Turntable Turns Waveform Generator

In need of a waveform generator for another project, [David Cook] crammed out the old turntable to modify it for a handy hack: By adding a simple reflectance sensor to the pickup he turned it into a waveform generator that optically plays back arbitrary waveforms from printed paper discs.

For his hack, [Dave] created a 3D printed mount, which attaches a LED and a photodiode to the pickup of the turntable. The mount also blocks most of the ambient light, allowing only the reflected light from the paper disc to reach the sensor through a small slit. A little breadboard circuit powers the LED and converts the photocurrent of the sensor into a measurable voltage by loading it with a trim potentiometer. The modified turntable allows [Dave] to play printed discs with greyscale patterns, generated by a little program he wrote. Square waves, triangular waves, sine waves, noise — everything you need in a lab is there.

Critics may argue that a cheap, off-the-shelf waveform generator would deliver a cleaner signal, but playing back a waveform from a turntable adds an irreplaceable, unique experience to any measurement setup. A little bit of noise here, some non-linearities there, none of this detracts from the feeling that you own the waveform. Enjoy the video, where [Dave] demonstrates his hack!

Thanks to [Aaron] for the tip!

23 thoughts on “Turntable Turns Waveform Generator

  1. Easier ways to achieve the goal, but that isn’t the point. I admire the thinking that has resulted in this hack!

    And for really out-there signals, add some good old ‘DJ scratching’ into the pot.

  2. I wonder if he could modulate the print darkness with sound. That would work but to get any decent time for the sound it would have to spiral in towards the center. I know maybe he could put a groove in the paper to guide the optical pickup and then … :)

    Like it, Oddly so.

    1. Old home projectors for 16mm used an optical analog soundtrack on the film that was just a modulated light/dark area. It’s definitely been done, so would be easier to adapt than coming up with a from scratch implementation of the idea and of recording.

  3. I used to DJ years ago. I had a set of records from Numark that had long tracks of continuous tones- square waves, synth leads, etc. You could scratch melodies by carefully controlling the speed at which you moved the record back and forth while scratching and create a wave envelope with the crossfader. It was a ton of fun.

    1. The Compton Electrone used metal electrostatic discs, similar to the Hammond ‘tone wheel’ design. Mattel manufactured the Optigan in 1970 which used clear plastic discs with the film style optical audio track printed on them. The discs were interchangeable, comprising what was in essence a mechanical sampler with real recorded rhythm, bass lines and lead sounds. They were very ‘lo-fi’ and cheaply made, which makes them kind of rare. They pop up on Ebay from time to time for a few hundred bucks, but the discs are $30 and up. Some aficionados even released a new Optigan disc a few years back.

      1. I was gonna mention the Optigan. Difference here being using reflective rather than transmissive sensor, so he doesn’t have to bother getting film developed, or printing on acetate.

        I wonder what sort of complex waveforms he could get if he tried? Would depend on the size of the sensor’s window. Also would be interesting to try playing photographs or pages of newspaper and the like, just to see if any sound nice.

  4. If all you have is LEDs, resistors and transistors, and who doesn’t have a box of them, you can binary encode your amplitude values and feed them into a resistor network to get an analogue waveform. Basically the disk becomes a lookup table. This will probably work a lot better as you are just sensing on and off values so that nonlinear response problem he is having will not arise. I would not bother with the turn table either as any motor that you can control the speed of will work, and be more versatile.

  5. Okay, how about using this as a lock out mechanism?

    You’d need not only the disc to read, but a track that you’d use for speed calibration. After speed calibration move the turntable arm to the appropriate track to read the key to unlock the mechanism. Only authorized users would know what the disc was for and what tracks made up the calibrate and key functions.

    Being made out of paper, it’d be easy to destroy as well.

    1. I was just thinking that a Raspberry Pi would make a fantastic workbench PC. Not just signal generation, but capture also, and with a slew of GPIO pins to control your projects. The USB and network connectivity don’t hurt, either.

  6. This reminds of an awesome hack I read about years ago (sorry, I can’t seem to find it). It was done long before arbitrary waveform generators became available.

    An engineer positioned a photodiode in front of an CRT oscilloscope screen, while blocking all outside light. This photodiode was connected to a analog integrator, the output of which was connected to the vertical amplifier of the oscilloscope, such that bright light would make the signal fall, and no light would make it rise. He would then block part of the oscilloscope screen with a paper cut-out of the desired waveform, and because of the integrator, the dot of the oscilloscope would hug the edge of the paper. The output of the integrator would thus follow the desired waveform, and he could pretty much make any waveform he could cut out of a piece of paper.

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