Building the Widlar Hassler

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!

The Widlar salute!

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

Hassler schematic

Hassler schematic

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:

Hassler deadbug construction

Hassler deadbug construction

12 thoughts on “Building the Widlar Hassler

  1. Pingback: Annoy Your Enemies with the Hassler Circuit | Hackaday

  2. What about the voltage? Can it be optimized for low power without making it less powerful?

    I’m an electronics student at a vocational training school, despite I’m 30yo and ADHD (and still failing most exams, I’m going to get a scholarship and spend most of it in a private tutor that gave me classes for the entry exam). People in class act like a sugar filled 12yo ADHD kid, despite there’s a dozen people with twenty-something of 26 students. Despite other students get upset, we finally give up and accept the chaos.

    Can I avoid enabling it from a voice of an older person?

  3. Add a high pass filter before the fet speaker driver, this will pass just the edges of the square wave and cut your dc current consumption.

    • Ah, great suggestion! After some quick calculations I’m pretty sure that a <50% duty cycle would let me use some more generic 2N7000's instead of the IRF510's too, even under worse-case conditions.

  4. Pingback: Annoy Your Enemies with the Hassler Circuit - Arduino collector blog

  5. Pingback: Annoy Your Enemies with the Hassler Circuit • W1GNL WebsiteW1GNL Website

  6. Pingback: Annoy Your Enemies with the Hassler Circuit | Hack The Planet

  7. I did read people loss hearing certain frequencies over age. If this is stadistally stable, I would be able to build the ultimate teenage hassler to deactivate the most annoying individuals (usually the younger ones, the rest are just imitating their behavior in a stupid way).

  8. Absolutely one of the best circuit walk-thru videos that I have watched in years. Very good information…..deliberated in a fun and playful, yet educated way.

    I am bookmarking this page…..waiting for further videos in the future.

    Thanks for taking the time to make it.!!

  9. Pingback: «Глушитель» Видлара | ТехноТрэш

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