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Hi - been lurking here for a while and finally came up with something worth posting. I made a Stuart PI detector which I modified quite a bit and it works well. I have a Hammerhead board but still have to build it.
I was experimenting with different ideas for a pinpointer and concluded a specialized coil for the PI detector is simpler than building a whole new detector and probably works better too. Tried a few experiments with ferrite bars and they worked but didn't seem to localize the signal very well. A 50uh 1" circular coil worked quite well - reasonably directional and would detect a dime at about 2".
Tried some small chokes and they worked as well. It occurred to me if I used a very small choke core I could put it in the end of a probe and this would really help pinpoint stuff. I had some small 10uh chokes that had ferrite cores about 1/2" long and 3/16" in dia. These are the type that have the leads glued in each end of the core and are wrapped with wire. I cut one of the leads off and removed the windings. I rewound with 30 gauge enameled copper wire, soldering one end to the remaining lead and winding down the length of the core and back - approx 50 turns. This coil measured 45uh. I soldered about 3 ft of shielded cable to the coil using a bit of heatshrink tubing to prevent the wires from shorting.
For the probe, I cut a piece of 1/4" hollow fiberglass tube to about 10" in length. Fiberglass tubing is commonly used in kitemaking - I suppose a section of an old fishing rod would work too. The inside dia of the tube is large enough that the choke coil and wire can slide down the middle.
I needed a nice hard probe tip since the fiberglass tube is strong but not very abrasion resistant. A hard tip helps to identify metal objects when you are probing as well. The solution was to use another piece of ferrite from a larger coil that was the same diameter as the fiberglass tube - 1/4". Ferrite also has the advantage of shaping the magnetic field and it will not mess up the detector as a metal tip would. I figured a bullet shape would be ideal - relatively easy to push thru the ground and with a thinned section that would slide into the fiberglass tube so it could be glued very solidly. I discovered that ferrite is extremely hard - none of my metal tools would touch it. What worked really well to shape the ferrite was a simple improvised lathe. I gently chucked the ferrite in an electric drill held in a vice and while it was spinning I shaped it with a Dremel tool and a cutoff wheel. The cutoff wheel will cut away the ferrite reasonably quickly and spinning the ferrite rod keeps the shape round. I strongly recommend eye protection and a respirator for this job - I don't know if ferrite dust is harmful to your lungs but I don't want to find out !
I assembled the prototype by gluing the ferrite bullet into the end of the fiberglass tube with superglue. I then slid the cable and coil through the other end of the tube so the coil was against the ferrite. I secured the cable with a bit of hot glue but I didn't bother to glue the coil in place since its a fairly snug fit in the tube anyway. I improvised a temporary handle with a wrap of duct tape.
I use RCA plugs and jacks on my PI unit so the pinpointer just plugs in instead of the regular coil. I search with a 7 inch spiral coil and when I find a target I plug in the pinpointer. The threshold must be readjusted for the pinpointer. I start by just touching the probe tip to the ground to see if I get a response. If the metal is deep you can poke it into the ground. With my PI unit adjusted for max sensitivity the pinpointer will detect a dime at around 2" from the tip. You can adjust the threshold for more accurate pinpointing if desired. With the threshold cranked way up my PI unit will sound off only when the tip is virtually touching the metal.
How effective is it ? VERY ! I don't like to dig big holes so I use a screwdriver for extraction. With the pinpointer I estimate the time to locate and extract the target went from a couple of minutes to about 30 seconds. Before I often couldn't find the target if it was small. With the pinpointer my extraction rate is now virtually 100%. Coins often pop out on the first pry with the screwdriver. You can dig a very small hole and then probe around inside to exactly locate your find. This thing works !
Rich
1st pic - right - original choke coil. left - rewound and soldered to cable
2nd pic - ferrite bullet probe tip (sorry about the fuzzy pic)
3rd pic - assembed probe with hokey duct tape handle ; )
4rth pic - asst stuff I found in the local schoolyard today using the pinpointer in about 2 1/2 hrs. This is about 10x my previous results !
I was experimenting with different ideas for a pinpointer and concluded a specialized coil for the PI detector is simpler than building a whole new detector and probably works better too. Tried a few experiments with ferrite bars and they worked but didn't seem to localize the signal very well. A 50uh 1" circular coil worked quite well - reasonably directional and would detect a dime at about 2".
Tried some small chokes and they worked as well. It occurred to me if I used a very small choke core I could put it in the end of a probe and this would really help pinpoint stuff. I had some small 10uh chokes that had ferrite cores about 1/2" long and 3/16" in dia. These are the type that have the leads glued in each end of the core and are wrapped with wire. I cut one of the leads off and removed the windings. I rewound with 30 gauge enameled copper wire, soldering one end to the remaining lead and winding down the length of the core and back - approx 50 turns. This coil measured 45uh. I soldered about 3 ft of shielded cable to the coil using a bit of heatshrink tubing to prevent the wires from shorting.
For the probe, I cut a piece of 1/4" hollow fiberglass tube to about 10" in length. Fiberglass tubing is commonly used in kitemaking - I suppose a section of an old fishing rod would work too. The inside dia of the tube is large enough that the choke coil and wire can slide down the middle.
I needed a nice hard probe tip since the fiberglass tube is strong but not very abrasion resistant. A hard tip helps to identify metal objects when you are probing as well. The solution was to use another piece of ferrite from a larger coil that was the same diameter as the fiberglass tube - 1/4". Ferrite also has the advantage of shaping the magnetic field and it will not mess up the detector as a metal tip would. I figured a bullet shape would be ideal - relatively easy to push thru the ground and with a thinned section that would slide into the fiberglass tube so it could be glued very solidly. I discovered that ferrite is extremely hard - none of my metal tools would touch it. What worked really well to shape the ferrite was a simple improvised lathe. I gently chucked the ferrite in an electric drill held in a vice and while it was spinning I shaped it with a Dremel tool and a cutoff wheel. The cutoff wheel will cut away the ferrite reasonably quickly and spinning the ferrite rod keeps the shape round. I strongly recommend eye protection and a respirator for this job - I don't know if ferrite dust is harmful to your lungs but I don't want to find out !
I assembled the prototype by gluing the ferrite bullet into the end of the fiberglass tube with superglue. I then slid the cable and coil through the other end of the tube so the coil was against the ferrite. I secured the cable with a bit of hot glue but I didn't bother to glue the coil in place since its a fairly snug fit in the tube anyway. I improvised a temporary handle with a wrap of duct tape.
I use RCA plugs and jacks on my PI unit so the pinpointer just plugs in instead of the regular coil. I search with a 7 inch spiral coil and when I find a target I plug in the pinpointer. The threshold must be readjusted for the pinpointer. I start by just touching the probe tip to the ground to see if I get a response. If the metal is deep you can poke it into the ground. With my PI unit adjusted for max sensitivity the pinpointer will detect a dime at around 2" from the tip. You can adjust the threshold for more accurate pinpointing if desired. With the threshold cranked way up my PI unit will sound off only when the tip is virtually touching the metal.
How effective is it ? VERY ! I don't like to dig big holes so I use a screwdriver for extraction. With the pinpointer I estimate the time to locate and extract the target went from a couple of minutes to about 30 seconds. Before I often couldn't find the target if it was small. With the pinpointer my extraction rate is now virtually 100%. Coins often pop out on the first pry with the screwdriver. You can dig a very small hole and then probe around inside to exactly locate your find. This thing works !
Rich
1st pic - right - original choke coil. left - rewound and soldered to cable
2nd pic - ferrite bullet probe tip (sorry about the fuzzy pic)
3rd pic - assembed probe with hokey duct tape handle ; )
4rth pic - asst stuff I found in the local schoolyard today using the pinpointer in about 2 1/2 hrs. This is about 10x my previous results !
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