Flickr member 5Volt designed and built an oscilloscope-esque VGA output board using an ATTiny84 chip. The board’s input signal is provided by a little red microphone/preamp daughterboard –

VGAWaveDisplayBoard_cc.jpg
From the MAKE Flickr pool

The circuit is very simple as this was my main target. I decided I would use one of the smallest AVRs I had, an ATtiny84. Physically small but large in memory : 8k FLASH is way larger than needed as my circuit fits 1/32th of that much. Anyways, just one 20MHz crystal, 5 capacitors, 4 resistors and one VGA connector are sufficient to generate a vertical line on a VGA monitor which swings left to right depending on the voltage at a micro’s input pin. Applying music at the pin the line dances at the music. A little bit kitschy ’70s prop.

More of the project’s backstory (and frontstory) plus code & schematics over at 5Volt.eu.