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Simple Low-Cost Digital PI-1

A

Anonymous

Guest
The test conditions are:
Pulse repetition frequency: 2000 Hz
Main delay: 20 usec
Secondary delay: 100 usec
Digital sample: Two ADC samples at each point, 10 usec apart
The samples are separated by about 10 usec.
 
Didn't understand it then and don't understand it now.
 
Georg,
That is a very nice looking PI. What kind of distance are you getting on coins? If you move the first delay to 12uSecs. then it would be of use for hunting gold nuggets.
Have you made a printed circuit board for this?
Randy Seden
 
What resistor controls the first delay? its value? Thanks
 
The distance is like original PI-1 by Carl Moreland.
12uSecs is problem for PIC16F676 ADC
TACQ = Amplifier Settling Time +
Hold Capacitor Charging Time +
Temperature Coefficient
= TAMP + TC + TCOFF
= 2
 
All control resistors are like original PI-1 by Carl Moreland.
 
Go and Read PI-1 project page.
PI-1 Article (PDF-760K, 300dpi)Rev0.5 - 11Mar2003
http://www.thunting.com/geotech/
 
Using the onboard A/D converter in the PIC will get you 20us delay at best because of its limitations. You could utilize an external faster A/D to overcome this and get a shorter pulse delay.
Good luck,
FJ
 
There are different ways of using a microcontroller in a PI that have different levels of difficulty.
1. The easiest would be to just have the microcontroller take over the timing functions. The micro does not have much work to do and the program is small and should be easy to get running. This would eliminate the timing chips from the design.
2. The next level of difficulty would be to have the microcontroller sample the signal at the output of the integrator (IC8a). If you use 2 ADC channels you can sample from both sides of C19 and get both motion and non-motion signals. The microcontroller does not have to be very fast because sampling at about 100 Hz would be enough. Then the microcontroller could add the threshold and chop the signal to make the audio output. The microcontroller would have to do a lot more work than the first case and there is more software to write, but it still does not have to do much math. This would eliminate all the parts from IC8b to IC9.
3. The next level is to sample before the integrator eliminating IC8a. This is much more work because now the sampling has to be done at the pulse rate or higher. And now there is a lot of math to do to filter the samples. An advantage of doing this is that you can take extra samples at different delay times to try to identify the target.
I am working on this third case but case 2 is a lot less work for only a small difference in the amount of analog parts. And I am not certain that I will be able to get the same performance possible from case 2. I hope that I can, but I will not know for sure until after I do all the work.
-Robert Hoolko
I will also work on case 3. Case 1 has been done in some of the other projects (GoldPic & Stuart), and case 2 is how Tesoro does the SandShark. If case 3 performs poorly, then back off to 2.
- Carl Moreland
 
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