DISCREET ALL BAND RECEIVER

If you ever wished to listen to radio signals which populate the air without being noticed, test some radio equipment, hear the noise produced by electronic devices or listen to in-flight radio transmission while traveling on an airplane, this project is what you're looking for.

It is able to pick up all sorts of AM transmission and sometimes even FM via multi-path reception. The circuit has been built directly inside a small tin box, used as a ground plane and shield; the small sliding hole on the side has been used to mount the headphones jack socket. Once closed it cannot be distinguished from a common candy box.

The circuit is basically a diode detector (experimenting shows that RF schottky diodes work even better than germanium ones, with a little bias) and the original high gain audio amplifier following the first stage has been modified using an lm386 amplifier in a low noise-high gain configuration, in order to avoid using high impedance earplugs. In the photo you can see the audio jack socket, the 10k trimmer for volume regulation on the left, along with the deadbug-style soldered amplifier; next to it, to provide a little bit of noise insulation, there is the first stage and diode detector, then the 12v battery, power switch and in the upper side of the box the antenna wire can be seen.

Originally, i used a stereo jack as follows: signal ground was connected to the box wall, left audio was used to connect both headphones speakers to the output of the amplifier and the right audio channel was used, along with a tiny coaxial (which was running parallel to the earplug wire) , to connect an  external antenna.

This project is inspired by the work of Charles Wenzel.