Here is a simple design for a DIY metal detector that uses a beat frequency oscillator.
“BFO ( beat frequency oscillator ) metal detectors use two oscillators, each of which produces a radio frequency. One of these oscillators uses a coil of wire that we call the search loop. The second oscillator uses a much smaller coil of wire, and is usually inside the control box and is called the reference oscillator. By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the search loop is moved over or near to a piece of metal. It has been found in practice best to make the search oscillator fixed say at 100khz and to arrange for the reference oscillator to be adjustable 100khz plus or minus 250hz. This gives a beat note of 250hz to 0 to 250hz. The beat note disappears or nulls when the two oscillators are about equal. This type of detector is most sensitive when the beat note is close to zero, about 5hz ( motor boating ) any slight change being noticeable.
Parts list
Power source:
Any 9v battery PP3 is ideal.
Capacitors:
2 off 220uF 16v electrolytic.
5 off .01uF polyester.
5 off .1uF polyester.
Resistors:
All resistors 1/4 watt 5%
6 off 10k
1 off 1K
1 off 2.2m ===== 2.2 Mega ohm
2 off 39k”
March 31st, 2007
Very neat. I spent some time looking at the “Treasure Act” webpage on that site. Good grief.
March 31st, 2007
It only detects objects about 120 mm down… Not as impressive, but still interesting. What would the detection depth on a retailed metal detector be?
April 3rd, 2007
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April 3rd, 2007
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April 10th, 2007
Its still pretty kewl in that its something anyone could do for the most part. I would love to use this as a fun weekend project for my kids.
September 22nd, 2007
hi, i made this bfo metal detector but the detector doesnt work, pls help me how i can be getting right and working it!!!
give me defenition of this metal detector, i think i made it following ur advice what pcb layout, electron element and making coil are, my mail address www.b_bayar33@yahoo.com. sent me more information how i work it
September 22nd, 2007
Hi bayara,
Looks like the site has moved. I updated the link to point to the new site location.
http://www.easytreasure.co.uk/bfo.htm
I would suggest that you contact the creator of the device for assistance. Let us know how well it works when you get it going.