Another plea for help diagnosing a Surf PI board

[gfcardi]

I need help diagnosing my Surf PI board. I am sure all the resistors and caps are the correct values. I connected a 5-inch (125 mm) SM PRO PI METAL DETECTOR COIL, which I purchased on eBay. The coil has the following characteristics:

COIL for SM PRO PI METAL DETECTOR. 5 INCH. Wrapped with TEFLON wire wrap. Shielded with SCOTCH 24 shielding mesh. Housing is custom 1/16 inch ABS. 36" marine grade COAX. Coax is 50 ohm stranded SILVER coated, TEFLON insulated with shield. 350 UH inductance +/-. The board seems to work, sort of...I get the audio "beep" upon powering up, and it detects (beeps) a soda can when it is swept in front of the coil, with the can located 2 - 5 cm in front of the coil. The can is also detected when placing the can in front of the coil and moving it away from the surface of the coil in a direction perpendicular to the coil surface. However, the can is not detected when stationary. And, the coil does not detect large metal objects, such as a pair of pliers or spool of copper wire, when stationary or moving. Here are scope trace images of testing at various ports/pins U3A Pin 1 Coil, Pin 1 U2, Pin 6 U1, Pin 3 U7D, Pin 11 U4D, Pin 11 U7A, Pin 2 U3B, Pin 7 U7B, Pin 4 Any ideas, or advice would be GREATLY appreciated! Thanks!

[chemelec]

Well I Believe the Surf is Only a Motion type Detector, so it will Not Detect in Non-Motion. (Stationary)

350 uh is a bit high.
300 uh is better.

Not sure I would use a Shielded Coil.

And What is the DC Resistance of this Coil?
I Believe around 2 Ohms is recommended.

Pliers should be detected, But a Spool of Wire MY NOT be easily Detected.

[gfcardi]

Measured the coil and it has a 400uH inductance and has 1.5 ohms resistance. I had built a simple ~6" coil using 20 turns of #26 AWG magnet wire that has a rather low 190uH inductance and a 1.5 ohm resistance. Both coils gave similar results.

[chemelec]

Curious, Where do you live?

[gfcardi]

Larger images

[chemelec]

Scope traces don't help me.
I have no good comparisons with my older scope.

[Kev]

Hi gfcardi,
The scope shot for U2 pin6, is this the output of the preamp? (sorry I'm not familiar with the Surf)
It appears that the recovery time in relation to the pulse is way too long.
Your damping resistor may be way too low, or the shielding is creating way too much capacitance. Are you certain there is a gap in the shield

But really you need a coil of around 300uH as chemelec says, and then the damping resistor may be correct.

Using both channels on your scope is good as you can compare the output of the preamp to the first sample pulse, then you can see exactly where on the decay curve you are sampling. The first sample should occur when the preamp output has almost returned to relative zero.

Hope you solve it you're nearly there.
Kev.

[Kev]

An extra thought.
For good sensitivity the first sample needs to occur around 10us after the transmitter is shut off, so you need to get that decay curve back to near zero before you sample.
It's a combination of good coil building technique and carefully selected damping resistance. I used to use a 10 turn 10k trimming resistor across a 1k resistor in place of the damping resistor, and then adjust this while observing the output of the preamp on the scope.
You can fine tune the setting and then replace this with 1 or 2 resistors in parallel to give critical damping for that frontend/coil combination.

Cheers
Kev.

[gfcardi]

Curious, Where do you live?

Northern Virginia, USA

[gfcardi]

The scope shot for U2 pin6, is this the output of the preamp? (sorry I'm not familiar with the Surf)

Here is the schematic that was used -

It was built with the values of all the components listed - no substitutions.

[gfcardi]

Are you certain there is a gap in the shield.

Reasonably certain or else I could not have detected the aluminum soda can, correct?

[gfcardi]

Using both channels on your scope is good as you can compare the output of the preamp to the first sample pulse, then you can see exactly where on the decay curve you are sampling. The first sample should occur when the preamp output has almost returned to relative zero.

Just to be clear, are you asking for the traces of U2_Pin6 and U7A_Pin2 simultaneously? If so, here it is -

[Kev]

Just to be clear, are you asking for the traces of U2_Pin6 and U7A_Pin2 simultaneously? If so, here it is -
[ATTACH]30636[/ATTACH]
[ATTACH]30637[/ATTACH]

That's correct, thank you.
Going by that if I'm reading it correctly there isn't enough delay after the pulse before you start the first sample. It appears to be about 4us which is too short for most coils.
Until you get other things sorted you need this to be at least 15us. Later on you may be able to decrease it to perhaps 8us if your coil is very well made.
Also the decay curve resembles one you will get with a pair of pliers sitting right on the coil, it's overloading.

There is something wrong with the damping maybe 390R is too low for this coil try 470 and 560 and see what the waveform does.
Were all the coils like this? If not try swapping the connections to your coil around, try connecting your shield to the ground on your circuit board.

I've roughly adapted your image to show what you need to aim for, although the decay in this image is still not good enough, but you should get the idea.

Cheers
Kev.

[VinceC]

..

[Kev]

Ah I see something which maybe contributing to the problem. Not all varieties of 555 have enough current sinking ability certainly not enough to charge a high side PFET arrangement like this.
Try connecting the gate of Q2 the PFET to the emitter of Q1. The bipolar variety of 555 may drive this circuit but it will use far more current.

This should turn the FET off much faster and not impede the decay. It will either way eliminate the device from the investigation.

Cheers
Kev.