chance pi build

[baum7154]

Fascinating stuff, thanks for the info!

[Tinkerer]

After doing some detection tests on 1/4" X 1/4" X .004" thick aluminum targets (with a TC of 1.5us per member GREEN) the question of arose of "Just how fast is CHANCE PI sampling at it's minimum Guard Interval setting of 10?"

Scope measurement shows that the TX pulse is a string of 15 shorter pulses 40us apart with the first short pulse 5us wide and successive pulses incrementally increase with the last short pulse being 16us wide. This is followed by a 120us long pulse, then flyback occurs. At a Guard Interval setting of '10', CHANCE opens the sample window 8us after the end of the 120us long pulse, when coil flyback begins. This sample is 12.5us long.

Guard Interval settings can be made in the range of 10 through 99...10 is the lowest allowed by the current Version 1.2.1 software. A test of Guard Interval set at 90 (an 80 unit increase) resulted in a change of 71us on my old Tektronix scope. This means that for practical purposes each unit of Guard Interval is about 0.9us. TEPCO pointed out that since the uC is clocked at 11.0592mhz that this is likely why it is .9us/GI unit.

CHANCE takes other samples during the time when the short pulses are being transmitted and they appear to be 12.5us also but do not appear to change their timing with changes in the Guard Interval.


Dan

The sample taken during the short pulses is probably for the Ground Balance. Minelab uses a similar method. Several closely spaced short pulses also act like a demagnetizer for the magnetic properties of minerals in the soil.

With the original coil (is it 400uH?) and 390R? for damping we get a long coil decay slope. Somewhere along this slope, the decay slope from magnetic targets crosses the coil decay slope. Using this crossing point as a pivot, we can discriminate magnetic metals.

[6666]

Thanks, I've just clicked it again from here in Oz, and goes nowhere just "connecting".

[baum7154]

I just tried it and it works fine. The link is to ACP Direct in Dallas, TX @ 800-232-2979

[6666]

The link in post #27 does not seem to work ?

[baum7154]

After doing some detection tests on 1/4" X 1/4" X .004" thick aluminum targets (with a TC of 1.5us per member GREEN) the question of arose of "Just how fast is CHANCE PI sampling at it's minimum Guard Interval setting of 10?"

Scope measurement shows that the TX pulse is a string of 15 shorter pulses 40us apart with the first short pulse 5us wide and successive pulses incrementally increase with the last short pulse being 16us wide. This is followed by a 120us long pulse, then flyback occurs. At a Guard Interval setting of '10', CHANCE opens the sample window 8us after the end of the 120us long pulse, when coil flyback begins. This sample is 12.5us long.

Guard Interval settings can be made in the range of 10 through 99...10 is the lowest allowed by the current Version 1.2.1 software. A test of Guard Interval set at 90 (an 80 unit increase) resulted in a change of 71us on my old Tektronix scope. This means that for practical purposes each unit of Guard Interval is about 0.9us. TEPCO pointed out that since the uC is clocked at 11.0592mhz that this is likely why it is .9us/GI unit.

CHANCE takes other samples during the time when the short pulses are being transmitted and they appear to be 12.5us also but do not appear to change their timing with changes in the Guard Interval.


Dan

[baum7154]

Good decision that you will not regret!

[Waikiki_Sweep]

I recommend that you do not use the 220 uh axial lead choke L1 that was provided in the original kit. I wound one on a toroidal form using 26 AWG enamel wire. It has much better current delivery than the original.

Glad to see you have the OP37 amp as it is a much better device!

Dan

When ordering parts from Digikey I saw such toroidal choke 220uH for $11. So I have wound my own to save money using AWG24. Looks bulky but will drive current for sure.

[baum7154]

I recommend that you do not use the 220 uh axial lead choke L1 that was provided in the original kit. I wound one on a toroidal form using 26 AWG enamel wire. It has much better current delivery than the original.

Glad to see you have the OP37 amp as it is a much better device!

Dan

[Waikiki_Sweep]

Does the board you have incorporate the changes required for the VERSION 1.2.1 software? If not it is the older board for
Version 0.8.4. and you will need to modify for the changes.

Thank you. Anyway I will check all connections on board because they are 30% incomplete without top PCB layer then I will follow instructions for PCB and software updates from Andy website.

[baum7154]

Does the board you have incorporate the changes required for the VERSION 1.2.1 software? If not it is the older board for
Version 0.8.4. and you will need to modify for the changes.

[Waikiki_Sweep]

It goes to be a messy business. I have started to solder parts on Chance PCB and have noticed on that PCB just missed whole ground layer on the top.
So I will have to make several wire jumpers to ground all components properly. Probably it was one of that epic failure PCB's from the first Cance with mistakes.

[baum7154]

AUDIO OUTPUT MODIFICATION

The audio output of the CHANCE is designed to feed a device floating off of system ground. I wanted the option to use headphones or the internal speaker. It is no secret that phones are much superior and I hear more nuance in the audio when using them. The problem is that the audio output should not be tied to system ground but nearly all headphone jacks have the sleeve at ground.

I found a TRIAD SP-42 miniature audio transformer nicely isolates the audio allowing it's 12 ohm secondary to be tied to ground on one side. It has a 50 mw power capability with a 150 ohm center tapped (75 ohm) primary and a 12 ohm secondary. The electronics wants to see 50 to 100 ohms on the primary so I used the 75 ohm center tap.

I installed a 1/8"/3mm switched stereo jack from Radio Shack so that when the phone plug is inserted the speaker is cut off. The headphones I use are 16 ohm mono/32 ohm stereo, switch selectable with independent left and right volume controls. There is plenty of volume with the CHANCE volume set at '1' and the independent headset volume controls really help with keeping it at the best level for me.

Dan

I am posting some pictures of this transformer modification of the audio output to allow output grounding. Also there are some pictures of copper shielding I placed over the transformer and internal speaker assembly. It seems that at higher audio levels required when using the internal speaker that there was an increase in detector noise due to higher speaker drive levels.

This image shows the speaker salvaged from an old LCD display with the transformer and switched stereo phone jack attached below it.


This image shows the copper shielding foil over the speaker, transformer,and jack.


Dan

[baum7154]

Took this coil on my CHANCE out today and buried a 10.8 grain piece of dental gold in a hillside at my prospecting site. Was able to detect the piece at a depth of 4.5 inches with a signal strength of 5 units and a solid ID of 'Gold' on the left end of the scale.

Dan

[baum7154]

This photo shows the battery pack installed and the car charging cord that is stored behind the end cap of the hollow PVC handle. The charging cord plugs into the rear jack of the enclosure.


This photo shows the underside of the enclosure and the boom attachment using PVC electrical saddle clamps. Also visible here are the internal speaker port and the headphone jack that cuts out the speaker when a plug is inserted into it. Note there his plenty of headphone audio drive at an audio setting of '1'.


This photo shows the headphones plugged into the jack on the underside of the enclosure. This arrangement keeps the plug out of the way and protects it.


These two photos show the overall view of the detector


These photos are of the 8" fast spider coil in various stages of construction. Details of this coil have been posted in the COILS forum under the title of CHANCE PI COIL.