Last wednesday some friends from hackteria and the SGMK visited us at the space. They organised a little workshop for the occasion, which they call make-away. One of the simple design consisted of a home-made PCB, NAND gates (4093), a piezo speaker and a ceramic capacitor. Schematics available on their wiki page.
We had a great fun talking with them and sharing experience with hackerspace and fablab, thanks guys!