Here's something completely different. I'm well known for my electronic designs, and here's one of the classics. It gives good performance and stability for a beat frequency operation (BFO) metal detector. BFO in itself, though, is not very sensitive -- yet it's easy and fun. This metal detector will pick up a 25 mm (1") diameter coin at 90 mm (3½"). The design is based on a simple inverter oscillator, which is attached to a Medium Wave radio aerial. When metal is brought near to coil L1, the frequency of the oscillator changes, bringing about a shift in the difference frequency in the MW radio speaker. Solder the circuit and take a wire to the MW radio aerial as shown. This should be screened, with the screen going to -6V (negative). Switch on both metal detector and radio, and tune the radio until a clear heterodyne (a whistle) is heard in the speaker (some heterodynes will work better than others). There will be a "band of silence" at the centre of most heterodynes (a zero beat frequency), and depending which side of this zero beat frequency is tuned in, the tone in the radio speaker will rise or fall at the presence of metal. It will also respond differently to ferrous and non-ferrous metals. OBSERVATION: Click on the diagram to blow it up. You can find instructions for the coil at http://thomasscarborough.blogspot.com/2008/05/bfo-metal-detector-2.html. A similar design reached the top of the charts at CircuitDB: http://www.circuitdb.com/. To follow the electronics trail on this blog, click on the "Electronics" category top left. Have you seen my electronics books? http://stores.lulu.com/store.php?fAcctID=980032
NOTE: You may re-publish this design, on condition that you acknowledge the designer (Thomas Scarborough) and this blog (http://thomasscarborough.blogspot.com).