BFO Metal Detector

[superabdo]

hello ^^

i'm working on this video ...
.

The buzzer is continuously beeping even with no metal near the coil and even without the two coils XD
what should i do ?

 

[Les Jones]

It is working as I would expect. BFO stands for beat frequency oscillator. The unit consists of 2 oscillators. One of them has its frequency controlled by the search coil and its tunning capacitor. The other looks like it is controlled by the coil near the battery. The way it works is you adjust the frequency of the adjustable oscillator to match the search coil oscillator. when the frequencies are EXACTLY the same there will be no beat between the two oscillators so there will be no audio tone. It is unlikely you will get them to exactly the same frequency so you get a low frequency tone. when the search coil comes near to some metal it's inductance changes which changes the frequency of the oscillator which means that the beat frequency (Audio tone.) changes. The frequency of the adio tone is the difference in frequency of the two oscillators. Note it is only the frequency of the audio tone that changed not the amplitude.

Les.

 

[jpanhalt]

I agree with Les, but didn't watch the whole 14-minute video. I did watch the first couple of minutes and last few minutes, and that is how they work. Please post your circuit. What is your question?
John

 

[audioguru]

It is not sensitive enough to sense coins or jewellery buried below the ground.

 

[cowboybob]

Welcome to ETO, superabdo!

Apparently, this circuit is from this site: http://www.easytreasure.co.uk/bfo.htm and Thomas Kim modified the construction a bit.

The schematic (note both oscillators are identical and both feed mixer Q3, as you'd expect):

I suggest that you go back and thoroughly check your construction of the circuit (check that each component goes where the schematic above shows where it is supposed to go) and that you followed the construction of the coils exactly as described.

And your post would suggest that this part of your circuit is working:

since you are getting some audio.

Generally speaking, errors in coil construction are the most common problem.

 

[chemelec]

Yes, It Definitely sounds like your Two coils are Not Balanced with each other.
Even though they are Different in Physical Size, they need to be the SAME Inductance

 

[JimB]

I seem to remember discussing that circuit before, in the dim and distant past.

I will see if I can find the thread.

JimB

 

[audioguru]

I suspect he is using a piezo beeper (with built-in oscillator) instead of a piezo transducer (speaker).

 

[cowboybob]

Even though they are Different in Physical Size, they need to be the SAME Inductance

Of course. Thanks chemelec. My brain was turned off in the post above.

I've edited it to reflect reality.

 

[JimB]

I seem to remember discussing that circuit before, in the dim and distant past.
I will see if I can find the thread.

Here it is:
https://www.electro-tech-online.com/threads/metal-detector-help-easytreasure.141123/

JimB

 

[superabdo]

thanks so much to all of you ^^
i'm still a student still learning
i'll try making the freq of the two coils equal , then i'll tell you what's happening

 

[MikeMl]

Actually, you want the oscillators running at different frequencies, such that the difference between their respective frequencies (the beat) is about 400Hz. The search oscillator may only be pulled a few Hz as the coil is moved near metal.

The frequency response of the audio amp and speaker will not reproduce anything below ~50Hz, so you need to make the initial beat ~ 400Hz, which you will be able to hear in the speaker. Turns out that human hearing is very sensitive to pitch change of a tone near A440 on the musical scale.

If you set the oscillators to almost the same freq, they might injection lock to each other's frequency due to inadvertent coupling (magnetic or through the power supply), in which case you will hear nothing...

 

[chemelec]

400Hz?

There are Better Ways, If you use Buffered Oscillators before beating the two.
You can go down to a Audible, Tick, Tick, Tick.......

 

[MikeMl]

400Hz?

There are Better Ways, If you use Buffered Oscillators before beating the two.
You can go down to a Audible, Tick, Tick, Tick.......

But not in that simplistic circuit...