Coil Winding Guide

[Skippy]

"I suspect this metal detector is a simple TR-type with amplitude detection" : That's exactly what it is ( see the schematic ) , the RX coil feeds the 5kHz directly into the 'mic' input. Just a couple of diodes to limit amplitude, and a coupling cap to block the DC output the phone is likely to supply for powering condenser mics.

[Rustybolts57]

There is also a simple audio circuit with a speaker added to it but not shown in the schematic.

[Rustybolts57]

" I am getting the impression that I should be at a substantially higher frequency." Not really ... a decent 'all-round' operating frequency would be the 10k - 12k range; lower freqs, like 5k - 8k would favour US 1c/10c/25c/ modern dollar coins in a park-hunting scenario, for example; 15k - 20k would favour smaller targets, and those made of metals that don't conduct electricity so well - including US 5c coins, most gold finger rings, drinks can ringpulls, and here in Europe, many bronze / lead / brass artefacts and older/ancient smaller coins.

Good information, thank you.