Goldscan IV, 4 position switch question?

[Alexismex]

more and more

[Alexismex]

more

[Alexismex]

Some more photos

[Orbit]

that certainly .

[N@SKO]

Hi Orbit!
Can you make more pic's on Goldscan pcb?
10x!

[Orbit]

D15 and D14 are not oa 85 they were originally 1n4148 signal is positive polarity.

[Reg]

One other thing I should mention is, when I refer to positive or negative responses, this can't mean the tone will change to a positive or negative signal, but, rather which of the two diodes will conduct on U13B.

Because of the fact the two signals from the diodes will ultimately result in a positive tone, then shutting off one of the two diodes will still have the same audio response but will simply cause the detector to ignore either high or low conductor objects.

In other words, the positive or negative response I referred to before has to do with which diode will conduct and what the polarity of the output of U13B will be.

Maybe, this will make more sense now.

Reg

[Orbit]

legendary pi detector goldskan 4

[bchampig]

Just realized that the minimum capacitor value stated in the spec for the HCF4538 that I used in the timing circuit, is 4nF. Oops.

[bchampig]

I made the mistake of measuring from the fall of the TX pulse to the fall of the first sample pulse. So, if I subtract the sample width of 15uS, I get a minimum sample delay of 15uS. I'm hoping to improve upon this with a two stage preamp.

thanks,
Brian

[Reg]

"My measurements were taken from the falling edge of the TX pulse, not the flyback signal. The decaying portion of the flyback signal looks OK in relation to first sample. I'm sampling at about 15uS minimum."

I don't understand the statement above at all. The falling edge of the TX pulse should be when the flyback signal starts. The 10 usec to 15 usec measurement should begin when the flyback starts.

Reg

[bchampig]

Reg,

My measurements were taken from the falling edge of the TX pulse, not the flyback signal. The decaying portion of the flyback signal looks OK in relation to first sample. I'm sampling at about 15uS minimum.

The suggestion for a smaller pulse width is a something I'll try as well.

Do my secondary delays look Ok? Also, I seem to have roughly 50uS between the two channels. Maybe I should make it 35uS as you suggested.

Thanks,
Brian

[Reg]

Hi Brian,

Your initial delay of 30 usec is way too long making it difficult to even ground balance, let alone differentiate metals by conductivity.

Adjust this delay down to between 10 and 15 usec and try again.

As for the 250 usec pulse, this will work but using a 100 usec pulse will do almost as good depth wise but use much less current. Also, the shorter pulse time will make it easier to obtain a working 10 to 15 usec initial delay.

Reg

[bchampig]

Reg,

It looks like my timings might be messed up. From the falling edge of the TX pulse(250uS wide), I get the following timings(approximately):

Q5: 300uS
Q6: 75uS
Q7: 350uS
Q8: 30uS

Sample widths: 20uS

The secondary delay on the GB channel looks long to me, but maybe that's ok?

-Brian

[Reg]

I would use a thicker piece of copper. Use a penny if you are in the US. If not, a simple copper coupling for pipe will work.

See if a nail about 3" long changes tones.

Reg