SURF PI PROBLEM

[daverave]

i was out detecting on the beach with my surf pi and a detector friend of mine tried my detector and he said the magnetic field of the earth cancellation was not working correctly and also when i swing my detector i do get a false signal at the end of each sweep..can anyone tell me how to put this right.

[kt315]

no way. we must work at distance.

br

[Qiaozhi]

i was out detecting on the beach with my surf pi and a detector friend of mine tried my detector and he said the magnetic field of the earth cancellation was not working correctly and also when i swing my detector i do get a false signal at the end of each sweep..can anyone tell me how to put this right.

If your detector is working correctly, you should be able to wave a whiteboard magnet a few inches away from the coil without detecting it.
Look at the schematic, and you'll see there are two sample gates (CD4066). The lower one (U7A) is the main sample, and the upper one (U7B) is the Earth Field sample. The idea here is that both samples will contain the Earth Field signal, but only the earlier sample will contain any significant target information. Hence, subtracting the late sample from the early sample will leave only the signal information. In other words, the Earth Field gets rejected. If you cannot reject the magnet, then you may have an error in your PCB. There is no adjustment for the Earth Field setting, and no need to include one.

[daverave]

what is a whiteboard magnet ??? and where do i buy one ????

[paku]

[Qiaozhi]

what is a whiteboard magnet ??? and where do i buy one ????

http://www.amazon.co.uk/Rybond-Offic.../dp/B00BCZGOMA

[Qiaozhi]

I think this needs some further explanation as to what we're looking at here.

Is this a modified Surf-PI? Can it only tell the difference between ferrous and non-ferrous when the target is actually touching the coil. That's what seems to be happening in the video.
Does this only work on the bench in an air test, or does it work in the field?
Can it discriminate ferrous targets of different sizes and shapes?

A more convincing demonstration would be a test with targets buried in real soil.

[greylourie]

DS Nano or clone interfaced.

[daverave]

ok thanks for the info about magnets.

[daverave]

how far away from the coil do i waive these whiteboard magnets ?????

[Qiaozhi]

how far away from the coil do i waive these whiteboard magnets ?????

I don't know with the Surf-PI, but I would guess somewhere between 3 to 6 inches.
The whiteboard magnet has a much stronger magnetic field than the Earth, so it will more than likely make the detector beep when it's close to the coil.
Using a PI with bipolar switching, I've managed to eliminate the whiteboard magnet when it's less than 1 inch from the coil.

[greylourie]

Does moving your Delay away from minimum change this behaviour at all ?

[daverave]

i received the whiteboard magnets today and guess what ??? i can go right up to the coil with the magnet and no signal !!! so looks like nothing wrong with detector....i tried my other surf pi with dual field coil and i get a signal from the magnet at about 1" from coil.

[Qiaozhi]

i received the whiteboard magnets today and guess what ??? i can go right up to the coil with the magnet and no signal !!! so looks like nothing wrong with detector....i tried my other surf pi with dual field coil and i get a signal from the magnet at about 1" from coil.

Just want to check that you're testing this correctly ... you need to keep the magnet moving and not hold it stationary, as the circuit can only detect a moving magnet.

[daverave]

i waived the magnet in front of the coil like i would a coin and no response from the magnet..but dual field detector gave a signal about 1" away from coil.