This essentially consists of an amplifier which, by means of a transformer, feeds the output back to the input, thus sustaining oscillation. On this basis, the author has named the detector a Coil Coupled Operation (CCO) Metal Detector. In fact the circuit would oscillate even without L2 and C1. However, in this case one would have nothing more than a beat frequency operation (BFO) detector. Coil L2 is added to bring the induction balance principle into operation, thus modifying the signal which is returned to the output, and greatly boosting performance. This does not mean, however, that we are dealing strictly with an IB detector type, since the design requires a beat frequency oscillator for detection.
Also, unlike IB, its Rx section (L2) is active rather than passive, being an integral part of a TCO. Nor is this strictly a BFO type, since its performance far outstrips that of BFO, and of course it uses two coils. Search oscillator IC1 oscillates at around 480 kHz, depending on the positioning of the coils on the search head. The presence of metal induces changes both in the inductance and coupling of the two coils, thereby inducing a shift in oscillator IC1’s frequency. The output (pin 6) is taken via a screened cable to a Medium Wave radio aerial. A crocodile clip termination would make a convenient connection.
The two coils are each made of 50 turns 30swg (0.315mm) enamelled copper wire, wound on a 120mm diameter former. Each has a Faraday shield, which is connected to 0V as shown. A sketch of the coil is shown in the separate drawing. The coils are positioned on the search head to partly overlap one another, in such a way as to find a low tone on the best heterodyne, which should match the performance mentioned above. Oscillator IC1 will sustain oscillation no matter which way the coils are orientated - however, orientation significantly affects performance.
The correct orientation may be determined experimentally by flipping one of the coils on the search head. Ideally, the coils will finally be potted in polyester resin. The CCO Metal Detector’s search head offers a wide area of sensitivity, so that it is better suited to sweeping an area than pinpointing a find. As with both BFO and IB, it offers discrimination between ferrous and non-ferrous metals, making it well suited to ‘treasure hunting’. And if you get fed up with searching, there’s always the radio to listen to.