SPI-EG Detector

[Tec]

Hi yes,
C1 is just a smoothing capacitor,
the voltage doesn't change quickly.

So... the voltage on C1 at the '-' input of the comparator is set.
When the '+' terminal drops below this, the comparator output goes negative and the 'D' of the flip-flop is logic 0.

So this circuit is looking at one particular point of the discharge curve, the design has now been changed to be a more traditional sample and hold... the advantage being it grabs a sample over a period of time and averages (integrates) it which is a little less noisy and also faster.
I am going to post this updated design. It is more conventional. (the comparator approach does work, but it is slower than the normal sample-and-hold approach also...

[Tec]

SPI-EG-2c amplifier and smaple-and-hold

I've attached the lastest circuit diagram.
Design has completely changed.
Not very new or special now, just an amplifier and a sample and hold.
Much faster and easier to examine the discharge curve of the coil using a PC connected to the USB.

Waiting for the PCB to arrive (next week).

Tested the prototype, was noisy and next-to-useless (because there are long wires going to the op amp , and sample and hold).
The PCB should be alot better.

But, I found a big shallow target, and dug it.
It turned out to be $3.85 in coins, and an American Quarter !
(That's got to be worth at least another 5cents Australian)

[Tec]

Re-inventing the wheel

I have been working on this, testing, improving
It is looking more like a 'proper' PI detector every day

It is now like other designs I won't mention.
More pulses per second (560), give more averaging and less noise.

Now it has a nice loud signal whenever it is swung east to west. (Earth magnetic field)..
So how does the Hammer-head or Whites surf master solve this ?..
mmm seems They sample in two places and take the difference...because both of these samples are affected by the earth magnetic field equally (it is slow changing signal).

So I'm just adding the second sample and hold..
I'm expecting great things. I'm using monolithic sample and hold ICs, not 4066 switches to capacitors.
This should give less noise, narrower cleaner samples, and no noise or variation while the AtoD is occurring.

I am interested to know how deep an AUS five cent coin can be detected with a good PI detector, typically. I'm having trouble detecting it at all.

I have attached a nice picture of what happens when there is no damping resistor across the coil.

[Tec]

Make a Coil without a former

Here's my latest coil (pictures attached) made without a former.

A circle of the correct size is drawn and hookup wire (jaycar WH3007) is glued down with drops of super glue around the circle.
Each loop can be used as guide for the next loop.
The wire is layered up .
This particular coil is: 29 turns: 5 layers consisting of (5, 5, 5, 5, 4) loops.

Drop more super glue over the whole coil to help keep it rigid.

Use two or three layers of fine fiber glass cloth and polyester resin (or epoxy)..
While the resin is still wet, tape it up tightly with PVC tape (insulation tape).
Be careful to keep the coil flat.

Final specs:
Outer diameter 22cm
Turns: 29

50uSec to Charge (reach max current)
~ 35 uSec to discharge

[mikebg]

Tec, don't waste time experimenting with monocoil. Try with induction balance to feel advantages.

[Tec]

Mono coil

When you say Induction Balance, do you mean 2 coils in a D configuration ?

Or a mono twisted in a figure 8 ('salt' coil)

[Tec]

Continued on SPI-EG4..

[mikebg]

When you say Induction Balance, do you mean 2 coils in a D configuration ?
Or a mono twisted in a figure 8 ('salt' coil)

Induction balance means M->0 or minimal mutual inductance M between TX and RX coil. Here is example where each RX coil is in induction balance with TX coil.