Bug detector with logarithmic scale on 12 LEDs and sound indication. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Bug detector with logarithmic scale on 12 LEDs 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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The field detector includes a high-pass filter, an RF amplifier, a diode detector, a logarithmic gain DC amplifier, a frequency-varying tone generator, and a 12-LED bar graph. The detector is capable of registering working radio microphones in the frequency range of 20-600 MHz. The schematic diagram of the device is shown in the figure.

(click to enlarge)

The signal induced in the antenna is filtered by the HPF on the elements C2, L1, C3, L2 and fed to a broadband aperiodic amplifier. The latter is made on a high-frequency transistor VT1 type KT3101. The amplifier load is an emitter follower on a VT2 transistor of the KT3101 type. The signal taken from the sensitivity controller - resistor R4, is fed through capacitor C6 to a diode detector assembled on a VD1 type D9B diode. High-frequency components are filtered by RC filters R5, C7 and R6, C8. The low-frequency signal is fed to an amplifier on a DA1 chip of the KR140UD1208 type. The gain of this amplifier is determined by the value of the resistor R9. With a low input signal level, the amplifier on DA1 has a large gain. As the signal increases, the diode VD2 of the KD522 type opens, the resistance of which changes according to a logarithmic law. This leads to a change in the feedback resistance also according to a logarithmic law. From the output of the amplifier on the DA1 chip, the signal goes to the LED indicator and the sound generator.

The sound generator is made on a VT3 transistor of the KT315 type and a DD1 chip of the K561LA7 type. Capacitor C9 is charged through resistor R11 to the opening voltage of transistor VT3. This leads to a change in the level of a logical unit to the level of a logical zero on the collector of the transistor VT3. In this case, the cathode of the VD3 diode of the KD522 type is connected through the resistor R18 to the minus of the power source. Capacitor C9 quickly discharges through the circuit VD3, R18, which leads to the closing of the transistor VT3. Capacitor C9 starts charging again and the whole process repeats. Rectangular pulses are converted by a piezoceramic transducer ZQ1 type ЗП-22 into sound ones. With an increase in the voltage at the output of the amplifier DA1, the charge time of the capacitor C9 decreases to the opening voltage of the transistor VT3, and this, in turn, leads to an increase in the pulse repetition rate of the generator. Thus, as the input signal level increases, the tone of the audio signal increases.

The basis of the LED indicator is a DA2 chip of the KM1003PP2 type. The KM1003PP2 microcircuit is specialized and performs the function of controlling the LED scale, providing a bar on the scale of 12 LEDs that light up in turn when the input voltage changes from the minimum to the maximum value. The brightness of the LEDs is kept constant. The input signal, through a voltage divider on resistors R13, R16, is fed to the input of the DA2 chip (pin 17). Pins 16 and 3 of the DA2 microcircuit are supplied with reference voltage levels, which determine, respectively, the minimum (LEDs are off) and maximum (all LEDs are on) values of the input signal. The device is powered by a 5,6 V power supply. The VD4 LED of the AL307 type serves to indicate the device is on.

All parts used are small. The details of the HPF are described above. The DA1 chip can be replaced with a KR1407UD2 or any other operational amplifier with its own correction circuits. Instead of a DD1 chip, you can use K561LE5. When replacing the VD1 diode with the GD507, the range of the device can be increased to 900 MHz.

Publication: cxem.net

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Comments on the article:

a guest
everything is fine except for two points: 1) it is not clear from 20 Hz or MHz; 2) the reference voltage, its change will lead to an error, is there another microcircuit that will ensure the ignition of the LEDs.

a guest
did not find the designation dd27

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