Tim Jacobs created this simplified version of a Stylophone, it is put together on a credit card sized PCB. The Stylophone was invented in 1967 by Brain Jarvis and could be consider the forerunner of the portable keyboards and possibly the synthesizer.
This is not a replacement for an commercial keyboard but my interest was piqued as it involved electronics as well as music. I am not a musician by any means although I have a good understanding of music theory, I just don’t have any rhythm. Early on I found that without rhythm, I could play a song note for note and it wouldn’t sound right. I do still play around with my guitar but you wouldn’t want to listen.
The Stylophone is a small box device with a metallic keyboard which is activated when a stylist touches the keys. The connection completes a circuit generating a waveform using a voltage controlled oscillator (VCO). The waveform is changed with each key as every key is connected in series with a different value resistor. By changing the resistance, the voltage provided to the VCO is also changed resulting in a changed time constant.
In essence the waveform is created by connecting a capacitor and a resistor in parallel. When power is applied, the capacitor will charge. Once the power is removed the power stored in the capacitor will discharge through the resistor. A lower resistance will discharge the capacitor faster and conversely a higher resistance will discharge the capacitor at a slower rate. Connecting a sense line to the RC circuit will produce a waveform. The waveform determined by the value of the capacitor and the value of the resistor will be at a certain frequency. The frequency of the waveform determines the pitch of the note.