My First BFO Detector (easy treasure)

[Almost EE Student]

Schematic





Now on perfboard



What I understand,

Dual oscillators. Get the freq of both oscillators close until you get audible noise from the speaker. This is your beat note. When you pass the search coil over metallic objects, eddy currents will be induced, thus producing a magnetic field. This magnetic field interfere's w/ the search coil's magnetic field & you hear this as a change in freq.

It's pretty simple. Two coils. Your beat note is your middle man. Just listen to what he's saying.

What I don't understand,

So the search coil acts as the transmitter & sends out a signal via magnetic field & the reference coil acts as the receiver? Well, where does this middle man (beat note) fall into place?

^I know this is incorrect, but in order to get a beat note, you need TWO signals. So each coil has it's own receiver, therefore giving you two frequencies. Then these TWO frequencies produce a third frequency, which of course is your beat note. I'm just not seeing how one coil or one receiver would get you a beat note. I have "TWO" stuck in my head. Two piano key notes, two guitar string notes, etc...

Things I've noticed,

Detector is only good for 5." You pretty much have to have the search head on top of the object, so it's more of a conversation piece than anything.

Get the search coil too close to the ground & the beat note will cut out. I'm not sure if this has to do w/ the high iron content in the ground. Will shielding the search coil help w/ this? Meh, wouldn't that actually prevent EM waves from getting out

Don't waste your time w/ a speaker driver. It's too quiet, even at 16 ohm. Either wire in a jack or splice in a set of crystal ear phones.

Everything HAS to be FIXED. You move one wire a quarter inch & your beat note either drops below 20hz or above 20khz & that's not doing the human ear any good.

The Engineer who designed this circuit made it clear to get the search coil to oscillate at 104hz. I'm only assuming this is because most ear phones or small speaker drivers will cut out anything below 100hz. This is something I was thinking about. -You want to tune this device so the beat note is at it's LOWEST audible frequency. When you pass the search coil over a metallic object, the beat note frequency increases. Now assume you want to increase the means of detection. Well, why not just have the search coil oscillate at a much lower frequency; let's say 40hz ...which is audible. I couldn't tell you if that extra 60hz (100-40=60) is going to help out much, especially comparing it to the audible range (20hz to 20khz), lol. It would be an increase in your detection range though ...even if you have to carry around a 6' transmission line enclosure for that lil' 2" speaker to drop down to 40hz

I tried increasing the number of turns on the search coil. To be completely honest, I couldn't tell the difference & at 30+ turns. I had a hard time trying to find find the frequency band (audible noise from the speaker). I was winding & unwinding the reference coil + or - 75 turns



I really posted this thread to get some help on another detector. I have not taken any electronics classes (yet), but I think I can handle something a little more advanced. I'd really like to get down to 6" ...a foot would be nice.

[Qiaozhi]

Have you seen this book?
Inside the METAL DETECTOR - Published September 2012

[ODM]

"Inside the metal detector" is a good book, telling more about why metal detectors work the way they do, and some guidelines for starting out your own projects. The book is also useful to go with a metal detector "kit" project, the information helps in debugging when (not if) something goes not as expected It's also worth the while to pick up a copy of some basic electronics book, to at least familiarize yourself with the concepts of capacitance and inductance.

An unshielded LC oscillator will be sensitive to external capacitances such as yourself and the ground. Easy improvements would be adding a faraday shield to your coil and cable. Shielded, or coaxial cable is usually used for the coil cable in order to guard it against external capacitance. These are instructions for an IB detector coil, which would have superior performance compared to a BFO, but the process for shielding the coil is the same. The importance of making a rigid coil is also worth a note. http://silverdog.co.uk/shop/image/da...CoilMaking.pdf

Another cure would be adding a comparator to the output, to make those sub-audible frequencies into distinct "clicks". You will hear a two hertz shift in the search oscillator far more easily when it's between 2..4Hz and not 102..104Hz. If you want to build it by discrete transistors only, you could make something such as this; http://en.wikipedia.org/wiki/Schmitt_trigger - Its output could be coupled to the crystal headphone by a capacitor.

For the next project you might consider the PICKINI, or some of the designs in "inside the metal detector". But with a little improvements, at least faraday shielding, you'll have an operable detector in your hands. The beach combers of the sixties didn't usually have much more than a BFO, still turning up a decent amount of finds!

[Carl-NC]

What I don't understand,

So the search coil acts as the transmitter & sends out a signal via magnetic field & the reference coil acts as the receiver? Well, where does this middle man (beat note) fall into place?

No, technically there is no receiver in this design, just 2 transmitters. The reference TX is fixed, should never vary in frequency. The search TX varies with nearby metal, ferrous makes it decrease, non-ferrous makes it increase. The beat frequency is effectively the difference, so if the ref TX is 100kHz (100,000 Hz) and the search TX is 100.1kHz (100,100 Hz) then the difference is 100 Hz, which you can hear.

Get the search coil too close to the ground & the beat note will cut out. I'm not sure if this has to do w/ the high iron content in the ground. Will shielding the search coil help w/ this? Meh, wouldn't that actually prevent EM waves from getting out

Probably mineralization. Shielding will help in wet grass, done right it doesn't affect the magnetic field.

The Engineer who designed this circuit made it clear to get the search coil to oscillate at 104hz. I'm only assuming this is because most ear phones or small speaker drivers will cut out anything below 100hz. This is something I was thinking about. -You want to tune this device so the beat note is at it's LOWEST audible frequency.

ODM is right, make the audio a hard square wave and you will be able to hear the "clicks" down to 1Hz. BFO users call it "motorboating." I like to run around 5-10 Hz, easier to hear subtle changes.

- Carl