New Homebrew Metal Detector

[willrich]

home brew, how do ya do, I'm fine, how are you?

hey, won't the bolt holding the coil to the lower rod, make a differance in the depth?
what if you use a plastic bolt and nut there?
I dunno, but yer pulse inductor is pretty cool.
I am new to this stuff, sorta, but been detecting along with my little non discriminateing compass's such as the 94b and 77ib
found lots of pennies in my life and battery replacement $
few gold rings and rotton live bullets(pretty dangerous to probe into)
but try yer depth with plastic componant's and tell me whatcha think
get back to me eh?

[zisme]

hey, won't the bolt holding the coil to the lower rod, make a differance in the depth?
what if you use a plastic bolt and nut there?
I dunno, but yer pulse inductor is pretty cool.
I am new to this stuff, sorta, but been detecting along with my little non discriminateing compass's such as the 94b and 77ib
found lots of pennies in my life and battery replacement $
few gold rings and rotton live bullets(pretty dangerous to probe into)
but try yer depth with plastic componant's and tell me whatcha think
get back to me eh?

I had a nylon bolt and nut orginally, but it didn't provide the support I wanted. I was always having to retighten it. When I switched to steel, I didn't notice any difference in depth. I did have to set the threshold control a little lower, but it was hardly noticable. The bolt is really off to the side of the coil's most sensitive area, and the BNC connector is almost in the same spot, so figured I would try it. My next homemade coil will be much lighter and will have no metal near it except for the wire.

Mike

[bugwhiskers]

Interesting

Hi Zisme,

Nice touch capacitively coupling the coil to the stage 1 amp, solves the offset and dual supply problem !

I too have been working on a PIMD with a few differences, eg Switch mode power supply to convert the 4 NiMH cells to +15.7(coil drive), +5 and -5(op amps). The timing pulses are generated by an ATMEL MCU. I use a MOSFET driver chip and it switches in 30nS and its inputs are cmos/ttl compatible.
Probably the biggest diversion is the coil...its a PCB coil with spiral tracks, easy to make....repeatable, light weight and ultra low capacitance allowing me to sample about 7 uS after MOSFET cutoff. This has made possible detecting a 0.1 gram nugget at 5 inches.

bugwhiskers
P.S. The Micro switches off the SMPS when sampling is taking place so it is as quiet as running on a standard linear power supply.

[omega]

@bugwhiskers

can you tell us more about your PCB coil? i wanna also make a PCB coil so can you give me more informations about the number of tracks, space between tracks and the dimension of the tracks.

Thanks, omega12

[bugwhiskers]

Coil info

I have 2 types of PCB coil, round and capsule shaped. The round is 190mm diameter, 2mm tracks with 1.5mm between them on a standard 1oz copper fibreglass PCB, 28 "turns".The DC resistance is 2.6 ohms.

The capsule coil is 285mm long by 73mm wide, 2mm tracks, 0.5 mm between tracks.

If you have access to AutoCAD you can create lines and arcs then "PEDIT" them to alter track widths and print to clear film to make a photo tool.

The round coil is most sensitive in the centre which is gr8 for pin pointing but doesn't allow much searching with each swing. The capsule coil has equal sensitivity along its length with a slight increase in sensitivity at the centre of the arc at each end.

[zisme]

Hi Zisme,

Nice touch capacitively coupling the coil to the stage 1 amp, solves the offset and dual supply problem !

I too have been working on a PIMD with a few differences, eg Switch mode power supply to convert the 4 NiMH cells to +15.7(coil drive), +5 and -5(op amps). The timing pulses are generated by an ATMEL MCU. I use a MOSFET driver chip and it switches in 30nS and its inputs are cmos/ttl compatible.
Probably the biggest diversion is the coil...its a PCB coil with spiral tracks, easy to make....repeatable, light weight and ultra low capacitance allowing me to sample about 7 uS after MOSFET cutoff. This has made possible detecting a 0.1 gram nugget at 5 inches.

bugwhiskers
P.S. The Micro switches off the SMPS when sampling is taking place so it is as quiet as running on a standard linear power supply.

Very good! You are the first person to point out the capacitive coupling trick I used to eliminate dual supplies. I thought it was a neat improvement over the designs that are currently being published. The switching power supply with synchronized sampling idea is really great. I used LiIon cells because I had several of them as a result of the work I do part-time with cellphones. I too, have used MOSFET driver IC's in other projects, but I didn't see any advantages to using one here. Maybe I will revisit that concept. As for PCB coils, I have tried making some using AutoCad, but I could never get the inductance I wanted. I would love to see what you have done in this area. I appreciate the comments and interest in the project. Please do e-mail me through my website. I would very much like to exchange ideas.

Mike

[omega]

Hi and thanks for your help bugwhiskers. i´m searching for a windows program that able to calculate pcb coils...have anybody a program?

Thanks, omega12

[Unregistered]

Help

I use ZTX415 Uc-200V , Ct-2nF for laser diode, pulse ~ 20ns, but this can be used for coil.I can't understand why everybody use analog schematic for time measurement?You can optimize method , if use counter to measure time for fadind of coil voltage.

Hi,
Could you please send me your circuit?
Thank you!
email: [email protected]