Surf Pi 1.2 metal detector - air tests

My home build effort Surf PI metal detector - using the Surf Pi 1.2 (Silverdog board kit) - kit was good, all components present & board well laid out; rest of the stuff I had to find so a bit rough round the edges; added test points & pots to adjust 'delay' & coil damping supposed to allow me to play with coil designs - the coil I'm using here is made of 25 turns of braided wire from an old monitor- think this is called ‘litz’ wire . Not sure if this stuff behaves differently to solid copper wire - found coil more stable when coil shielded with a single wrap of foil tape.
Added a tiny piezo speaker which can be switched on or off wired in // to a 1/2 watt Cree LED - gives a massive change in brightness with increasing audio signal strength.