TrigPulser Mini- DIY squarewave CMOS synth (incl. schematic)

Here's an older video of it in action:

It's relatively simple, and only uses a single IC (the CD4093, pictured below), plus a few knobs and a few passive components, and runs on a 9V battery. Should make a good beginner's project. The one in the video is housed in a plastic VHS tape case, and there's room to spare.

Here's a shot of the original module's circuit, this version only uses one IC, so it would be even smaller.

...and here's the schematic, if you want to build it yourself.

If you have any questions, feel free to ask in the comments, and I will try to answer. Also, let me know if you're interested in seeing more of this kind of thing.