As done on the Stylophone 350!
Here’s some info on the Stylophone from that source of all information – Wikipedia:
The DÃ¼breq Stylophone is a miniature stylus-operated synthesizer invented in 1967 by Brian Jarvis. It consists of a metal keyboard played by touching it with a stylus â€” each note being connected to a voltage-controlled oscillator via a different-value resistor – thus closing a circuit. Some three million Stylophones were sold, mostly as children’s toys.
The Stylophone was available in three variants: standard, bass and treble, the standard one being by far the most common. There was also a larger version called the 350S with more notes on the keyboard, various voices, a novel ‘wah-wah’ effect that was controlled by moving one’s hand over a photo-sensor, and two stylii.