THE circuit shown in Fig. must represent the limits of simplicity for a metal detector – yet the design works surprisingly well. It uses just one 40106 hex Schmitt inverter i.c., a capacitor, and a search coil – and of course the batteries. A lead from IC1b pin 4 needs to be attached to a medium wave (MW) radio aerial, or it should be wrapped around the radio. As shown, the metal detector gives a respectable range for beat frequency operation (b.f.o.) – up to 90mm for a bottle-top. In fact, for the ultimate in simplicity, capacitor C1 may be omitted. In this way the author achieved an amazing 150mm range for the bottle-top. However, with the frequency then being raised to more than 4MHz, instability becomes a significant problem.
Simple metal Detector circuit
As shown, the circuit oscillates at around 230kHz. One may also experiment with the frequency by changing the value of C1. A Faraday shield may be added to reduce ground effect and capacitive coupling, and this is wired to 0V. Since an inductor resists rapid changes in voltage (called reactance), the charging of capacitor C1 is slightly delayed as the logic level at IC1a pin 2 changes. This sets up a rapid oscillation, which is picked up by a MW radio. Any changes in the inductance of the search coil (through the presence of metal) bring about a change to the oscillator frequency. Although 230kHz is out of range of the Medium Wave band, an MW radio will clearly pick up harmonics of this frequency. IC1b serves as a buffer. The making of search coil L1 allows a lot of room for error, and is far from critical. The author used seventy turns of 30s.w.g. (0·315mm) enamelled copper wire on a 120mm diameter former.The metal detector is set up by tuning the MW radio to pick up a whistle (a harmonic of 230kHz). Not every such harmonic works well, and the most suitable one needs to be found. The presence of metal will clearly change the tone of the whistle.
Thomas Scarborough,
South Africa
About Vivek R