I’ve found it hard to debug my non-working circuits with my two hands, and simultaneously play the guitar with my other two hands. I needed some source of audio signal to get all my four hands free for probing. One good solution is an audio oscillator, which does nothing but produces audio signal in static frequency. Something like this:
This simple circuit produces audio signal in frequency of 1000hz. Works with 9V battery for ages, and has a volume control knob. It’s good that it has, because this little bastardo has the power. You really need to beware to keep this one’s nose clean. Otherwise missus’s porcelains will be broken.
I didn’t want to waste enclosure for this, so I built it inside broken mouse:
This is also a good tool to get somebody to confess something if forced to listen continuously. So, multitool this is.