This circuit uses a 15-kHz oscillator coil. Whan metal placed ixi the energy field is withdrawn, the oscillator voltage is rectified and compared to a reference. A drop in oscillator voltage therefore operates comparator IC2 and D4 (LED) extinguishes.
L1= 500 turns enamelled copper wire. 0.2 - 0.3mm diameter. on ferrite rod 200mm long and 10 mm diameter.