This looks very similar to the wand used to detect tones on UTP(Cat5 etc). I have been looking for a design for the probe as although they aren't lost as often as the tone sender they still go missing. I have used my tone wand to debug but not for the last 20years. the senders can be any oscillator circuit, transistor relax, 555, 8051 anything.

— Paul on 18.08.2015 at 08:15 (UTC)

I have received a few questions about the power usage. With the 70 ohm speaker element I'm using, the circuit works at:

3.0V  11mA
4.5V  16mA
10V   30mA

Biasing needs to be adjusted with the trimmer depending on voltage range, but does not need to be particularly accurate. The circuit seems to work down to 2V but the audio volume is too low to be usable, atleast with this speaker.

— Petteri Aimonen on 18.12.2015 at 15:51 (UTC)