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Simple BFO metal detector
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.
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.
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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 off 39k
Transistors:
All BC 183B. Just about any small signal npn with a gain of 250+ will do. There are hundreds to choose from.
Audio output:
A 2.5 inch 8 ohm speaker will work but headphones or earpiece are preferable the higher the impedance the better.
Many of the above parts could be salvaged from a broken transistor radio.
Once the components have been obtained the circuit can be built in a few hours using copper clad stripboard, or if you the facilities make a printed circuit board using the layout below. The original layout as below should print out at about 50mm x 100mm.
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 off 39k
Transistors:
All BC 183B. Just about any small signal npn with a gain of 250+ will do. There are hundreds to choose from.
Audio output:
A 2.5 inch 8 ohm speaker will work but headphones or earpiece are preferable the higher the impedance the better.
Many of the above parts could be salvaged from a broken transistor radio.
Once the components have been obtained the circuit can be built in a few hours using copper clad stripboard, or if you the facilities make a printed circuit board using the layout below. The original layout as below should print out at about 50mm x 100mm.
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have the B-, and if possible where the two electrolictics are going to connect.
Best regards,
Paulino Alves paulino_al@iol.pt
p.s. i´m trying to build your project, but i don´t understand what i mencioned!
is this good to get started with?
please replie
thanks for this instructable. Can I use this method to make a security system like those we see in airports, those which look like doors that passengers pass to check them.
I am working on project of metal detector .
Could we find out that how far it can detect.
if we want more distance metal to detect what I have to do for it.
Thanks
this is where it was stolen from. There is also a video of it working here
http://www.youtube.com/watch?v=aSqmFWtoXYw
If you check it out on my site there is nothing to pay its free.
OK I'm finished here it is!!
http://www.coilgun.info/mark2/inductorsim.htm
and I have both inductors within .007mh difference, using 9v battery and tiny headphone. I hear nothing n i got voltage on both coils. i can't find what's wrong. I used radioshacks transistors " general" use npn. I've spent3 days and nothing.
is my home page