ATtiny85 Function Generator with OLED User Interface
If you need to generate functions like square and sawtooth waves, you could buy a piece of equipment that will likely do the job quite well, or you can build your own.
This well-documented design by David Johnson-Davies (aka Technoblogy) is based on an ATtiny85 chip, and features an encoder with a push button for control and a small 12C OLED display for user feedback. All of this, along with the necessary resistors and capacitors, fit nicely on a small breadboard.
In action, the rotary encoder is used to adjust the waveform frequency, and different waves can be selected at the push of a button. The OLED shows both the frequency in hertz, as well as a handy icon for each wave so that the user can identify what’s going on at a glance.