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!