(* BFO: “Beat Frequency Oscillator” a technical term describing the most common operating method used by metal detectors.)
The previous post mentioned the metal detector our students made during the winter vacation.
Following are the technical details:
A BFO type was chosen for its simplicity of operation and the ease of construction of the search coil. I had tried an “IB” (induction balance: another method of detecting metals) design earlier but most people were confused about the controls and couldn’t operate it without a lot of explanation and practice. Also the construction and balancing of the two search coils was beyond the level of understanding and skill of most students. This one can hear a Taiwan one dollar coin (about the same size as 1 cent U.S.) up to 100mm away.
The circuit is much like many others one can find on the web. Both oscillators use common npn transistors. One is fixed at 455 KHz with a cheap ceramic resonator. The other is adjustable by means of a trimmer capacitor and a fine tuning pot on the front. A 78L05 regulator I.C. provides a stable supply voltage until the battery voltage falls down to around 6 volts. This gives us a very stable beat note. A simple LM386 audio I.C. provides plenty of audio output. The input components of this I.C. are chosen to give high gain at low frequencies and very little at high frequencies. That’s because in operation it’s best to set the detector to a very low beat note where small changes are most easily heard. That low beat note needs to be loud but you don’t want to be deafened by a high-pitched screech every time you walk past a big target.
The whole thing was laid out roughly the same as the circuit diagram, to make it easier for students to understand. A large piece of blank double-sided PC board was used. The connecting pads were made by cutting off long strips with a hacksaw then chopping them into short lengths with a strong pair of side cutters. These were glued on with three second “superglue”, though in later models I used two-part epoxy glue. It was easier to handle.
The search coil was mounted between two plywood disks (135mm dia.) and held together with plastic cable ties. The coil was wound first as a long rectangle (on a suitably sized book) and pulled out into a 120mm diameter circle. It was fitted over a 120mm cardboard disk and sandwiched between the plywood pieces. The coils were attached to the shaft by gluing on two pieces of wood and using the body of a ballpoint pen as a pivot. A slot was cut lengthwise in the pen body to allow it to be squeezed through the hole and provide some friction (so the head will stay at the angle you choose.) A small section of larger water pipe was put on the end to make it stronger at the pivot point. The box used was a common plastic electronic project box 15cm x 9cm x 5cm with 18mm holes drilled at both ends for the water pipe shaft, which has an outside diameter of 18mm.
The search coil is shielded with copper tape, being careful to leave a small gap. One of the coil wires and the outer conductor of the shielded cable are soldered onto it. You could also use aluminum foil for the shielding but the connection to it would have to be made by wrapping tinned copper wire around it (several times to make sure it maintains a good connection.)
It’s best to tape the coil connections and glue them down with epoxy glue so nothing moves around. It’s also a good idea to paint the whole head assembly with a clear varnish in case it gets wet in field use.
The trimmer capacitor (6-60pf) inside needs to be adjusted for a zero beat note when the fine tuning pot on the front panel is set to the middle of its range. This should give you plenty of room for adjustment on either side of the zero beat. If no beat note is heard at all, it may be that there is some difference in the coil or other components. I found for example that a different shielded coil cable had quite a different capacitance and I had to compensate for it by reducing C8 to 680 pf. Another one had slightly thicker wire in the coil (26 swg) and it was easier to tune it with 21 turns instead of 22. (A multimeter with a frequency counter is very useful for finding out if it’s oscillating and on what frequency.)