metal detector using colpitts oscillator

Thread Starter

lephuduc

Joined Oct 11, 2012
5
I'm studying about metal detector using colpitts oscillator, can anyone help me to explain how the attached circuit work? can it detect metal easily? and the function of capacitors?
 

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wayneh

Joined Sep 9, 2010
17,163
...can anyone help me to explain how the attached circuit work? can it detect metal easily?
How do you know it works at all?

It may detect, but I see no way for it to indicate anything. No sound or light output.

Metal near the coil changes the impedance of the coil, thereby changing behavior of the circuit. Probably changes frequency of oscillation.
 

Thread Starter

lephuduc

Joined Oct 11, 2012
5
Thanks for help. actually I just want to design a simple circuit, the circuit I attached is just my beginning. If you understand how it work, please explain it?
 

radiohead

Joined May 28, 2009
508
Basically, the circuit will generate a signal through the coil. The signal will penetrate the ground. The coil will have to be tuned to balance with the ground so no sound will be heard in the earpiece (for a beat frequency oscillator type). When the coil passes over a ferrous metal, the electrical balance of the coil will change and the frequency difference will be heard as a tone in the earpiece (or deflection in a meter). There are hundreds of simple metal detector schematics on the web. Some are better than others, but most simple ones won't detect more than a few inches. Below are some links for history and theory of operation for VLF, pulse induction and BFO types.
http://electronics.howstuffworks.com/gadgets/other-gadgets/metal-detector.htm
http://inventors.about.com/od/pstartinventions/a/Metal_Detector.htm
http://www.hobby-hour.com/electronics/metal_detectors.php
http://www.discovercircuits.com/S/s-metal.htm
 

vk6zgo

Joined Jul 21, 2012
677
Yes,on looking at it,& Googling,I see the resemblance.

The strange thing is,that most of the Colpitts circuits I have seen over many years,have been common anode (yes I date back that far) or common collector,with the cathode or emitter returned to the capacitive tap.

These are fairly rare on the 'Net,& I don't instantly recognise the other,in real life,(in my experience) less common forms.
 
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