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.

Since Widlar’s original schematic was never published, I set out to re-create this infamous circuit myself:

Audio from the electret microphone is amplified, half-wave rectified, and low-pass filtered by U1a. Capacitor C3 converts the rectified audio into a nice flat DC voltage.

U1b serves as a voltage controlled relaxation oscillator, whose duty cycle increases as the control voltage, supplied by U1a to the hysteresis resistors R6 and R7, increases.

The oscillator’s output is fed to the gate of Q1, a power MOSFET which drives an 8 ohm speaker with ~850mW of power.

The circuit consisting of U2 and Q2 is not necessarily required, but makes the circuit more power efficient. Q2 keeps Q1 normally off so that the speaker isn’t being driven all the time. Q2, and subsequently Q1, are only turned on by a Schmitt trigger (U2, a TL081 op amp) once the rectified audio rises to a threshold of ~8.5 volts.

Both the circuit sensitivity and oscillator frequency range are configurable via trim pots R2 and R9 respectively.

A detailed circuit description, demonstration, and deadbug porn follows: