Whirly-Whirler LFO

Spinning plate toy and magnetic sensor as a mechanical Low Frequency Oscillator.

This uses magnets on a rotating plate and a magnetic sensor to generate a waveform to modulate an analog music synthesizer.

Toby Paddock - March 28, 2001

The magnetic sensor is an Allegro A3517LUA hall effect monolithic sensor. See Magnetic Sensor as a Control Voltage Source for more info. The plate is probibly from the 1960s (hey, it's vintage) and has the name Whirly-Whirler on it.

why is it called 'double-back' tape? Why not 'double-front'? sensor (nice focus on the carpet) of course there's duct tape OK, let's take it for a spin Click on images for bigger images.

Two very strong magnets were attached with double back foam tape to the top of a Whirly-Whirler (tm) spinning plate. One was 'north up' and the other was 'north down'. They were covered with duct tape to offer a little protection from anything magnetic in the neighborhood trying to smash into them. The white tape seen in the picture was from earlier photocell/laser tinkering.

The sensor was attached near the end of a pointy stick and wrapped with duct tape. During 'spin-up' the rim of the plate is hitting the sensor, so maybe a little more protection would be a good idea. The sensing axis is horizontal with the stick vertical. The magnets form a spinning horizontal field.

With the stick leaning THIS way. With the stick leaning THAT way. Click on waveforms for real waveforms.

The raw output was kind of a distorted sinewave of about 2 to 3 V pk-pk centered about 2.5 VDC. Waveform changed depending on the angle of the stick. Cool, a bonus variable. I don't know where that spiky looking noise is coming from. The sensor itself, power frequency magnetic spikes in the room, or the oh-my-God long unshielded wires. I didn't check.

Maximum frequency is somewhere around 5-10 Hz and takes about 5 minutes to stop spinning. It has a nice 'exponential-ish' decay in frequency. The sensor works down to DC so it doesn't care how slow it goes.

Future possibilities:
A simple op-amp circuit for offset and gain.
A comparitor for a gate pulse.
More plates.
Try opto sensing. Ir emitter/detector (or laser/photocell) and reflective stripes on plate. More stripes = higher frequency.
Blowing air along the edge might keep it spinning.

Suggestions, corrections, comments, and general abuse are more than welcome.

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© 2001 Toby Paddock