Eric's specs on Hammerhead applied to Gary's pi?

[Darren]

I came across a post especially for the Hammerhead written by Eric Foster. His recommendations for a large 1m coil (rated at 300 to 500uH and 1.5 - 2.0 ohms total resistance) are as follows:

Frequency: about 100hz (same as Gary's)
Pulse Width: 500uS (can't do it - maxes out at 166uS)
Delay: 60-100uS (no problem)
Sample: 45-50uS (no problem)

I just built Gary's pi. It is a very good detector with small coils. Also rumored to be with large coils, but Gary confesses he's never experienced with large coils. With a 1 meter square also built by Eric's specs (11 turns of 20 awg stranded), it currently detects about the same as my regular metal detector. It detects a 16" cast iron pot lid at about 11 inches. Any further and it fades pretty quickly. I played with the controls a while and this was the best and deepest I could get. My goal is large caches. I am going to swap resistors out to increase the pulse width and test it.

Have any veterans of Gary's machine used a large coil? Can you tell me what approx settings you used to detect large objects at depth?

[Carl-NC]

Re: Eric's specs on Hammerhead applied to Gary's pi?

In general, the same specs should work with Gary's design. Also, increasing the pulse width on the Hammerhead design is pretty trivial.

--Carl

[Darren]

Re: Eric's specs on Hammerhead applied to Gary's pi?

Thanks, Carl. Trivial, eh? So you don't think increasing pulse width would expand the magnetic field enough to go deeper? I guess I'm perplexed how a coil that size wouldn't detect deeper with the right settings. What's the point in building it? I may as well stick with my VLF with a 11" coil! Any ideas?

[Gary]

Re: Eric's specs on Hammerhead applied to Gary's pi?

Darren, Like I said, I know some others have put Large coils on my unit with Good Results.
And I don't believe they changed any settings, and if so, I really doubt the pulse width.

Just not sure why its not working for you. Try an 8 inch coil and see what results you get.

[FJ]

Re: Eric's specs on Hammerhead applied to Gary's pi?

With a one meter coil on a target of the size you are talking about, you should be getting much more depth/range than 11in. Something is not right. Certainly if 11in on either coil was all you would get, then there would be no point in building a large coil but you should be getting much better performance on the larger coil vs. the smaller coil on that size of a target. Something is not right.

Carl's point about trivial does not refer to the expected increase in performance, he is referring to how simple it is to increase the pulse width of the transmit pulse. Just a resistor value change. The longer transmit pulse can help out with depth increase but keep an eye out on other things like too much heat in the FET, it might require more attention to cooling. However in your situation, you need to figure out why you aren't getting more depth with the 1meter coil as things are before worrying about increasing the transmit pulse. That is not why you aren't getting better performance. You should be getting several feet with the large coil and the pulse width set at 100 to 150uS.
FJ

[Carl-NC]

Re: Eric's specs on Hammerhead applied to Gary's pi?

I meant that expanding the pulse width on the HH design is trivial... as FJ said, it's one of the resistors on the 555 timer.

TX pulse width is essentially the "charge time" for developing the coil's initial magnetic field. A coil with higher series resistance and/or higher inductance will need a longer charge time to develop a peak field. If you use a shorter pulse width, then the coil won't be fully "charged", and the resulting switch-off transient (di/dt) won't be maximal.

Also pay attention to the damping resistor. A big coil likely has a different parasitic capacitance, so the damping resistor might need tweaking.

--Carl

[Darren]

Thanks to all - I'll troubleshoot, modify, and post again n/t

nt