[vonPongrac] stumbled upon this handy guide, a free eBook on 50 555 Circuits, which contains many cool project ideas, including a simple metal detector ... varies the output sound, alerting ...

The detector circuit uses a miniature solar cell and a JFET wired up in a small brass tube to block most of the light and to offer some EM shielding. This, in turn, is attached to a small ...