K-26 Metal Detector: Sensor Coil Rewinding

There ought to be a survey marker pin at the front corner of the lot where it’d come in handy for locating the edge of the yet-to-be-contracted driveway paving, but if it’s there it’s been pushed below ground level. So I mooched a homebrew metal detector based on the Elenco K-26 PCB

K-26 Metal Detector PCB
K-26 Metal Detector PCB

The kit included 45 feet of  22 AWG enamel wire that should have become a 5 inch diameter coil with 30 turns, but the as-built detector had a coil wrapped around a 1 foot diameter cardboard form. The coil inductance sets the oscillation frequency, which turned out to be around 300 kHz: far below the nominal 1000 kHz. So I wound 40 turns of 22 AWG magnet wire around an old CD-ROM spindle case (which is, quite coincidentally, just over 5 inches in diameter), and taped it atop the cardboard form.

The datasheet recommends a nonmetallic handle, so I swapped in a plastic umbrella support for the original metal mop (?) handle.

Rewound homebrew metal detector
Rewound homebrew metal detector

The K-26 schematic looks like a common-base Colpitts oscillator, with only the most utterly absolutely vital essential components:

K-26 Schematic
K-26 Schematic

In round numbers, the oscillation frequency varies inversely with the number of turns:

F = 1/(2π√(LC)) (for a simple tank)

L = stuff × N2 (stuff = various constants & sizes)

F = stuff / N

The rewound coil oscillated at 350 kHz, so I spilled off a few turns at a time to produce these results and a tangle of wire on the floor:

L – µH Freq – kHz
330 350
186 535
107 711
65 840
42 1140

For the record, the coil in the photo corresponds to the last line and has 12 turns.

Contrary to what the instructions imply, trimpot P1 does not adjust the oscillation frequency. It tweaks the transistor bias for best oscillation, so it’s more of an amplitude control than anything else. I adjusted P1 while watching an oscilloscope connected across the negative battery terminal and the emitter of Q1, but you could probably use a small sniffer loop to good effect.

It draws about 2 mA, so the battery should last quite a while; labeling the switch positions should help a lot.

The oscillator produces an unmodulated carrier, so I tuned a Kenwood TH-F6A HT in LSB mode for maximum squeal. Any variation in L changes the carrier frequency and thus the pitch of the demodulated audio; an earbud just barely in one ear makes this almost tolerable.

As you should expect from the picture, that metal detector lashup is mightily microphonic, to the extent that touching a blade of grass wobbles the audio pitch and bumping the cardboard plate against an object can detune the whole affair. A bit more attention to rigid coil mounting would certainly help, but this isn’t the most stable of designs to begin with and I doubt anything will help very much at all.

The coil can detect a chunk of rebar sticking out of the ground at a range of maybe half a foot, but it’s not clear how well it will cope with buried treasures (like, oh, let’s say a survey marker pin). In any event, I must mow the grass down there before going prospecting.