Surf Pi Help - Poor Sensitivity

[jhu]

Hello Everyone,

First of all thanks to ApBerg and silverdog for making this kit happen. After browsing the surf pi threads for the last few weeks, I still have been unable to find an issue like my own, so I thought I'd start a new thread. My board is functional but has lower than desired sensitivity on fairly large objects. My goal is to find small to large iron objects underwater. For my application, depth and search width are the most important things.

So, first some specs.
Surf Pi 1.2 from Silverdog
Coil #1: 8" Diameter 24AWG 20 turns 2.0 ohms
Coil #2: 12" by 15" elliptical 24AWG 30 turns 3.8 ohms

Troubleshooting so far: my board does not work with the offset at 0V at TP2 (with any configuration of delay, tune, volume). It does work fairly well around 2.4V offset but the delay seems to have no effect. Tune and Volume do not seem like independent variables...the board behaves as if a certain "energy" threshold is required to activate the speaker, and many combinations of volume and tune (high volume low tune, high tune low volume) will satisfy. To get the board working I usually set volume high, increase tune until I hear a constant tone and then decrease until the tone goes away. After that, I usually get an audio response to metal somewhere between 2-4" away from the coil, no performance variation between the two coils really. I do have a scope, and monitoring the coil shows that it's working just fine. Consistent pulses, and the decay is visibly slower when metal approaches the system.

Any suggestions? Any alterations that could be made for me iron specific application? Are my coils ok? Any help would be greatly appreciated!

[Qiaozhi]

It seems suspicious to me that it only seems to work when the null-offset is set to approximately half the supply voltage. Have you checked the negative supply voltage? In fact, is it possible to adjust the offset to a negative value?

[jhu]

The offset ranges from -2.6 to +4.1V. I'm not sure what you mean by negative supply voltage....I have a 12 volt power supply connected to power and ground on the board...is this wrong?

[Qiaozhi]

The offset ranges from -2.6 to +4.1V. I'm not sure what you mean by negative supply voltage....I have a 12 volt power supply connected to power and ground on the board...is this wrong?

The null-offset should be adjusted to give 0V at the output of the preamp with the coil disconnected.

[jhu]

Well, my issue is that when I do set the null offset to 0V the whole thing doesn't work at all. (For confirmation, output of the preamp is pin 6 on the 5534, aka TP2?) The offset I had been measuring was with the coil attached. With the coil disconnected, the working value at TP2 is about +5V, and the range is from -3.2 to +5.2V. I don't understand why it doesnt work at the configuration where it was designed to work. Is there any way I can troubleshoot other points on the board to see why this is the case?

Thanks for your patience...I'm new to both metal detecting and electronics.

[Qiaozhi]

Well, my issue is that when I do set the null offset to 0V the whole thing doesn't work at all. (For confirmation, output of the preamp is pin 6 on the 5534, aka TP2?) The offset I had been measuring was with the coil attached. With the coil disconnected, the working value at TP2 is about +5V, and the range is from -3.2 to +5.2V. I don't understand why it doesnt work at the configuration where it was designed to work. Is there any way I can troubleshoot other points on the board to see why this is the case?

Thanks for your patience...I'm new to both metal detecting and electronics.

Have you seen this excellent guide? ->
http://www.adrianandgenese.com/blogg...rom-a-diy-kit/

In one of the comments at the bottom of the page, someone mentions that the offset had to be adjusted to 1.37V. Perhaps that's an issue with certain coils, or it could be that the wrong value damping resistor is being used.

[jhu]

Thank you for your help before I put the project down for a while but I'm picking it back up.

So I'm doing some self-troubleshooting and have run into a few things. I checked the +/- supply voltages from the 5V voltage regulator and the ICL7660. I'm getting +5V and -4.63 Volts for positive and negative supply voltages.

I used this guide and followed for tuning (http://www.adrianandgenese.com/blogg...rom-a-diy-kit/) (step 4). I turned the volume knob on and then turned the threshold pot and tried to get a just above silence noise. But instead I get an oscillating beep maybe a little more than once a second. Eventually I was able to get a steady above silence tone by playing with the offset pot along with the threshold. (also turning the volume knob affects whether this oscillating occurs or not.)

I then tested TP2 and turned the offset pot ulti TP2 to ground was 0V. Now I'm not sure if this is supposed to happen but when i turn the threshold pot the voltage at TP2 changes pretty drastically. (anywhere form .5V to 2.25V) Is this supposed to happen?

Even changing TP2 to 0V and not touching the threshold pot the coil doesn't even detect a large metal stool from 2 inches away. I also played with the delay to but it didn't help make it detect. But when the TP2 value is approx. 2.4V I can detect the large metal stool from a couple inches away.

On top of this which resistor number is the damping resistor and what should its value be?

Thank You guys for any help.

[Pete B1]

Hi jhu, if you're building the silverdog kit then R6 is the damping resistor and it's value is 390 ohms.
I can't really help with the rest as i'm new myself. good luck, Pete

[Tepco]

Well this is interesting behavior. First, to sort out offset stuff, you have scope, so you can adjust “baseline”, part of signal when nothing happens to 0V. For someone without scope, only DMM, most precise way is this: temporarily desolder and disconnect middle terminal of IRF 9640, (drain), to disable pulses influencing measurement, then adjust 5534 output to 0V, will be same with or without coil, then reconnect drain pin, voltage will now change somewhat in presence of pulse, but offset is precisely adjusted, same way like with scope. Problem is only if change is abnormal, up to almost 5V but in your case this is OK. From now leave offset adjustment as is, if you have problem, it is somewhere else.


Interdependence of trsh. control and volume usually can have two sources. One is very close proximity of speaker or cable to the coil, now coil will pick up field produced by speaker when it sounds and whole thing starts oscillating. Another is power source, triggering current limit if powered from PSU, or unregulated PSU, or bad battery, not just empty but old, with high internal resistance. Especially in combination with low impedance speaker, then, when it sounds, current consumption rises, battery voltage drops and this affect detector. All this for otherwise working detector (and correctly oriented 1000uF cap), another errors can cause this too.


By reading this, I think most probably trouble is somewhere in timing part, around 4093 or Q1, misplaced part or something alike. You have scope, so this is relatively easy to find. If you have two channel scope connect one to pin 3 of 555 and use as a reference, if just single channel, switch to “external trigger”, connect ext trig. Input to pin 3 and adjust whatever needed to get nice waveform at 5534 output. From then, probe about 4093, first pin 10, pulse must be visible, position will vary with delay adjustment, referenced to point when 555 pin 3 goes from low to high. After this, pin 3 of 4093 must produce pulse, after that, few times longer pulse is on pin 11, and finally, at pin 4 pulse is produced, exactly same as on pin 3 just bit latter. If any of this is missing, or happening in wrong time, this is the problem. I cannot be more detailed because your scope type or your experience in using it is not known, but good starting point, try to improvise something along these lines.