BFO metal detector

Thread Starter

kriti gupta

Joined Dec 8, 2014
4
Hello, I need help with this circuit.
I understand the overall working of the detector but I do not understand the working of the circuit components and their roles.
Thank you for the help!
 

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Thread Starter

kriti gupta

Joined Dec 8, 2014
4
The buzzer is continuously beeping even with no metal near the coil. Adjusting the variable resistor didn't help.
With the metal near the coil, there is no variation in the tone of the buzzer.
 

Kermit2

Joined Feb 5, 2010
4,162
you should add dots to all points at which you have made connections on the schematic then re-post it. the drawing you posted is confusing about where some components are connected and where the lines cross but do not connect. i suspect this is where you have a problem.
 

alfacliff

Joined Dec 13, 2013
2,458
it appears to be an old style bfo detector, it uses an am radio station as a reference, tuning the pot to "zero beat" the carrier of the oscilator with the radio station. when metal enters the field of influence of the coil, the oscilator is detuned and the mixed signal sets off the buzzer.
 

Thread Starter

kriti gupta

Joined Dec 8, 2014
4
I found the basic mechanism on the internet, I do not understand the contributions of the individual circuit elements.
Also, There were 58k resistors available instead of 56k, could it be because of this that the circuit is failing?
 
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