What is it?

Chomper plays a rousing, familiar tune, accompanied by a minimal LED animation.

Why did you make it?

I wanted to see if you could fit a chip tunes melody onto just an attiny13 (1k of flash) that plays reasonably loud. Turns out, you can.

Usage instructions

Press the button to play a tune. If the tune is no longer desired while still in progress, press the button a second time to stop the melody in progress.

The badge uses an ultra low power sleep mode while not activated.

Comes fully assembled.

Design notes

Normally you can drive a piezo off of one microcontroller pin. The results may vary however, and will probably not be loud enough to compete with any other noise in the room.

A piezo buzzer is like a capacitor: current doesn't really flow through it, what happens is the piezo disc physically deforms according to a voltage differential across it. So voltage makes it louder without adding much current.

So next step is to drive the piezo off of two microcontroller pins that generate an AC square wave. Rather than one pin of the piezo being high or low and the other pin being ground, instead each side alternates high and low on separate microcontroller pins. Using two pins on the same port register we can swap them virtually simultaneously. This doubles the effective amplitude of the voltage of the square wave.

Next, using a really basic charge pump square wave voltage doubler circuit we can further double the voltage across the piezo, minus the voltage drop of the diodes. That's the design used here. You should be able to chain more voltage doublers in series to keep making it louder, in theory.

This drives a piezo at a reasonable loudness using just a 3V coin cell. Aaaand it turns out that a magnetic buzzer works is also much louder with this circuit, although in my tests sounded if anything a bit overdriven.

See github link for further technical details, source code and some design files. The reset pin is not used for IO so the attiny can be re-programmed, but the board does not have an ISP header broken out, but you can still use an SOIC8 clip to connect in-circuit.