Single IC BFO Metal Detector
Thomas Scarborough
www.250.org.ua
Here's something completely different. I'm well known for my electronic designs, and here's one of
the classics. It gives good performance and stability for a beat frequency operation (BFO) metal
detector. BFO in itself, though, is not very sensitive -- yet it's easy and fun. This metal detector will
pick up a 25 mm (1") diameter coin at 90 mm (3½"). The design is based on a simple inverter
oscillator, which is attached to a Medium Wave radio aerial. When metal is brought near to coil L1,
the frequency of the oscillator changes, bringing about a shift in the difference frequency in the MW
radio speaker. Solder the circuit and take a wire to the MW radio aerial as shown. This should be
screened, with the screen going to -6V (negative). Switch on both metal detector and radio, and tune
the radio until a clear heterodyne (a whistle) is heard in the speaker (some heterodynes will work
better than others). There will be a "band of silence" at the centre of most heterodynes (a zero beat
frequency), and depending which side of this zero beat frequency is tuned in, the tone in the radio
speaker will rise or fall at the presence of metal. It will also respond differently to ferrous and
nonferrous metals. OBSERVATION: Click on the diagram to blow it up. You can find instructions for
the coil at
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Here are instructions for the search coil of the metal detector above. It requires 70 turns of 30 swg
(22 awg) enamelled copper wire on a 120 mm (4¾") diameter former. Neither the winding of the coil
nor the wire gauge are critical. Wind the coil around the former, and temporarily hold it together
with stubs of insulating tape. Then bind it tightly with insulating tape all round. Scrape the enamel
off the ends of the coil's enamelled copper wires. Then add a Faraday electrostatic shield, for
stability. Prepare some long, thin strips of tin foil. Twist a 100 mm (4") length of bare wire around
the coil, over the insulating tape. This provides electrical contact with the foil. Now wind the foil
around the circumference of the coil -- beginning at the base of the bare wire -- yet the foil should
not quite complete a full 360°. Leave a small gap as shown. Now tightly bind the whole Faraday
shield with insulating tape. Attach the coil to IC1 by means of microphone cable, with the cable's
screen(s) going to the Faraday shield at the one end, and to -6V (negative) at the other. The coil may
be mounted on a rigid plate, and a suitable handle attached. OBSERVATION: Click on the diagram to
blow it up. You could wire the coil directly to the circuit if you're in a hurry.
NOTE: You may re-publish this design, on condition that you acknowledge the designer
(Thomas Scarborough) and this blog
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