Using Arduino to enhance a VLF metal detector

Hello! First of all i have to say that I've only just started exploring the world of Arduino and was startled by the possibilities! Making an EMF detector got me thinking about Arduino's capabilities...

I have a rather basic, but highly praised VLF metal detector. Its a Tesoro Compadre if anyone is interested... It does have a discrimination circuit, but it works on exclusion principle so by turning up the discrimination pot you're blocking off iron first, then supposedly "foil" then pull tabs etc. VLF detectors use phase shifting to discriminate between metals (http://electronics.howstuffworks.com/gadgets/other-gadgets/metal-detector3.htm) and whereas more advanced models can tell you what the signal is likely to be, mine has no visual indicators on it.

Turning the discrimination up reduces sensitivity, so instead i was wondering if it would be possible to use Arduino to either analyse the amount of phase shift as the phase demodulator does to then tell me what the signal is likely to be, or to use it in conjunction with the built in discrimination circuit to do the same but identifying rather than discriminating...

So, plausible in theory?

There are several other threads about using an Arduino for metal detection, I'm not sure if you've seen them, but they may be worth reading, as it may cover the same theory etc.

Hey Roger, yup had a look on the forum before i posted and found a couple of discussions on making a metal detector but the talk there is mostly about pulse induction detectors and starting from scratch and i couldn't find anything VLF discrimination related. I have a rather basic knowledge of electronics so starting from scratch isn't really an option and i already have a basic metal detector, just thought maybe it could be as simple as jumping a few wires from the detector circuit to the Arduino and getting it to do some signal processing... But even if its not as simple as that, i would still rather try to add the identification function to my metal detector than attempt to build one...

Anyone? Could i maybe use the ADC function since the detector circuit is analog? Am i getting warmer?

Hey Squirly,

Any luck with your idea? I'm in a similar situation, just a different metal detector brand/model. I have general experience with microcontrollers and have read about the topic and think I know just enough to be dangerous.

You could use the ADC to detect the zero crossings of the search and receiver signals, but may need to raise the overall voltage of the signals so top (+) to bottom (-) swing is seen by the ADC. But with one ADC, you'd have to switch between the search signal (which I think would be pretty uniform and would require sampling less often) and the receiver signal, measure the times at which the signals crossed zero and from that determine the phase shift. Also, the voltages involved might need to be limited to 5v or 3.3v max- I'm not yet familiar with the arduino ADC.

Skimming over some MD patents, an easier way to do it might be to take in the signals from above, convert them to binary so the arduino (mc) only receives the positive side of the signals when they reach a certain lower voltage. This makes the ADC unnecessary. Then when the search signal input goes from low to high, an interrupt is called, the mc starts counting until the receive input goes low to high. If no receive low to high is received, the mc will reset the count when the search signal repeats. At this time it could use the count to determine the time period of the search signal. I would think this would be enough to solve for the phase shift of the receive signal and hence the target in the ground.

Thoughts?