approaches a metal object, the oscillation amplitude in it will decrease, or the generation will fail. In this case, the negative voltage at the output of the detector will decrease and current will begin to flow through the transistors VT3, VT5. The LED will light up, a beep will sound, indicating the presence of a metal object near the contour. Moreover, with a sound signaling device, the sensitivity of the device is higher, since it starts to work at a current of fractions of a milliamp, while a much larger current is required for the LED.

Instead of those indicated in the diagram, transistors KPZ0ZA (VT1), KPZ03V, KP303G, KP303E (VT2), KT315B, KT315D, KT312B, KT312V (VT3-VT5) with a current transfer coefficient of at least 50 can be used in the device. LED - any with a working current up to 20 mA, diodes VD1, VD2 - any of the KD503, KD522 series. Capacitors - KLS, K10-17 series, variable resistor - SP4, SPO, tuning - SPZ-19, constant - MLT, S2-33, R1-4. The device is powered by a battery with a total voltage of 9 V. The current consumption is 3...4 mA when the LED is off and rises to about 20 mA when it is on. If the device is used infrequently, then the SA1 switch can be omitted by supplying voltage to the device by connecting the battery.

The design of the self-oscillator inductor is shown in fig. 2 - it is similar to the magnetic antenna of a radio receiver. On a round rod 1 made of ferrite with a diameter of 8 ... 10 mm and a permeability of 400 ... 600, paper sleeves 2 are put on (2 ... (3 turns) and L2 (0,31 turns) - 1. In this case, the winding must be carried out in one direction and the coil leads must be correctly connected to the oscillator. In addition, coil L60 must move along the rod with little friction. The winding on the paper sleeve can be fixed with tape.

Fig.2. The design of the oscillator inductor

Most of the parts are placed on a printed circuit board (Fig. 3) made of double-sided foil fiberglass. The second side is left metallized and is used as a common wire. The piezo emitter is located on the reverse side of the board, but it must be isolated from metallization with electrical tape or adhesive tape.

Fig.3. Printed circuit board

The board and the battery are placed in a plastic case, and the coil is installed closer to the wall (Fig. 4). To increase the sensitivity of the device, the board and battery must be placed at a distance of several centimeters from the coil. The maximum sensitivity will be on the side of the rod on which the coil L1 is wound. It is more convenient to detect small metal objects from the end of the coil, this will allow you to more accurately determine their location.

Fig.4. Device design

The model of the device had the following detection parameters: large metal objects - 8...10 cm, pipe with a diameter of 15 mm - 6...8 cm, M5x25 screw - 4...5cm, M2,5 nut - 3...2,5 cm, screw M10x1 - 1,5 ... XNUMX cm.

Set up the device in the following sequence. First select the resistor R4. To do this, temporarily unsolder one of the terminals of the VD2 diode and install a resistor R4 of such resistance (the maximum possible) so that the collector of the transistor VT5 has a voltage of 0,8 ... 1 V. In this case, the LED should shine and the sound signal should sound.

Then set the slider of the resistor R3 to the lower position according to the diagram and solder the VD2 diode, and solder the L2 coil. After that, the transistors VT3, VT5 should close (the LED will turn off). By carefully moving the slider of the resistor R3 up the circuit, they achieve the opening of transistors VT3, VT5 and turn on the alarm.

After that, the engines of the resistors R1, R2 are set to the middle position and the coil L2 is soldered. When L2 approaches close to L1, generation should occur, and the alarm should turn off. Coil L2 is removed from L1 and the moment of disruption of generation is achieved, and it is restored by resistor R1. In this case, one should strive to ensure that the coil L2 is removed to the maximum distance, and the resistor R2 could be used to achieve a breakdown and restoration of generation. Then the generator is set on the verge of failure and the sensitivity of the device is checked.

Author: I. Nechaev, Kursk; Publication: radioradar.net

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