TP4 waveform wrong?

[Carl-NC]

Another question is that how much sample pulse width is important in detection depth?
In low frequency PIs (65-100Hz), sample pulse width is high (45us) and if we reduce it, Their reaction to the targets will be slow...

The integrator has 2 time constants... one when the switch is "on" for charging the cap, and one when the switch is "off" for discharging the cap.

Cap charging is determined by the charge current (voltage applied to the feedforward resistor), the cap value, and the amount of time the switch is on. Discharging is determined by the cap & feedback resistor, and the amount of time the switch is off, which is roughly the pulse period. So the slower you run the pulse rate, the more integrator discharge you get. Reducing sample width weakens the cap charge, so you end up with poor integrator performance.

- Carl

[1843]

Sometimes, this adjustment is critical, meaning just a little adjustment will make a big difference.
This takes practice to find the right adjustment.

Hi Reg,

You are right! Why frequency is such critical??? Some PIs become stable only when they are set to a critical frequency...

Another question is that how much sample pulse width is important in detection depth?
In low frequency PIs (65-100Hz), sample pulse width is high (45us) and if we reduce it, Their reaction to the targets will be slow...

What is your idea?

Best regards

[Reg]

Hi Exciter,

I suspect that Carl is right and the noise you hear is external noise caused by some interference nearby. PI type detectors such as the Hammerhead are affected very easily by external noise.

You should try Carl's suggestion of reducing the sample width. This is R 46. By reducing this pot, you reduce the width of the time the signal is sampled. This could reduce the noise some.

Now, you should also try adjusting the pulse frequency a little. The pulse frequency adjustment is R2. Adjust this control slowly and listen carefully to the noise to see what setting is best and has the least noise. Sometimes, this adjustment is critical, meaning just a little adjustment will make a big difference. This takes practice to find the right adjustment.

One other thing you can try is what I mentioned before and that is to make a figure 8 coil.

Cheers,

Reg

[exciter]

Thx NC-Carl!
Is there a way to modify the Threshold to get TP9 more negative, so that the speaker makes no noise?
Or to get the voltage on Tp9 more stable?
I´ve controlled all values, but maybe some resistors tolerance is to big?

[Carl-NC]

Could be interference. I usually run the sample width (not the pulse width!) at the min setting.

[exciter]

hey,
im back from the airfield.
found some small stuff...
Sometimes i cant get my HH quiet with threshold. it just beeps sporadically but i can still find objects.
also HH beeps not for a short time but long, when i wave an object.
if i point my hh into the sky, he still beeps...
when i measure TP9 i can see the curve isnt still and if it gets to an voltage point, speaker beeps. also if theres no metal near the coil.
what about main/second delay width?

[exciter]

Yeah,
i did it like u said.
I leave 680Ohm for now.
but my TP4 seems to look good?? not inverted?
My plastic shielded wire coil seems to perform better like the one i build out of copper isolated wire. Like this:


its not so noisy.
Antistatic bag shield is working better as aluminium.
I also shielded my case with it.
outside it worked pretty good. Will take it with me to the old airfield today.
also i will corrode a new PCB, because my is in bad condition....because of much soldering to find errors and so on.
I used ?hard-paper? plate instead of epoxy. will go for epoxy next time.

[Carl-NC]

If you are using a screw-type terminal block for the coil, you can easily swap damping resistors without soldering. I have also used pin sockets for components that I want to swap out. With either, you can also hold the damping resistor in your fingers (without touching the leads) and touch it to the coil wires, while watching the oscope decay. In just a few minutes you can find the right value.

- Carl

[Reg]

Hi Exciter,

You are correct if you increase the damping resistor too much the circuit will oscillate. However, if you are careful you can increase the value a little and not have any oscillation. Sometimes, just increasing 50 ohms or less and even as much as 20 ohms makes a big difference. If this resistor is too low in value, it takes too long for the signal to recover and doesn't allow for a short sample time. So, if the resistor is too low of a value, then the signal droops and takes a long time to recover.

So, the key is to find the right resistor that lets the signal rise or ramp back up to 0V as fast as possible. This can take some experimenting.

For the time being you probably shouldn't change yours, but you will not be able to reduce the delay as much. That is ok if you are hunting relics and coins.

If you can find a Philips NE5534A, you will probably see things are a little better. So, this is one way you can improve your PI. You might also try the ON Semi NE5534A also to see which ones seem the best.

Just so you will have an idea of what can happen with just a little damping resistor change I have attached a pic of will happen, or can happen. You will see that the original pic has the white trace. The red and yellow traces show what happens if you change the damping resistor. The if you increase the resistance, you should be able to get a pic that looks closer to the red trace. If you lower the value of the damping resistor, the signal will droop more like the yellow trace. If you increase the resistance too much, then you will get the oscillation you mentioned.

Ideally, you want to sample when the trace is at or near 0V, so you can see that if the signal rises faster, then it is possible to sample sooner. That is the reason for trying to adjust the damping resistor.

However, for the time being, you can leave the damping resistor alone and concentrate on the rest of the PI

Ok, now the HH PI, like other PI's is very susceptible to external noise. So any form of noise from transmitters or even machinery can cause problems. This is why you need to test the PI away from such noise sources to reduce problems. It is very difficult to use this type of detector in the city. Also, it is difficult to try to use or test a PI near an operating TV or computer. If there are businesses near where you live, then it is possible they have equipment that will cause problems. Even fluorescent lights can cause too much noise.

Now, if you make a larger coil, form it into an oval, pinch the oval in the center along the long axis and then twist the top half over one half turn so it is in a figure 8, you will find that the noise level will reduce. You can even try this with your coil, but the two halves will be small and the depth will be reduced because the two halves will be small coils. This is one way to see if the noise is caused by external sources. You might try it and see what happens. It it reduces the noise a lot, you may have to make a larger coil and twist it into a figure 8 and use it in the noisy areas. There are other designs that will work, but this is one of the easier ones to do.

Shielding the electronics also helps. It is surprising just how much noise the circuitry can pick up.

Cheers,

Reg

[exciter]

hello!
when i increase damping resistor, there will be ringing.
many sinus-waves.
now it looks like this:

[exciter]

okay,
have to go to university in 5h
Its a TI you are right.
Many thanks for your help.
i will be online tomorrow. maybe i get this thing working correctly

Regards
Steffen

[Reg]

Hi Exciter,

I have to go to work now, so I may not be able to respond much for a while.

Anyway, my guess is your coil is about 450 uh coil. That is fine for what you want to do. You might want to increase the value of your damping resistor some so the signal recovers faster. You might try a resistor value of maybe 750 oms to begin with and see if the curve still looks ok.

Also, I am not sure which NE5534 you are using. By this I mean, which brand. If you are using a TI, you might want to try to find one made by Philips. They are faster and work a little better.

I have to go now.

Reg

[exciter]

My coil is 25cm in diameter. 26 turns of 0,5mm plastic shielded wire. 0,5mm without isolation.
im using RG58U to connect it to my HH.
Damping resistor is 680ohm.
I mainly go to search old wwII stuff or coins...

such a wire:

[Reg]

Ah,

All looks ok now. Also, your pics of the TP without the coil connected explains Carl's pics.

Thanks for clearing that up.

Yes, your pic looks correct.

Now, I am assuming you used aluminum foil tape for your shielding. Is that correct? That would explain the droop or what is the long delay for the signal to rise to 0V.

What is the value of your damping resistor? Also, was is the inductance of the coil. If you don't know the inductance, then let me know the specifics of the coil such as the diameter and the number of turns.

You will probably have to change the damping resistor it if you are trying to make the detector as sensitive as possible. But, that shouldn't be done until you decide on the standard coil you think you are planning to use.

If you are not sure, then what do you plan to hunt with this detector?

One common size is a 300 uh coil. If you decide to use this value then you can buy coils such as those built for Minelab PI's that would work on yours quite easily. Also, this value allows for a short delay better than a higher inductance coil.

Reg

[exciter]

one coil i use aluminium tape
this coil actually connected is shielded by antistatic-bag material.