As University of Western Australia electrical engineering student Darcy Johnson writes:
There’s a certain intrigue in the unpredictability and complexity of analogue electronics — something digital circuits can’t quite replicate. After building several digital projects with microcontrollers and logic gates, [he] wanted to dive into the raw, hands-on world of analogue circuit design.
For Johnson, this meant building a Stylophone-inspired monophonic analog synthesizer from scratch, powered by none other than the NE555 oscillator (of “just use a 555” fame). For his aptly named “TouchTone555,” the NE555 provides an oscillating signal that drives an LM358 op amp to power the device’s speaker. Sound is played by connecting a probe to the conductive keypad built into the unit’s PCB. This emits different notes based on the key touched and its corresponding resistor.
The TouchTone555 also features an RC oscillator for a vibrato effect, and it is powered by a 9V battery. The design went through three PCB iterations (plus breadboard testing) before settling on the final(?) version. All this work paid off, however, as it looks and sounds awesome in the demo video below.
Notably, the unit’s bare PCB is laid out and annotated in such a way that someone can learn about its operation just by looking at it. Johnson certainly gained a massive amount of experience in analog electronics and music synthesis during the build, including the troubleshooting required to get it to work well. KiCad files and more info are available on GitHub if you want to make one yourself and/or further examine the design.