three 20comp 20metal 20det

THREE-COMPONENT METAL DETECTOR -

TM-HJHnPIHIAL WMOTEIS

The Circuit shown in Fig.l must represent the limits of simplicity for a metal detec-tor - 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 ff om iClb pin 4 needs to be attachedto a medium wave (MW) radio aerial, or it should be wrapped around the radio.

As shown, the metal detector gives a respectable rangę for beat freąuency opera-tion (b.f.o.) - up to 90mm for a bottle-top. In fact, for the ultimate in simplicity, capacitor Cl may be omitted. In this way the author achieved an amazing 150mm rangę for the bottle-top. However, with the freąuency then being raised to morę than 4MHz, instability be co mes a significant problem.

As shown, the Circuit oscillates at around 230kHz. One may ais o experiment with the freąuency by changing the value of Cl. A Faraday shield may be added to reduoe ground effect and capacitive coupling, and this is wired to 0V.

Sinoe an inductor resists rap id changes in voltage (caIle d reactance), the charging of capacitor Cl is slightly delayed as the logie level at iCla pin 2 changes. This sets up a rap id oscillation, which is piekę d up by a MW radio. Any changes inthe inductanoe of the search coil (through the presence of metal) bring ab out a change to the oscillator freąuency. Although 230kHz is out of rangę of the Medium Wave band, an MW radio will clearly pick up harmonics of this freąuency. iClb serves as a buffer.

The making of search coil LI 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 120 mm diameter former.

Detector Circuit.

The metal detector is set up by tuning the MW radio to pick up a whistle (a harmonie of 230kHz). Not every such harmonie works well, and the most suitable one needs to be found. The presenoe of metal will clearly change the tonę of the whistle.

Thomas Scarborough, South Africa