Cow PI

[Silver Dollar]

Well still looking for info. I found another source that gave some typical timings;

TX rep rate 100hz to 1000hz
TX width 50us to 150us
TX to first sample 10us to 100us
RX pule widths (both same) 15us to 50 us
First sample to Second sample 200us + - Set to as close to new TX pulse as possible

I'm still trying to figure out how the fancy multiple pulse timing could be used.

I thought we might be able to use the ADC and take one sample at each RX and see if we
can find any metal. There is the other method and just measure from TX start to a specified
comparitor level, different metals will give different times.

I found a cheap LCD so might look into adding that to this project;

http://www.ebay.com/itm/Serial-UART-...:X:RTQ:US:1123

[Qiaozhi]

First sample to Second sample 75us to 350us This one seems a bit funny...

The second sample is sometimes referred to as a "late sample". The idea here is that the majority of the target response will have died away by the time the second sample is taken (e.g. at 200us), so any signal detected here will be due to an external source such as movement of the coil through the Earth's magnetic field. By subtracting this signal from the first sample, the Earth Field Effect (EFE) can be eliminated. Without this second sample, there will be an audio response at the end of each sweep of the coil.

[Silver Dollar]

According to the Hammerhead article we should shoot for the following specs;

TX rep rate 900hz to2300hz
TX width 10us to 75us
TX to first sample 10us to 75us
First sample to Second sample 75us to 350us This one seems a bit funny...
RX pule widths (both same) 3us to 50 us

On another project I found them using port B for the pulses. You can have up to 8;

0 - MOSFET control
1 - MOSFET 2
2 - Ch1 - 1
3 - Ch1 - 2
4 - Ch2 - 1
5 - Ch2 - 2
6 - Gnd Ch1
7 - Gnd Ch2

I'd like to have a button and some LED's for mode select, and a button and pot for data entry and
possibly a bargraph like display to show relative value being input and finally a save button.

We need to figure out what resources we need on the PIC before starting the code.

I need to study some more to figure out how to make the pulses. There are some PWM's
available and some timers. We could trigger the TX off a PWM and then generate the pulses
in reference to that. We might use a timer and a PWM to generate them too...

[Silver Dollar]

I wanted to make a PI detector for the beach (though my modified TGSL can be used there now);

http://www.geotech1.com/forums/showt...-Quad-Sampling

I have a Hammerhead I board half built up so will using that for initial development. I searched around checking out various PIC's and software's to use on this project and came up with this;

I don't know C and it will take a long time to learn so assembler or Basic would be better. I found the Great Cow Basic compiler;

http://gcbasic.sourceforge.net/index.html

It will allow faster development and outputs assembler with Basic commands and comments in it so you could then optimize the assembly or work totally in assembler once the framework is written.

Thus the name "Cow PI".

I will use the 18F4550 as it's newer and has lots of features. Carl had already designed a board. See the attachment for Carl's schematic and and ExpressSCH schematic I drew with some mods. We won't be using USB yet as the software is difficult unless your using C.

No display on this one either as it adds complexity and cost and uses more battery.

I envision a couple of push buttons and some LED's to set it up. I need to figure out what a typical PI timing might be for shallow/deep and small large targets.

One mode might be a button and a pot. Press the button and adjust the pot and the PIC will widen the sample pulses and move them apart to set the working pulse pattern.

Another might continuously vary the pulses from shallow to deep then stop when a target is found.

Later we might add wide and narrow pulses like the big guys use or other fun things.

Most of these projects die quickly or progress to high end hardware and go commercial.
I hope to keep this one as a hobby detector and just use it for my own personal use.

I will be using Microchip's MPLAB IDE and the Great Cow Basic. I have a IDC3 programmer and can program the PIC in circuit (most PIC programmers can do the same). The 40 pin 18F4550 might be easier to work with but these schematics are based on the 44 pin TQFP as I obtained a board already designed for this. The programming pins attach differently on the 40 pin (see 40 pin version). You can use the same pins on the 44 pin but the alternate ones free up some pins for circuit use...

I could use some input on typical timing for a beach PI for small/shallow and large/deep targets and any other suggestions for capabilities or implementation help.

I'll be back in my lab in a few days to start working on the hardware.