The Stylophone is a miniature analog stylus-operated synthesizer invented in 1967 by Brian Jarvis and going into production in 1968. It consists of a metalkeyboard played by touching it with a stylus — each note being connected to a voltage-controlled oscillator via a different-value resistor – thus closing a circuit. The only other controls were a power switch and a vibrato control on the front panel beside the keyboard, and a tuning control on the rear. 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 styluses.
Info gleaned from Wikipedia