Simplest PI detector possible?

[Eddy71]

Oh, my God..


Look in archive.

http://yadi.sk/d/Yqq1nXl7227MK

[brucester]

Wohoo!

I did a rewire of the op amp as the timer circuit and speaker circuit were fine.

Now when i turn it on, the speaker makes a humming sound like a metal detector.

Its not very loud. Sounds like an out of tune radio.

When i put metal near the coil, the sound doesn't change.

[brucester]

Ok, I'll leave the build for a while and get back to theory.

I understand the whole circuit except for op2.

The default setting is around 72Hertz and OP1 amplifies the final moments of the flyback and feeds it into op2 inverting input.

I know op2 is a comparator but i dont understand what it is comparing the signal to.

Op2 outputs a square wave so i dont see how the presence of metal can change the beating output other than always on or always off.

If some could explain this comparator to me i would be forever in their debt.

Thanks

[bernte_one]

hello eddy71

the layout files yu have uploaded contain nothing
cant see any pcb if i open the with sprint layout 5

can you please check


last pcb layout from russian forum was something like that

thanks

[6666]

All
I have knocked up the first part of the circuit and after taking a few measurements there is a small design flaw if you simply replace the transistor with a IRF740 FET.
If you are running the circuit from say 9 volts with the voltage divider of R5/R6 you are only putting about 4.2 volts on the gate
this will not be turning the fet on fully,
4.2 volts would be ok for a tranny but not a 740.
I will do a few more tests and post results.

[brucester]

I am working on a better sound system. This thing is so quiet.

Need more amps and higher frequency

[Eddy71]

PCB file (with russian OPamp) http://yadi.sk/d/jY-MvpOm23NhQ

[6666]

Thanks Eddy for the coil info.

[WM6]

Maybe useful to someone (two pinky lines are jumpers):

PCB dimensions 5.5"x2.2". Variable resistor VR1 = 20k.

[brucester]

Thanks WM6

Looks good but is your second opam (IC2b) correct?

Shouldnt the +/- be reversed?

[brucester]

Unless it doesnt matter with comparators in which case, ignore me :P

Offer on the table, I will give $1mil US to whoever explains to me how op amp 2 works and what the signal gets compared to and why.

Thanks

[Qiaozhi]

Unless it doesnt matter with comparators in which case, ignore me :P

Offer on the table, I will give $1mil US to whoever explains to me how op amp 2 works and what the signal gets compared to and why.

Thanks

Carl already answered this question previously, so you can send the 1/1000th of a US dollar to him.

The input signal is split into two paths, one slow and the other fast. Note that one of the opamp inputs is charging a 1uF capacitor, and the other is charging a 10uF capacitor. This causes a differential voltage to appear across the comparator, and hence a pulse appears at the output. If you keep the target stationary, then the inputs will eventually equalize, and the output will turn off. Therefore this is a simple self-adjusting threshold (SAT).

[brucester]

Wow, thanks It has been killing me.

I thought Carls answer was referring to my wiring error. Ie, the wiring error made it a motion detector.

Oh well, you and Carl can split the millidollar. :P Thanks Q, you're too kind.

Progress update, downloaded Q's brilliant coil inductance calculator, discovered my coil was only around 100us, wound a new test coil, 42 turns of 0.5mm with diameter of 10cm and success

It detects a 30cm cast iron pot lid when touching the coil Horrible performance but am so happy to have something working after months of research.

Here's a pic of the finished product:



Will solder it up tomorrow.

Am going to make a mini kids sized detector and hang it on my wall as a reminder of how far i have come when i finish my super detector that bankrupts ML :P

[brucester]

First detector down, just ordered a surf PI kit from SilverDog then get started on a micro PI.

Huge thanks for the patience and knowledge

[WM6]

Thanks WM6

Looks good but is your second opam (IC2b) correct?

Shouldnt the +/- be reversed?

You are right. Corrected: