Colpitts oscillator metal detector
I've built a metal detector using a colpitts oscillator. I used two 1uF capacitors in series. I've been measuring the voltage output from the center tap of the two capacitors. When I bring metal close to the coil, the amplitude of the oscillations goes down to zero.
Can anyone please explain why this happens? Someone told me that it has the same effect as placing a resistor in parallel to the inductor, and in simulation this works very well..but i still don't understand the reason for this effect!
I've attached three pictures to help explain what i mean.The circuit without the parallel resistor shows the circuit and waveform before any metal is brought close to the coil. The other two show the effect on the waveform as metal is brought slowly closer to the coil.Please can someone explain how bringing metal close to the coil gives the same effect as adding a resistor in parallel.. I need to explain it in a project report today!
Dear Shmosep: the effect you see is due to eddy current induced in a conductive object close to your coil. Inducing the eddy current effectively adds a loss to your resonant circuit; if the object loads it too much, the oscillation stops.
In the application like metal detector, you can detect metal object close to your coil, evaluate their distance and conductivity, etc.
To avoid stopping oscillation, you can add a buffer amplifier with another pickup coil and an amplitude detector to evaluate the damping effect due to eddy current induced in a tested object.
That's one strange Colpitts schematic, not even sure it is one in fact.
The explanation above is correct, but IMHO your oscillator is running at WAY too low a frequency anyhow. To effectively detect anything other than ferrous material aim for around 10KHz.
Dear Sir
I wish to interface metal detector with PIC16F877A
Which detector suits best and how 2 connect with it sir?
Dear Sir:
I am not familiar with PICF microprocessors. I do not see why to link your Colpitts oscillator with it.
As far as I know, metal detectors use two beating oscillators and an acoustical indicator (human hearing) for the best sensitivity.
Visual indication (with a mA meter) is added to possibly distinguish different objects sensed by one of the two oscillators.
A training of an user is the best way to improve detection capability. The metal-detection system combines a human user hearing sense with the electromagnetic sensor for the best results. No other devices could improve it.
Best to go HERE > Geotech Forums and have a look around the schematics section.