Hey Guys wondering If i can get some help with a metal detector bfo circuit?

Thread Starter


Joined Sep 6, 2017
I have a bfo metal detector and have a couple of queries regarding my schematic, I am still a novice only 2nd year and need some help I have gone through site after site before signing up to this !
If you are active and can help please reply.



Joined Apr 5, 2008

Wich schematic do you use?

Here is a list of metal detector articles from the chemelec site:

A BFO Metal Detector"
Not the best, but Simple, and Now with a PCB.

A BETTER BFO Metal Detector"
Much Better and Simple, Parts and PCB Available.

Metal Detector Coils, for my PI Detectors

Small Detector Coils, with Ferrite Cores for my PI Detectors

Testing Metal Detector Coils,
for Free Air Resonance and Interwire Capacitance.

A Metal Detector, for Pin Pointing.
"Possibly More info to come later".

My "SUPER SENSITIVE", Pin Pointing Metal Detector.

"Metal-AV, A Pulse Induction Detector."
A PI Detector with "Audiable & Visual" Indications of Targets. Suitable for use on some Conveyor Belts.

Metal Detector, A Pulse Induction Detector. "Pulse-1"
Hundreds of this Detector have been built with Very favourable Feedback.
This is "the Most Most Popular Project on my site".

Pictures of My Pulse-1 & 2 Metal Detector Project.
These Detectors have been "Built by Various Persons",
And I included their Comments on it's performance.

Metal Detector, My "Pulse-2" Induction Type Detector
This is a Revised Version of the Pulse-1 above with Additional
Controls for Possibly More Flexibility.
This Detector is "Definately Not for the Beginner".

My Metal Detector Coil Jig
This is Not a Project, But this is how I make the Flat Radial Coils.
I have Shown this here because of Numerous Requests.

"Metal Detector", based on the CS209, Now Discontinued.
Especially Suited to find studs in Walls.
Note: this IC is discontinued and getting hard to obtain.

"Metal Detector", based on the TDA0161.
Preliminary Circuit Article...More Info to Come.


Thread Starter


Joined Sep 6, 2017
Hey guys thank you for your replies this is my schematic, I am aware of what the individual components are doing ( low pass filters, low voltage audio power amplifier etc) but I am just wondering what the JFET and Differential amplifier are doing, I get that a small current from the base terminal of q3 allows a larger current to flow from collector to emitter but other then that im a little lost. Also I am trying to sketch this schematic into Tina but the lm386 is not available on this software does anyone know an alternative so I can see what waves I am getting at my speaker output? thank you .



Joined Jun 22, 2012
Q3,Q4 are being used as a Mixer to produce a Hetrodyne signal, which is amplified by the fet Q5, and final amp IC1 to the speaker.