Tone Maker is an ATmega328p-based Arduino Uno-compatible square-wave generator designed specifically to take the place of an expensive function generator to drive a small 8-ohm speaker for resonance experiments in my physics courses. The default program generates audio frequencies corresponding to C4 (middle C on a piano), D4, E4, F4, G4, A4, B4, and C5. A push-button interface allows a user to rotate through this octave of frequencies and a seven-segment display shows which note is currently being produced. A barrel jack accepts battery power ranging from 9 V to 12 V. A slider switch provides two output volume levels, and banana plug jacks allow easy speaker hookup. An 6-pin ICSP header is provided so the MCU can be reprogramming using the Arduino IDE and a programmer like the AVRISP mkII.