A simple compact radio bug detector with two LED indication. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling
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This one differs from other designs in small dimensions, a small number of parts used and, at the same time, a sufficiently high sensitivity. This field detector uses a new circuit design. It is well known that the measurement of RF voltages less than 0,5 V is difficult because already at an alternating voltage of less than 0,2-0,3 V, all semiconductor diodes become ineffective. However, there is a way to measure small AC voltages using a balanced diode-resistive bridge, which allows you to measure voltages less than 20 mV with a uniform frequency response up to 900 MHz. A schematic diagram of a device using this method is shown in the figure.
The basis of this device is a DA1 chip of type KR1112PP2. This microcircuit includes a device for determining the balance of an electric bridge with an indication. The microcircuit has a built-in reference voltage source.
The signal induced in the antenna is amplified by a broadband aperiodic high-frequency amplifier based on a VT1 transistor of the KT3101 type. The amplified high-frequency alternating voltage through the capacitor C3 enters the diode-resistive bridge on diodes VD1-VD4 of the GD507 type and resistors R3-R5. A small (about a few microamperes) direct current flows from the reference voltage source (pin 3 of the DA1 chip) through resistors R3-R5 and diodes VD1-VD4, which improves detection conditions and increases the sensitivity of the detector. Only diodes VD1 and VD2 are involved in rectifying the measured alternating voltage, while the other two - VD3, VD4 - form the adjacent bridge arm, on which the initial voltage is created, balancing the bridge, and at the same time serve for its thermal compensation.
All diodes are selected with the closest possible current-voltage characteristics. Capacitor C4 filters out the AC component of the rectified voltage. Resistor R4 is used to accurately balance the bridge. When well balanced, the device will only respond to the voltage resulting from the rectification of the measured signal.
The rectified voltage and the voltage balancing the bridge are fed through resistors R7 and R8 to the inputs of the DC amplifier located in the DA1 chip. Depending on the state of balance of the bridge, the indication signal is sent to one of the LEDs VD5 or VD6 - type AL307. Thus, when the bridge is balanced (no signal), the VD5 LED is on, and when there is a signal (bridge imbalance), the VD6 LED is on. As diodes VD1-VD4, you can use any high-frequency diodes. LEDs can be of any type.
A 2,5-5 V DC source is used as a power source.
Publication: cxem.net
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