One of my personal favorite Bob Widlar pranks was his “hassler” circuit, which he used to combat office noise.
If you spoke too loudly in the presence of the hassler, it would emit a high pitch tone, which would decrease in frequency the louder you spoke; this had the effect of causing an annoying ringing noise in your ears. As soon as you noticed the ringing and stopped talking in order to identify the source, the hassler would shut up too.
The Widlar salute!
Although generally not as stable as their harmonic relatives, relaxation oscillators are quite handy little circuits for low frequency generation, and can be found in everything from turn signal indicators to joule thief circuits.
Relaxation oscillators generate a changing voltage at a particular frequency by charging and discharging a capacitor through a resistor, and are often built around an operational amplifier:
Relaxation oscillator, Art of Electronics, p. 285
Before designing an oscillator for ourselves, let’s take a look at how this circuit functions.