In fact, it is a signal detector which indicates the presence of high-frequency oscillations in a circuit. With it, we can check if an oscillator or a transmitter is working, and even locate transmitters spies of the type of FM, for example.
Easy to assemble, the circuit uses only 3 components and can be installed in a small plastic box. You can take it along with the other features that are used in electronic espionage in a special briefcase. In Figure 1, we have the diagram of the device.
The coil consists of about 30 to 50 turns of fine wire in a ferrite rod and the instrument is a microammeter of 50 uA at 200 uA full scale.
To use it, simply approach the coil of the oscillating circuit or operate with RF signals so that an indication of its operation is given if the power is enough. The ideal is to approximate L1 of the coils of the circuit under test.
Material list
L1 - Coil - see text
D1- 1N34 or equivalent - germanium diode
M1 - 50 uA at 200 uA full scale - microammeter
Miscellaneous: wires, welding, mounting box
