metal detection questions for robots- hardware sources, interfacing etc.

[R.O.V.E.R]

I am building an all terrain robot based on a 12v dc system and micro atx pc board that is 10" x 12" using RC moster truck tires.  It will be controlled via wireless networking and will send landscape images back to the controller PC.  The posts here about metal detection have sparked my interest.  What if my Rover bot was sitting on a rolex just below the soil and I never knew?   

Can anyone recommend a small lightweight unit for around 100 dollars or preferably less that could be hacked?  I saw some toys linked- I would like to use something capable of determining trash from precious metal, but I also am aware a top end detector is several hundred dollars and looks sort of hefty for my application.  Does anyone know of a detector that already interfaces with a PC by usb or something?  (one less thing to hack/code, hehe)

What about other sources- I hate scavenging new products for 10 bucks worth of circuitry and coil and junking the rest. 
Perhaps I could get a replacement parts sheet for a well known detector and just order what I need?

I have seen little hobby kits that you construct yourself in the past but again those seem to fall under the toy catagory.
any input is greatly appreciated, this is something that I can develop and add to my bot over time and would certainly be useful and give another fun aspect ot the project.

if you see a guy in the park chasing a computer on wheels with a laptop and a shovel its me!

[dunk]

what about those DIY gadgets you use to look for pipes and cables before you drill through the wall?
no USB interface but sounds like they would have the sensor you are looking for.

dunk.

[Militoy]

I would like to use something capable of determining trash from precious metal

Discrimination of trash from treasure is the toughest challenge in metal detecting.  Most published metal detector schematics available are of the old BFO (beat frequency oscillator) type, and though they are fine for determining the presence of metal objects, they are useless for separating iron and beer tabs from coins or gold rings. Generally speaking, the higher a detector’s price, and the harder it is to use, the better it will be at separating trash from treasure. The better discriminator models all use a microprocessor in their transmit, receive and discriminate circuits, and it will likely be easier to hack an existing circuit, than to build and program your own. Even the highest-price detectors (>$3K) have a difficult time telling the difference between an aluminum beer tab and a gold ring, since they work by measuring eddy currents induced in the resistance of the metallic object – and aluminum and 14K gold are quite close to each other in that respect. That’s why many experienced detectorists just dig up ALL targets.

For a low-price discriminating detector to hack, I would look at the Chinese copies of the Bounty Hunter detectors that Harbor Freight Sales offers.

[Cognaut]

I recently read up on metal detectors and found that the technology is just starting to progress and specialize.  Some are using almost 100 frequencies - even underwater.  One actually generates a video display.

However, none are cheap, except those old BFO's.  I too took the time to study schematics.  I'm going to wait a year or so and check again.  I'd like to dig up some meteorites.  There's a decent market for those.

[R.O.V.E.R]

thanks for letting me pick at your brains guys, I appreciate it.

I may be wrong but I think a stud finder and a metal detector would utilize different sensors, as a stud finder will pick up a wooden 2x4 where a metal detector would not.

I looked at harbor freight's website and they carry a few detectors, the better of which are priced 39 dollars and 59 dollars.  I was thinking about going all out and grabbing a 59.00 unit but it calls for 3 9v batteries and the 39.00 unit uses a few AAA- I'm guessing the 59 dollar special has a greater detection depth.  Either way I go those will be suitable sources of hardware within my budget, I just have to decide which will be easier to adapt.

As far as a quick interface so far have thought of just connecting the detector circuit's  audio output to my robots mic in and listening for  a signal, or writing a program that listens for me and throws up a graphic and notes GPS data when something is found- that may be easier said than done:)

[Militoy]

I'd like to dig up some meteorites.  There's a decent market for those.

Meteorites are a little bit different to detect than relics, coins, gold, etc. Most that you will find are of several "stony iron" types, and a detector will respond to them much like the "hot rocks" that cause gold detectorists such grief. Kind of a "null - boing!" response. I use my detector for meteorites - but I've found most of them by just spotting them on the surface.

I may be wrong but I think a stud finder and a metal detector would utilize different sensors, as a stud finder will pick up a wooden 2x4 where a metal detector would not.

I believe most of them work by using ultrasonics - crudely measuring the density of the wall, as opposed to detecting metal.

[Cognaut]

They've got new detectors that are specialized for meteorites.

This area of technology is going through some changes right now.  I did some reading on it recently and decided to wait a year before making a purchase.