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

A

Anonymous

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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?
 
Hi Darren,
The figures I gave were what I would start with, if I were modifying a HH to run with a 1m coil. These figures are also based on my experience in designing detectors for your type of application. I have looked at the circuit diagram and from gains and characteristics of the various stages, would have anticipated much more than the 11in you are getting with the 16in pot lid. At a minimum, I would have expected 3 - 4 feet. Maybe the TX pulse width is reducing this, but even at the max figure you quote, you should exceed 11in. Try it with a target of similar area, but of a different metal, say mild steel or aluminium. Cast iron does have a fast decay, and if it is thin, this might be reducing the signal when sampling at 60us. Have a look at the signal decay on a scope.
Eric.
 
I don't know how similar Gary's PI is to the HH, as I have not looked at that schematic. There may be some other reason why it is not working well.
Eric.
 
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