Anyone have any info on The "Minipulse"an early Eric Foster PI.

[adam(Va)]

Was told there was some Mods to make it more sensitive to small gold. Any info appreciated.
Thanks!

[yourvibes]

hi adam. . .

all the info i got is on the mini pulse thread. .

[mikebg]

Was told there was some Mods to make it more sensitive to small gold. Any info appreciated.
Thanks!

Adam, to detect small gold, the machine should use TWO COIL loop configuration (separate TX and RX windings) placed in induction balance. Here is the circuit diagram of Eric Foster:
http://www.geotech1.com/forums/showp...8&postcount=49

[adam(Va)]

Thanks for the help Mike, I am going to assume the circuit you posted in the link is a an add on ciruit to the "minipulse" ?

[mikebg]

Was told there was some Mods to make it more sensitive to small gold. Any info appreciated.
Thanks!

Adam,
For small gold detecting, you should use DOD loop configuration:
http://australianelectronicgoldprosp...2346/#msg12346
In that drawing:
L1 is TX winding.
L2A and L2B are RX windings connected in opposite direction to suppress GND signal.
M is a small ferrite core or iron rod excentric mounted inside a knob. It is rotated by operator to adjust induction balance.

[adam(Va)]

Mike, I probably should have been more detailed in my original question. A friend of mine had heard that there was a simple mod to lower the pulse delay setting on his "Minipulse" so it would be more sensitive to find smaller gold jewelry on the beach (mostly chains and bracelets, not so much as grains or small nuggets). His detector has since quit working and I am in the process of now troubleshooting it (not very fun without schematics), but thanks again for taking the time to reply to my question.

[yourvibes]

part numbers

hi adam. . . got pm but no photo of circled parts. . . send me photo with part numbers wanted. . . yourvibes

[adam(Va)]

Resent, stupid me forgot to attach the photo.

[adam(Va)]

Here is the photo

[porkluvr]

I will stick my neck out here and make an assumption that your Mini Pulse timer approximately follows the topology of one half of a Goldscan IV.

If that assumption is correct, and IF you get the unit working you could shorten the primary delay by reducing the resistance in the RC network on the first stage of the one-shot chain.

It looks like you have a TLC555 master timer sending a clock signal to one of two MC14538 one-shots. Each MC145138 has two identical channels so you would need to trace the clock to the first one-shot in the signal chain. In my diagram it is an "A" side, represented by U9.

Notice the 10k resistor R10. That could be replaced with a 5k1 fixed resistor and a 5k trimpot and that would make it possible to reduce the minimum delay. Your "R10" could either be connected to pin 2 or pin 14, depending on which half of your IC is relevant. Note that, if you reduce the minimum delay, you will also reduce the maximum delay on the CW end of your REJECT control (obviously) but you could possibly add a small amount of capacitance to get that back.

Beware that you cannot get any benefit from shortening the delay beyond a certain point if your coil is not fast enough so that the flyback pulse settles before the primary sample is taken. All that would do is cause instability.

One approach that Eric Foster used for reducing his flyback recovery period was adding a series resistance between the coil and the FET switch.
That may seem inefficient because it can severely limit the maximum amount of coil current for a given voltage input - how much depends on the resistance used.
Unfortunately I believe that it probably takes a substantial resistance before you reduce the flyback period appreciatively.

A series resistance, besides limiting coil current, also means that a longer pulse is required for a given coil current to be attained. You could increase pulse width to build to a higher coil current, and increase pulse frequency to try and gain back lost output power.
In my diagram R7 and R6 adjust PW and PPS and those should not be toyed with unless you have a scope (and have a fair idea of how the 555 works). Recommend you simply try reducing primary delay before you play with adding a series resistance because adding a series resistance could get involved.

There are several ways that your circuit could be different from what I've shown here. Don't try and draw a comparison if none exists.

[yourvibes]

extra ic

hope this helps

[adam(Va)]

Yourvibes, Yes that was exactly what I needed, thanks very much for all your help!

[adam(Va)]

porkluvr, I 'll do some circuit comparisons and try to absorb all the info you posted. I am in the process of tracing out the oneshot chain and your diagram will give me a baseline for what I should be seeing. Thank you for taking the time to explain how the circuit works. I seemed to have found part of the problem with the one I am working with. It seems that one of the 1.5 nanofard caps (similar to c2, c5 and c7 in your diagram) in the one shot network has an internal break that is intermittent. Currently it is working just a little noisy. It looks to be a polarized cap. Would I be able to replace this with a nonpolarized ceramic disc cap (don't have any polarized on hand)?

[porkluvr]

Looking at YourVibes' photo, it looks like...

What you have there are not strictly speaking polarized caps, but polystyrene film caps that make use of the foil connected to one terminal (marked with the bar) to shield against interference. PS is low-loss and temperature stable dielectric and you should not replace those with just any old ceramic cap except for temporary.

You should only use either polystyrene, polypropylene, or perhaps COG/NPO ceramic! If using PS, you would normally connect the 'bar' end to a low impedance point. That would be pins 1 & 15 because even if there is no external PCB trace for a connection, those pins are internally connected to MC145138 pin-8 (Vss).

Good luck!

edit: Last time I checked, you could get polystyrene capacitors from Mouser.

[adam(Va)]

Thank you very much, I had no clue what type of cap I had or what type to replace them with. As of now, it seems to be working like it was before. With all the info you gave me it will give me a great starting point for tweeking the pulse delay. I think I'll start with winding a new coil with 30 Awg teflon wire to see if that helps some. Thanks again!