A Very Simple "Beat Frequency Oscillator" type of Metal Detector. These are about the Simplest of all Metal Detector types, But still quite useful for many Detecting applications. And although this one is particularly simple, I find it works very well.
For use outside on the ground the coil "MUST have a Faraday Shield", or it will not be of any use because of Ground Effects. Note: I may Elaborate more on the coil Construction and a Farady Shield. But for now, Only the Schematic, PCB and A Basic Coil are Presented here.
L1 is a Standard RF Choke. My choke was made by "Delivan", and I can supply them if needed. But any simular choke should be OK.
Depending on the Accuracy of the Inductance of L1, L2 and other parts, you may need to add or remove a turn or two from L2, to get both oscillators operating at the same frequency. Based on an Inductance of 220uH and the parallel capacitance of .005uF, (Two.01's in series) the operating Frequency is about 151 Khz.
As an Alternative to adding or Removing Turns, You can wind the coil with Various Taps, starting at 25 turns, than 26 turns, 27 turns and 28 turns. Than Pick the tap that works Best. OR CHANGE THE SHAPE OF THE COIL, SEE BELOW!
In Most BFO Detectors, L1 is either an adjustable choke with slug tuning. Or in parallel with a Variable AM Radio, Tuning Capacitor of 365pf. This eliminates the 10K pot, but variable chokes are more difficult to obtain and 365uF tuning caps typically much bigger than this 10K Pot.
I find this 470pf and 10K Pot Arrangement, VERY Simple and Quite Satisfactory. I just used a Trimpot on my Prototype, but you will want to use a Standard size pot for Ease of Adjustment.
The Frequency of the Fixed Oscillator is Aproximately 150 Khz.
My Origonal Proto, was built on a "Bread Board", and it Detected a "Canadian Quarter at about 5 inches in Air".
Because of Reader Requests, I have now created a PCB for this detector. And If you can't make this PCB, It is Available from me.
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Complete Working Unit.
My BFO Detector Schematic.
BFO Detector PCB.
PCB, Parts Overlay.
MORE, ABOUT MAKING COILS. See HERE, How the Inductance of a Coil changes, when you change the shape of it. And reguardless of the Shape of the coil, it will work in this circuit, IF you have the Correct Inductance.
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