This simple device is a basic signal generator that can be used to debug audio and AM radio frequency circuits. It produces a (roughly) 1 kHz periodic wave output that has enough harmonic frequency content to go into high MHz range – this will make the signal audible even when injected into RF and IF radio circuits.
The schematics is based on a simplest possible two-transistor multivibrator that can be powered by one 1,5 battery (AA or AAA) but will also work fine with a higher voltage source, for example a 3,6V lithium coin cell. It draws around 300 mA at 1.5V. The components values can vary, also pretty much any pair of low power transistors will work. In case of PNP transistors, the battery should be flipped (+ as the ground and – as the power rail). It should start working right away and doesn’t require any tuning.
The output waveform and frequency will vary depending on the tested circuit impedance, without the load it will be similar to this:
Github source: https://github.com/badscrew/RadioTester
I simply soldered mine on a perfboard and it proved itself very useful: