Minipulse Plus Rev My time has come!

[godigit1]

Even can use them for the coil form itself. They are cheap in second hand stores and and they come pretty big.

[mushaba]

Been following your thread. Been thinking of making a coil for larger targets with some AWG19 enamel coated wire I have. Including a chart I posted awhile back comparing signal strength vs distance with three different diameter coils assuming everything else remained the same. Wondering what needs to be done to make the MPP a good large target detector(learning exercise).What targets are you looking for? What is the largest coil you would like to use?

Hello green.Generally in Europe hunters go for stashes of rebels and nazi WW2 gold.Usually they putted the gold in metal ammunition boxes,in cans of food and stuff.The searching will take place mostly at mountains ..My plan is instead of looking for gold directly to look for metal but not for small ones (nail,pultabs etc)..Since PI detectors cant reject iron i was thinking , as i said to George some posts ago , to go bigger on coil so i will have depth and reject the small items.Thats why i was thinking to try a 1m x 1m square coil.

Tested the coil i made today with the MPP and noticed the following things (i didnt remove the coil from the MDF waiting for the epoxy to harden):

1) Coil gained me a little detection range-distance on large items and lost a little at small.As it should.But in close proximity it still detects small items.For example with the 3dss an gold coin (22mm OD) was detected at 27-30cm) with this coil at 18-20cm.
2) Larger coil should be more "slow".Damped around 2k it has a very nice curve and can sample at 9us.
3) Watching the preamp output i was getting some noise that had period,moments it had a normal curve about 800mV and at other moments the noise went up to 2V.Some wird interference i cant explain
4) Working too long with a coil at 500 pps and Tx pulse at 300us gave me a headache..

[godigit1]

The wedges are actually ply wood, even the more impressive you coould not have done that with a router.
How do you plan on sealing your MDf. If I may I would suggest a liquid thin epoxy Or a wood hardener used for restoring rotted wood.
Pc Petrifier would work and is not that expensive. I have a wood shell and thats what I was going to use and then clear coat but mine is maple ply wood so I was going to make a woody style coil.
This is a way you can do a easy form for that style of coil and that is to use a embrodrary hoop glue on 1/8 ply and you have a form for a laminate router.

[green]

I wonder if it's possible to use the MPP with a coil 1m x 1m square like Delta pulse,the damping procedure will be the same I believe Did anyone try it?

Been following your thread. Been thinking of making a coil for larger targets with some AWG19 enamel coated wire I have. Including a chart I posted awhile back comparing signal strength vs distance with three different diameter coils assuming everything else remained the same. Wondering what needs to be done to make the MPP a good large target detector(learning exercise). What targets are you looking for? What is the largest coil you would like to use?

[mushaba]

Went to my best friends router and engraved the coil patern 10mm deep and wide (The coil measures about 6mm OD).He blew my mind with these tinny wedges made with his laser cutter also from wood to keep coil parallel to the form sides so there is room for epoxy to fill it all the way through!Im worried about if the epoxied coil easilly will "go" out of the form.

[mushaba]

Scratch that about DC...capacitor is a barrier for DC...but only DC makes sense because if the main sample and secondary was ac waveforms should compare at the same time and cancel each other...

[mushaba]

As a rough guide, when searching for gold nuggets (8-10us delay) the coil must be shielded. This will provide electrostatic shielding between the coil and the ground, and also stops electrical noise.
Use > 15us for the beach, and 30-50us for coin shooting.
Longer delays (typically transmitting at 100-400pps) do not need to be shielded.

Why longer delays do not need to be shielded??There is "more time" to compare the 1st with 2nd sample ??

The embarrasing question of the day:the sampling interogator is configured as differential amp (U3a)??The inputs are dc voltages or ac waveforms...probaply dc because the 470nF acts as decoupling??

If your coil has an inductance of 300uH and a series resistance of 5 ohms (including the series 3R3 resistor) then the tau during TX-on will be L/R = 300u/5 = 60us. To allow the coil current to build up to maximum requires 5*tau, which equals 300us. In this case I would recommend using a TX pulse rate of 500pps. However, in practice, there is little or no difference to using 1000pps with a TX pulse width of 185us. With 300us the energy in the coil will be 600uJ, and with 185us the energy is 536uJ.

So R10 on the Tx circuit its kind of current regulator,it determines how much current the coil will draw??The difficult is to achive more current without spikes so the mosfet wont go avalance..More current=more strength on the magnetic field created??

I've just finished a simple 36cm OD and man thats hard to keep it rigid...I wound 18 turns of 0.4mm double enameld wire,stabilised with electrical tape,then plastic spiral wrap and then another layer of electric tape..Still did not figure out how to make it rigid,tommorow i plan go to a friends router and engrave a circular pattern on wood,stabilise it in there end then epoxy it...

Wind spiral wrap on top of the coil before adding the electrostatic shield.

I wont dare the shielding at the moment but i thing i will have to because the inductance read about 400uH,shielding will lower that right?

[Qiaozhi]

I didnt have the time to make a new coil but i increased the 1st sample delay to 60us (i think it as far as it can with R39).I lowered the total frequency at about 500Hz and left the Tx pulse at 120us,with a quick scan at my trashy backyard seems the detection became less sensitive to small items.Do you have any recomendation on timming values?

As a rough guide, when searching for gold nuggets (8-10us delay) the coil must be shielded. This will provide electrostatic shielding between the coil and the ground, and also stops electrical noise.
Use > 15us for the beach, and 30-50us for coin shooting.
Longer delays (typically transmitting at 100-400pps) do not need to be shielded.

If i find the correct timmings i will have fixed values resistor to "replace" pots (R5-R14-R37) via a switch,i will think it through some more later..

If your coil has an inductance of 300uH and a series resistance of 5 ohms (including the series 3R3 resistor) then the tau during TX-on will be L/R = 300u/5 = 60us. To allow the coil current to build up to maximum requires 5*tau, which equals 300us. In this case I would recommend using a TX pulse rate of 500pps. However, in practice, there is little or no difference to using 1000pps with a TX pulse width of 185us. With 300us the energy in the coil will be 600uJ, and with 185us the energy is 536uJ.

As for the coil my plan is to try a 40cm OD coil with double enameld 0,4mm wire,the shielding is that frighttens me because i've never succesfully done one..I was reading Carl's notes on Hammerhead article about coil shielding,the Scotch24 is the optimal choice,unfortunatelly they dont have it in stock...I have 2 adhesive tapes,one with aluminum and one with coper but when i tried once to shield my coil its detection ragnge went down dramatically..How much space should be between coil windings and shielding material?

[ATTACH]46340[/ATTACH]

Wind spiral wrap on top of the coil before adding the electrostatic shield.

[waltr]

First delay time should be a user control. Many PI detectors have this as a control so the user can adjust as needed.
As needed can be due to the location, field verse salt beach, or which coil is connected, small fast verse large slow, and what object you are targeting, small gold jewelry verse coins.

I never found shielding to help on the coils I have built. So my three coils (30cm mono, 25cm DD & a 20cm Concentric) are all unshielded.
I had a thread to discuss this but it never went anywhere and everyone just quoted what has been written.

I also test coils with temporary mounting, foam board and wood, before doing all the work to encase them. This allows changes and re-testing.

[mushaba]

There is no need for a fast coil if you're searching for items larger than a coin. To find larger items deeper, then go for a bigger coil. That will also help you to ignore small targets.

When the MOSFET is abruptly turned off, the magnetic field in the coil collapses. This generates eddy currents in any nearby metal targets. These eddy currents will die away more quickly for thin, small, and low conductivity items. Presumably these are the targets you want to ignore. Therefore, sampling with a delay that is short enough to pick up these items is just pointless.
In conclusion:
You should increase the sample delay, use a bigger diameter coil, and increase the TX pulse width. You may also need to lower the TX pulse rate. It should be obvious that Litz wire, basket weave coils, etc., do not provide any advantage in this situation.

Ok George,i wasnt a good student after all

I didnt have the time to make a new coil but i increased the 1st sample delay to 60us (i think it as far as it can with R39).I lowered the total frequency at about 500Hz and left the Tx pulse at 120us,with a quick scan at my trashy backyard seems the detection became less sensitive to small items.Do you have any recomendation on timming values?

If i find the correct timmings i will have fixed values resistor to "replace" pots (R5-R14-R37) via a switch,i will think it through some more later..

As for the coil my plan is to try a 40cm OD coil with double enameld 0,4mm wire,the shielding is that frighttens me because i've never succesfully done one..I was reading Carl's notes on Hammerhead article about coil shielding,the Scotch24 is the optimal choice,unfortunatelly they dont have it in stock...I have 2 adhesive tapes,one with aluminum and one with coper but when i tried once to shield my coil its detection ragnge went down dramatically..How much space should be between coil windings and shielding material?

[Qiaozhi]

At my air test I found that a fast coil has greater detection depth that a jumble wound(without shield) that's why I stayed with fast coil.Do not want to detect items smaller than a coin but also I don't want to compromise the detection range.George suggested to have a larger main sample delay...I wonder if a larger coil will ignore small items and the MPP will operate as it should(no change of original delays)..

There is no need for a fast coil if you're searching for items larger than a coin. To find larger items deeper, then go for a bigger coil. That will also help you to ignore small targets.

When the MOSFET is abruptly turned off, the magnetic field in the coil collapses. This generates eddy currents in any nearby metal targets. These eddy currents will die away more quickly for thin, small, and low conductivity items. Presumably these are the targets you want to ignore. Therefore, sampling with a delay that is short enough to pick up these items is just pointless.
In conclusion:
You should increase the sample delay, use a bigger diameter coil, and increase the TX pulse width. You may also need to lower the TX pulse rate. It should be obvious that Litz wire, basket weave coils, etc., do not provide any advantage in this situation.

[mushaba]

Fast is to detect target with very short TC. A target's TC is dependent of composition (type and alloy of the metal which determine the conductivity) AND the size.
Lower conductivity => shorter TC
smaller size => shorter TC

At my air test I found that a fast coil has greater detection depth that a jumble wound(without shield) that's why I stayed with fast coil.Do not want to detect items smaller than a coin but also I don't want to compromise the detection range.George suggested to have a larger main sample delay...I wonder if a larger coil will ignore small items and the MPP will operate as it should(no change of original delays)..

[waltr]

I thought "fast" had to do with fast response of coil so we can sample earlier to find non megnetic metals like gold,didnt know that could be that sensitive and detect all theese little quantity of metal..

Fast is to detect target with very short TC. A target's TC is dependent of composition (type and alloy of the metal which determine the conductivity) AND the size.
Lower conductivity => shorter TC
smaller size => shorter TC

Also when we place a can in front of the coil,we hear audio and seconds later the audio fades away,is that proof of EFE circuit working?

No that is not EFE circuit effect. It is the SAT circuit (U4a) effect of C9 & R6.

[mushaba]

I wonder if it's possible to use the MPP with a coil 1m x 1m square like Delta pulse,the damping procedure will be the same I believe Did anyone try it?

[mushaba]

If you want to make the detector less sensitive to small items, then just increase the sample delay.
Also, you don't need to spend time making a super fast coil, a simple jumble wound coil using enamelled copper wire will be perfectly ok.

I thought "fast" had to do with fast response of coil so we can sample earlier to find non megnetic metals like gold,didnt know that could be that sensitive and detect all theese little quantity of metal..

Also when we place a can in front of the coil,we hear audio and seconds later the audio fades away,is that proof of EFE circuit working?