Bug detector with a linear scale of eight LEDs, sensitivity adjustment and sound indication. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Bug detector with a linear scale of eight LEDs, sensitivity adjustment and sound indication. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling

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This device has some similarities with those described above. So, there is an RF amplifier and a detector on a balanced resistive-diode bridge. A distinctive feature of this field detector is: a high-pass filter at the input, a DC amplifier on two operational amplifiers, a sound generator, a linear LED scale and a low battery indicator. All this makes this device undoubtedly simpler and more convenient to use. Schematic diagram of the field detector is shown in the figure.

(click to enlarge)

The signal received by the antenna is fed to a high-frequency filter on the elements C 2, LI, C3, L2, which is necessary to suppress signals with a frequency of less than 20 MHz. This is necessary to reduce the level of low-frequency signals that usually make up the background radio emission. With a high-pass filter, signals with a frequency of more than 20 MHz are fed to the input of an aperiodic high-frequency broadband amplifier assembled on a VT1 transistor of the KT3101 type. From the load of the amplifier - resistor R2 - the high-frequency voltage through the capacitor C5 is supplied to the diodes VD1, VD2 of the GD507 type, which are part of the resistive-diode bridge. Resistor R4 is used to balance the bridge. The operation of the bridge has already been described in detail above.

The detected low-frequency voltage, smoothed by capacitor C6, is fed to a DC amplifier based on two operational amplifiers DA1.1 and DA1.2, which are part of the K1401UD1 microcircuit. From the output of the DA1.1 element, a constant voltage is supplied to the audio frequency generator, made on the operational amplifier DA1.3. The generator frequency depends on the DC voltage level at the non-inverting input of the DA1.3 element, which, in turn, depends on the input signal level. Thus, the higher the input signal level, the higher the frequency of the audio frequency generator. From the output of the generator, the sound signal is fed to the base of the transistor VT4 of the KT315 type, in the collector circuit of which the piezoceramic transducer ZQ1 of the ZP-1 type is connected.

Chips DA2 and DA3 type K1401UD1 form the basis of the linear scale. The operational amplifiers included in these microcircuits are connected according to the voltage comparator circuit. The non-inverting inputs of these comparators receive a reference voltage from a line of resistors R14-R21. The other inputs of the comparators are connected together, they receive a constant voltage from the output of the DC amplifier DA1.2. When this voltage changes from 0 to the maximum value, the comparators switch, at the output of which the VD5-VD14 LEDs are turned on, forming a linear light-emitting scale. The higher the signal level at the input, the more LEDs are turned on.

To reduce the current consumed by the LED scale, the principle of dynamic indication is used. To do this, the base of the transistor VT2 type KT315 receives pulses from the audio frequency generator DA1.3, causing the alternate closing and opening of the transistor VT2. When the transistor VT2 is closed, the positive voltage of the power source through the resistor R32 is supplied to the cathodes of the VD5-VD14 LEDs, which leads to the locking of the latter. No current flows through the LEDs and they go out. When the transistor VT2 is opened, the cathodes of the LEDs are closed to the minus of the power source, and those LEDs, on the anode of which there is a positive voltage, light up. Due to the inertial properties of the human eye, the blinking of the LEDs becomes imperceptible. The battery discharge indicator is made on the DA1.4 element and LEDs VD13, VD14. When the power supply voltage decreases, the current flowing through the VD15 zener diode and the VD13 LED decreases and, accordingly, the voltage at the VD13 anode. This causes the VD14 LED to turn on. The trigger level is set by the tuning resistor R33 during tuning. The whole device is powered by a stabilizer assembled on the elements VT3, VD15, VD13, R34, C8.

The device uses resistors of the MLT-0,125 type. LEDs VD5-VD14 can be anything. Diodes VD1-VD4 - any high-frequency germanium. Coils L1 and L2 are frameless, 8 mm in diameter, wound with 0,6 mm PEV wire. Coil L1 - 8 turns, coil L2 - 6 turns. Resistor R4 - any variable resistor with a linear characteristic. Transistors VT2-VT4 can be type KT3102. The VD15 zener diode can be replaced with KS147, KS168, KS170. Piezoceramic transducer ZQ1 - any. You can also use a dynamic head with a resistance of more than 50 ohms, while the resistor R36 can be excluded from the circuit.

Setting up the scheme has no special features. Before starting work, it is necessary to set the detector to maximum sensitivity with resistor R4. By rotating the slider of the resistor R4, 1-2 LEDs are lit and the sound alarm is turned off. The device is ready to work.

Publication: cxem.net

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