This device has some similarities with the one described above. Thus, there is an RF amplifier and 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 with two operational amplifiers, sound generator, linear scale led and battery indicator. All this makes this device undoubtedly more simple and convenient in operation. Schematic diagram of the detector field is given in the figure.

The bug sweeper with a linear scale of eight LEDs,sensitivity adjustment and audible indication

The signal received by an antenna is supplied to a high-pass filter on the elements 2, LI, C3, L2, necessary to suppress the signals of frequency less than 20 MHz. It is required to reduce the level of low-frequency signals, usually constituting a background radiation. With HPF signals of a frequency greater than 20 MHz is input to the aperiodic broadband high-frequency amplifier, assembled on the transistor VT1 type CT. With load amplifier - R2 resistor - high-frequency voltage across the capacitor C5 via steps onto the diodes VD1, VD2 type GD included in the resistor-diode bridge. For balancing the bridge is used, the resistor R4. The work of the bridge has already been described in detail above.

Predetection low frequency voltage smoothed by capacitor C6, is fed to the DC amplifier has two operational amplifiers DA1.1 and DA1.2 included in the chip CUT. From the output of the element DA1.1 constant voltage is supplied to the audio frequency generator realized by operational amplifier DA1.3. The oscillator frequency depends on the level of the DC voltage at the noninverting input element DA1.3, which, in turn, depends on the level of the input signal. Thus, the higher the input level, the higher the frequency of audio frequency generator. From the sound generator output signal is supplied to the base of transistor VT4 KT315 type in the collector circuit of which includes a piezoelectric transducer ZQ1 type ZP-1.

Chip DA2 and DA3 type CUT form the basis of a linear scale. Operational amplifiers included in the composition of the chips is connected in the circuit of voltage Comparators. The non-inverting input of the comparator receives the reference voltage from the line of resistors R14-R21. The other inputs of the Comparators are connected together, they received a constant voltage from the output of the DC amplifier DA1.2. Changing this voltage from 0 to a maximum value of the switching Comparators, the output of the enabled LEDs VD5-VD14, forming a linear light-emitting scale. The higher the level of the input signal, the more LEDs included.

To reduce consumption led current scale uses the principle of dynamic display. To do this, at the base of the VT2 transistor KT315 type received impulses from the generator of sound frequency DA1.3, causing the alternate closing and opening of the transistor VT2. When closing the transistor VT2 positive supply voltage through resistor R32 is supplied to the cathodes of LEDs VD5-VD14, which leads to the locking of the latter. The current through the LEDs is not flowing and they go out. With the opening of the transistor VT2 cathodes of the LEDs are closed to the minus power source and the led, the anode of which there is a positive voltage, light up. Due to the inertial properties of the human eye blinking LEDs becomes invisible. The battery indicator is made on an element DA1.4 and the LEDs VD13, VD14. When the voltage of the power source decreases the current flowing through the Zener diode VD15 and diode VD13 and, accordingly, the voltage at the anode VD13. This causes the led on VD14. Trigger level set trimming resistor R33. The entire device is powered by a stabilizer collected on elements VT3, VD15, VD13, R34, C8.

In the device used type resistors MLT-0,125. LEDs VD5-VD14 can be arbitrary. Diodes VD1-VD4 - any high-frequency germanium. Coils L1 and L2 frameless, with a diameter of 8 mm, coiled wire sew 0.6 mm. Coil L1 - 8 turns, the coil L2 - 6 turns. The resistor R4 is any variable resistor with a linear characteristic. Transistors VT2-VT4 can be type KT3102. Zener VD15 can be replaced by X, X, X. Piezoceramic transducer ZQ1 - any. You can also use the dynamic driver impedance than 50 Ohms, the resistor R36 you can from the diagram to be deleted.

Setting the schema has no peculiar features. Before you start, you need to configure the detector for maximum sensitivity resistor R4. The rotation of the engine resistor R4 achieve emission 1-2 LEDs on and off sound alarm. The device is ready for operation.

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