Created: "Sept 11, 2016" Latest Update: "Sept 12, 2016"
Another, BETTER and Very Simple "Beat Frequency Oscillator" type of Metal Detector.
BFO Detectors 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 Pin Pointing Coil is Presented here.
This circuit is Quite Stable, except the Frequency will change with Battery Voltage goes lower over using it. Measured Current Draw at 9.5 Volts is 10.4 mA. Measured Current Draw at 12.6 Volts is 15.4 mA.
Construction:  The PCB Circuit should be put in a Metal Box, Or a Plastic box with a Metal Shield liner. (Possibly usng Aluminum Foil.)
Based on an Inductance of about 220uH the operating Frequency is about 151 Khz. Note:  However this circuit can Easily tune to an Inductance of between 100uH or also up to 2000uH (2mH).   So the Coil is NOT very critical.
In this BFO Detector I created a CD4069 Oscillator with a Buffered Output for the Reference Frequency. The Frequency is Tunable with both a Coarse Tune Trimpot and a Fine Tune Potentometer.
Setting it up First Set the Fine Tune potentiomer to a Middle Position. Than adjust the Coarse Trimpot to get the LOUDEST Zero Beat Frequency. Now the Fine Tune Potentiometer will allow adjustments Either Side of this Zero Beat.
So it can be set so Non-Magnetic metals cause an Increase in the Tone or a Decrease in the tone. Or the same for Magnetic Metals.
A PCB, and/or all the Parts are Available from me.
"Back to my Projects Page"
My BFO Detector Schematic.
BFO Detector PCB.
PCB, Parts Overlay.
Actual Hookup Picture.
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