Need a Mini Metal Detector

Phil Pilgrim (PhiPi)

A friend of mine brought by a bag of pheasant and chukkar breasts from birds that he had shot. Before preparing anything for guests, I need to ascertain that they are all free of birdshot. 'Don't want to inflict any broken teeth! A miniature metal detector that I could sweep over each breast would fill the bill nicely. Any ideas for a simple circuit that could detect ferrous (or non-ferrous?) BBs at close range?

Thanks,
-Phil

Hal Albach

Possibly one of those circuits that uses two oscillators to generate an audible beat frequency and when one of the coils (or rod it's wound on) gets close to a metal object the tone may change. I'm just not sure how well lead interacts with magnetic fields. I have seen detectors that differentiate gold from other metals and lead seems to be somewhat like gold.
And there's always X-Ray, if you know somebody.....

RDL2004

There's a commercial solution called Little Wizard Bird Shot Detector, but the few reviews I looked at were kind of mixed. Most suggestions were to just hold the meat up to a bright light and look for dark spots, but that apparently only works for breast fillets.

Beau Schwabe

Phil,

What is the actual diameter of the buck shot you are looking for?

Lead vs Ferric metal shouldn't make a huge difference (in my personal experience)

One thing to watch out for as a general rule of thumb is that the detectable distance that the coil can detect metal approximates the coil's diameter. So if a coil has a diameter of 1 inch, the detectable depth is also about 1 inch. One way to improve that is to create a coil with a high Q in a resonant LC tank where it is "tuned" either so that the presence of metal increases the output or decreases the output. <- your choice depending on the remaining circuit you have.

Hal Albach

This was mentioned on a website regarding finding birdshot. They also mentioned a Garrett hand wand that TSA uses, but those are very pricey.

http://www.harborfreight.com/metal-detector-97245.html

garyg

HI
I've been under the impression that lead shot has not been in use for a few years now.
I'm thinking that gun people are using some type of steel shot.
If the birds were taken within the last few days, it might pay to talk to your friend about what type
of shotgun shells he was using.
I have not been hunting since the lead shot was, I thought, pretty much eliminated, at least in duck hunting in Wisconsin.

Just a thought.

Phil Pilgrim (PhiPi)

My friend assures me now that it was steel shot -- possibly copper-coated.

-Phil

Martin_H

Beau, that's neat, a schematic would be helpful to understand how the sense coils cancel each other out.

kwinn

The coils are connected in series so that the voltage induced in one coil is opposite of polarity to the other one. Think of it as a transformer with 2 separate secondary windings. Connect them in series one way and the voltages add, connect them the other way and they cancel.

Phil Pilgrim (PhiPi)

Thanks, Beau! That's very helpful. Does the BB have to be centered in the small coils to be detected, or is anywhere inside the large coil okay?

-Phil

Beau Schwabe

Phil,

Centered ... With further amplification you could "dial in" much further since the noise floor is pretty quiet, thanks to the 455kHz filters throwing virtually everything else out. This allows you to amplify the signal much further. .... Also, the small coils would still benefit from being tuned to the same resonant frequency as the large drive coil. You might even get an order of magnitude increase in your signal to noise ratio.

EDIT:
You could probably just increase the number of turns on the small coils also.... I just picked an arbitrary value of 10T. That's only about 4 inches of wire detecting a BB from 1/4+ inches away

Phil Pilgrim (PhiPi)

Beau Schwabe wrote:

Centered ...

Oh. That's going to entail a lot of scanning to capture one. Hmm, I might have to rethink my approach. Maybe I could put the breasts in the microwave and look for sparks.

-Phil

Beau Schwabe

Might try increasing the size of the smaller coils and tuning them so that they are at the same resonant frequency as the larger coil. I somehow thought you needed more of a pin point detection. The key is that the two series coils are the same geometry.

kwinn

I wonder if a circuit along the lines of that used in a grid dip meter would be better. Use a single coil and capacitor LC circuit at the resonant frequency and look for a dip in the peak voltage of the resonant circuit when a pellet absorbs energy.

doggiedoc

Phil Pilgrim (PhiPi) wrote: »

Oh. That's going to entail a lot of scanning to capture one. Hmm, I might have to rethink my approach. Maybe I could put the breasts in the microwave and look for sparks.

-Phil

Phil just send them all to me and I'll radiograph them for you.