Chance PI issues

[joop]

I have just assembled the Chance PI electronics and assembled a coil, which measures about 380 microHenries, resistance is about 1 Ohm (0.8 mm wire). Using the six buttons I can set the Barrier, Volume and Mask. The little speaker also works.

Trouble is, nothing happens when moving a metal object in front of the coil.

I have been looking at the coil voltage with a scope and I see a pulse train of about 20 pulses which lasts about 500 microSeconds alternated by a single 120 microSecond pulse. At the end of either pulse there is a large negative spike of 110 Volts. Just wondering, is this correct?

The top of this spike is 10 microSeconds flat and decays almost instantly.

Any idea where I should be looking? Thanks.

[joop]

Studying the signals with an oscilloscope I found that the delay between the end of the coil pulse and the start of the ADC sample is to big. The coil is already complete discharged when there is a measuring sample.

Studying Andy's website, I found that his coil measures 1.7 Ohms. With a 390 Ohm resistor in parallel this makes about 1.69 Ohms. My coil measures 1.0 Ohm, even though I am using the same 0.8 mm wire and the same number of windings.

I made a new coil with a smaller diameter wire. I measures 3 Ohms. It is working now!!!!!!!

Hope to take the detector outdoors later this week.

[joop]

One last follow up...

I have flashed version 1.2.1 of the chance software (been using 0.8.4). This allows setting of the "guard interval", which is the delay between the pulse and the sampling point. This will allow using faster coils with this PI detector.

[baum7154]

Chance troubleshooting/fix

Update on Chance troubleshooting /fix
I finally got some time to continue troubleshooting my Chance. After rechecking the xmtr I found that it was not transmitting because Q2 was not installed in the proper way. Transmitter immediately started working as seen on my scope using a second coil to pick up the Chance output through the air. However the Chance receiver constantly buzzed and troubleshooting the Damper stage showed good signal traces all the way to the gate of Q9. It turned out that Q9 (IRF9640)was open and it was replaced. The Chance immediately started working again. Sweet relief!

I earlier wound an 8" fast basketweave spider coil of 400mh, 1.2 ohms and it works pretty well with it even though I used too large of solid copper enameled wire (20 ga). With the Guard Interval set at 21 to 23 and the Barrier set at 4 or lower I get some intermittent audio chatter and the detector will solidly detect a glass vial of gold flakes totaling about a match head in volume at about 3.5". 1/4 of a crown from a tooth is detected at about 6" in air.

I need to spend some time with this detector learning what it is trying to tell me. The unit is highly responsive to US coins at about 8-10" and popcans are seen at 20" with this coil. Gold is best detected at the lowest possible Guard Interval setting in a range from 21 to 25 and the Barrier set as low as possible below 4, Iron Mask was set at 1. I should mention that this unit is not integrated into a completed detector yet, just a bare circuit board not yet in a shielded metal housing, no boom or coil housing

I bought a Chinese L/C meter on Ebay for $36 to help in my coil winding and it works very well. I think it is a model LS-200. It saves so much doubt and time in trying to get a working coil and, in future experimentation on other coil designs.

I'm thinking of winding an elliptical 6"X8" spider coil to get a 'Blade' like search pattern and I think 24ga solid copper wire or Litz wire will be the best choice. First I need to integrate the detector into a complete unit. More later as progress is made. Summer is here and the dredge piles are calling me!

Dan

[Orbit]

I hope that you will not be left crown of gold in your experiments I'm joking of course !

[baum7154]

One last follow up...

I have flashed version 1.2.1 of the chance software (been using 0.8.4). This allows setting of the "guard interval", which is the delay between the pulse and the sampling point. This will allow using faster coils with this PI detector.

If you have not already done so get rid of that 390 ohm damping resistor spec'd by the author. Build the resistor network detailed in BB Sailor's 'How to make a Fast PI mono coil' and use it to determine the proper damping resistor for your coil. If your coil is 'Fast' enough you should be able to get down to a Guard Interval setting of '10'. This should help with small gold detection (and best of all, aluminum!) The quick and easy way to get the guard interval down is to set it to '10' and sweep through the network pot range to see if the detector stops buzzing at you and begins to function. If not, then step up to a setting of 11 or 12 or more until it does start to function. If you run out of range on the pot you'll have to change the values of the network fixed resistors to get more range. Then replace the network with a fixed damping resistor of the same value. This process allowed me to get from a minimum '16' Guard Interval setting using the 'required' 390 ohm resistor, all the way down to a GI setting of '10' using an 1150 ohm damping resistor. BIG difference!

Regards,

Dan

[joop]

Thanks Dan, will try this out!

[baum7154]

I'm sure that you will see much improvement on small gold and other small aluminum targets operating at the lowest Guard Interval.

Good luck and keep me posted on your results.

Dan