TP4 waveform wrong?

[Reg]

Hi Exciter,

The LM318 should work. If you can't zero the amp, then as you said, you might need to change the values of the resistors you mentioned. You could reduce the value of the 1 meg. This large of a resistor could cause extra noise also. So, you might try a lower value of one, maybe a 100K or so.

Keep in mind that the LM 318 will be noisier than the NE5534A. So, when trying to hear the very weak signals, the noisier IC will cause more problems.

Reg

[exciter]

My R17 and R19 are 100k with the NE installed.
Me and my friends took the HH to a test today on an old WWII airfield, but i only found old nails etc....
also the HH sometimes beeps around random seconds.
i could only use some puls-width settings, because its beeping every time on some.
or does my shielded coil is the error here? i uses Al-Tape.

[exciter]

what i also figured out is, that im almost unable to trigger on any signals???
they all jumping around and im not able to measure any pulse.

[Reg]

Hi Exciter,

You should make the pulse frequency an outside adjustment so it can easily be changed. Sometimes just a little adjustment will cure the problem.

Now, if the problem is caused by other things, then those will have to be fixed also. As an example, if you increase the gain, then the battery voltage becomes a critical issue and a good regulator could be needed. I use a 14.4V Li Ion battery and regulate the voltage down to 12V. This keeps things stable.

Now, as for your use of the aluminum tape for shielding, I suspect it is causing problems. You need to look at the signal at the output of the preamp to see just how much droop is occurring. Usually, the use of aluminum tape causes the output signal to droop because the tape is detected as a target. This causes the signal to droop for some time.

One big problem is there is no simple material that is cheap and easy to get that can be used for shielding. Some of the conductive paints work well. I have used them.

I have also used some of the thin conductive mylar film and wrapped it around the windings. I have also used some simple conductive ribbon I purchased at Walmart.

My preferred tape is a conductive polyester plated tape that has a very thin coating almost like paint. Unfortunately, it is very difficult to come by.

I used to buy it in single rolls, but it is not available that way any more. So, I have been trying to buy 10 rolls from Schlegel, but so far have been unsuccessful in getting them to send it to me.

Reg

[Reg]

Hi Exciter,

When you change the pulse width, you also change the pulse frequency. You probably should set the pulse width at a fixed value, maybe 100 usec and then adjust the pulse frequency to see if things get better or worse. If things are jumping around, you need to look at the +5 and -5V as well as the battery voltage to see if they are jumping around also. I would measure them with a scope and a meter. Any one of them changing much at all can cause a problem.

What isn't known right now is whether the jumping around is because of external noise or internal problems or internal instability. External noise can make a detector problem look like the problem is internal. If you can remove the coil and the noise or jumping problem goes away or reduces significantly, then external noise is probably the main problem. However, external noise can point out internal weaknesses also.

If you built your PI so you can use a DD type coil, then I suggest you built a noise canceling coil that is ground canceling also. If you didn't modify your PI, then you can simply take the coil you have and twist it into a figure 8. This should reduce the external noise. To make a figure 8, simply pinch the coil in the middle and twist the top half one half turn. This makes half of the coil up and the other half down.

This type of coil reduces the effects of external noise.

Now, the audio can cause problems also if you have the adjustments too high. By this I mean, if R35 and/R41 are set for minimum resistance or near minimum resistance, they can cause the detector to be unstable at times because the high current draw of the audio can react with the pulse. I would recommend you set R 35 and R 41 for about mid position to start to see if that helps. I suspect your audio will be weak, but that is ok to begin with.

I am not sure what you mean when you say you can't measure any pulse. Are you saying you can't stabilize the scope signal so you can measure things?

Reg

[exciter]

Hi Reg!
I mean i cant [IMG]file:///C:/DOKUME%7E1/ADMINI%7E1.EXC/LOKALE%7E1/Temp/moz-screenshot.jpg[/IMG][IMG]file:///C:/DOKUME%7E1/ADMINI%7E1.EXC/LOKALE%7E1/Temp/moz-screenshot-1.jpg[/IMG] stabilize the scope signal.
It just jumps...there ist only one timebase setting to get it standing still on the scope.
maybe i should try it without any shielding.
Or i should go for an shielded case...
willl try it in a few hours

[exciter]

so i checked my voltages. they are fine, also battery (12,22V).
u said pulse width 100us. Am i right to measure it on TP1? pulse width is that what i have marked in this picture or not?
this was with both pot (freq, width) in middle position. timebase 0,1ms, so one div should be 100us.
But i cant tune pulse-width to get 100us???

[exciter]

i can get 150us, with tuning freq and width. its the smallest value i can get.
what i dont understand is, that when i change pulsewidth to 150us and after this play around with freq. pulsewidth will also change or not?
i asked because you said i should tune width to 100us and then tune the freq...
but this will change pulsewidth, so it isnt any longer 100us...

[Carl-NC]

Sorry, been out-of-town with very limited access... Reg, thanks for helping out.

I agree, my TP4 waveforms look wrong, but I'm sure they are what I measured. Not sure why there is no positive pulse, I'll have to look again.

Yes, PW will change slightly with frequency change... nature of the 555.

- Carl

[exciter]

hi, thanks.
but my understanding of pulswidth is right, isnt it?
is 150us okay???
my pulswidth changes much with freqency for my understanding.

[Carl-NC]

What you have marked on the oscope pic is closer to the period, not the pulse width. It shows a period T=300us, which is a frequency f = 1/T = 3.33 kHz. This sounds way too high for a mid-setting of the pots. Check the component values around the 555.

The pulsewidth is the width of the skinny part of the pulse... looks to be 10us or less, which is too low for a mid-setting, and too low in general.

- Carl

[1843]

exciter,
Check R1...R4 and C8 and make sure their values are right.

[Reg]

Hi Exciter,

I gave you some bad advice when I mentioned you might start with a 100 usec pulse width. I have modified my HH so I have a wider range of pulse widths than what is originally designed so I can adjust for a wider pulse width than originally designed.

My simulation package indicates what 1843 indicated and that is the pulse width on the HH can be varied between about 10 usec and 75 usec or so on a standard HH with no mods.

So, to begin with, simply adjust the pulse width for somewhere between 50 usec and 75 usec, rather than the 100 usec I originally mentioned. This still gives you a good starting place to check the rest of the detector.

Sorry about the confusion.

The minimal width of about 10 usec or so is too short and using it will result in a detector lacking sensitivity. The 50 usec to 75 usec range will work just fine for most applications.

Cheers,

Reg

[exciter]

Okay thank you.
so pulse-width are these little lines.
will check this.

[exciter]

where do you measure the sample time????
directly on the pots??
R42 - is main sample time?
R45 - the second ""

tunes my pulse width to 50us like you said.

Best regards.

I´ve shielded my coil with antistatic-fail now.