Signal Generator Mini-Kit MK105
All the parts and the PCB for a signal source based on the 555 timer. The schematic and assembly instructions are here. Runs on a 9-volt battery and delivers fixed 1 KHz at up to 100 millivolts output. Good basic tool for troubleshooting pedals and amplifiers. Square, triangle or approximated sine output waveform is selectable with a jumper. I figure most people will want a panel-mount pot rather than a trimpot for the output level, so that's included.