built a surf pi - design using irf 9640 for pulsing the coil, need help calibrating it.

[steve36]

Hi
I’ve just built a surf Pi detector using the mosfet irf 9640 for pulsing the coil design/version.

its working ok but limited range about 6” at best for a pound coin in the air.

is it supposed to make a clicking sound when no metal then a tone when metal, or be completely silent when no metal?

could anyone offer any advice on setting the the surf pi please and any voltage settings.

(for example trimmed null offset on pre amp output so pin 6 is around 50 mV)

maybe my coil is wrong, 28 turns of 28awg 8” diameter.

Thanks in advance

Steve

[Carl-NC]

P2 is the threshold pot, increase until you hear a faint hum.

[ivconic]

Hi
I’ve just built a surf Pi detector using the mosfet irf 9640 for pulsing the coil design/version.

its working ok but limited range about 6” at best for a pound coin in the air.

is it supposed to make a clicking sound when no metal then a tone when metal, or be completely silent when no metal?

could anyone offer any advice on setting the the surf pi please and any voltage settings.

(for example trimmed null offset on pre amp output so pin 6 is around 50 mV)

maybe my coil is wrong, 28 turns of 28awg 8” diameter.

Thanks in advance

Steve

I don't know what kind of alloy that coin is made of, but the result is not bad at all.
Don't be too ambitious.
Coil specs don't mean much without inductance, you didn't specify what it is?
The IRF9640 is an excellent choice, given the limited selection of P-channel MOSFETs.

[ivconic]

Based on your specs, the inductance of your coil from 28 turns of 28 AWG 8" diameter wire is approx 0.29mH.
This is a "super fast" coil for that type of detector.
Wind the second coil with an inductance of about 0.4mH.
Maybe even a bit higher than that.
It's been a long time since... so I forgot the best specs for it.

[steve36]

Thanks,
Built it on 6 vero boards, 6 separate modules.
just one coil.
cant measure inductance no meter.

[ivconic]

Geee...!
Lot of noise!
Your results are then very good!

[ivconic]

I found an old video on my channel, this is a Barracuda not a SurfPI but the two detectors are quite similar.
However, I went one step further and used Atmega328P for timings.
Coil is 18cm.
However, my pcb was also with poor S/N.
I would probably have better results if I designed a better pcb.
I don't remember at how much distance the coins are detected but not too much more than what you achieved.
One should not expect miracles from a simple PI design, especially not on coins.
It also depends a lot on the alloy of the coin.

[ivconic]

On one of the many schematics for the Surfmaster, I found data of 0.35mH for the coil.
Maybe it's the original data, I'm not sure.
With your wire and the diameter of the winding body; math says you should wind 30-31 windings.
So you are very close!

[ivconic]

"Original Surfmaster PI loop:
20t, #24, 9" Dia.
#24 American Wire Gauge = 0.5 mm metric
Mrand"
Math says it is 0.35mH too!
Also from some old posts...