Simplest PI detector possible?

[geoscash1]

Thanks for the help WM6...

I'll be messing with a few things today on it... at least it was super cheap to build so no worries'
if it never makes it off the bench but it would be nice to see it work. I'll do the 555/pin 3 test today
when I get time to go mess with it.. Silver Dollar mentioned the pulse width, so it may be the 3k3 resistor
I used in place of the 1k2. we will see....

[Harp]

geoscash1,
I have never seen this way of making tiles
But I'm happy because now coming to the fore simplicity of design of the PCB.

Be sure to check the length of the pulse, because that was my main changes!
The original scheme of Eddy http://www.geotech1.com/forums/attac...5&d=1358762518

and after consultation here on the forum, I threw out part of the transistor, and made directly manage IRF740, exactly along the lines of "surf pi" scheme.
I think someone checked in the simulator "ltspice" that has the same characteristics as originlna scheme.
I made its the test plate, but it worked very poorly, only to 10cm max, so I gave up to study why not, because I do not have sufficient knowledge in electronics that I find out what the problem was.
It would be extremely nice to someone who is perceived, and there is a will, that the model of the Eddy PI, do indeed Simplest PI, which will run from the first.


Anyway, I'm interested in why you're on opamp (background tiles) merged capacitor between pins 1-8?

[geoscash1]

Hey Harp,

Well that schematic is quite different than the one I used although they are quite similar...not sure what you mean by "throw out part of the transistor"..

Anyways, I will continue to mess with it, it is a fun little project and perfect for me to learn with, I am very limited also in my electronics knowledge so
am learning as I go. Its nice to have something new to gorge the brain with and since I got "Inside the Metal Detector" (already read it cover to cover twice)
things are really starting to make a hec of alot more sense.

As far as the cap across pins 1 & 8 (Vdd/Vss or +/-), I was told quite a few years ago that in most basic or IC circuits (was working with camera controllers at the time) to use a small
.1uF cap to help with noise, sort of a filter I guess. I have used it like this on many pic micros (12F629, 12F675, 555's etc) and op amps and it seems to quiet things down
quite a bit. The schematic I have also called for it, even though it didn't specify cap size, I had previous experience with using it like that.

(My mistake, I just looked at my schematic to verify and it shows it should be across pins 4 & 8, not 1 & 8 so I'll fix that too....lol...)

Pretty much running blind really without a scope to check things out... I'll pick another one up here soon hopefully..

[Harp]

>not sure what you mean by "throw out part of the transistor"..

by that I mean that whole part of the original scheme (PNP transistor BC557 and accompanying resistors), I was completely ejected - which must result in different settings of time adjustments 555 chip - so added a diode and values of resistors are changed.

>it should be across pins 4 & 8, not 1 & 8 so I'll fix that too....lol...)

that is why I'm asking.

by!

[geoscash1]

Here are the voltages I get on the NE555 and the LF412 along with C3. Using a 12v supply (8 AA duracell batts), it is a bit low now though at 11.7v but should still be plenty to power this board without issues.

NE555
pin 1 - .4V
pin 2 - 10.78
pin 3 - 3.6
pin 4 - 11.02
pin 5 - 7.35
pin 6 - 10.76
pin 7 - 4.5
pin 8 - 11.02

LM358/LF412
pin 1 - 6.26V
pin 2 - 6.26
pin 3 - 6.27
pin 4 - .8mv
pin 5 - 3.1
pin 6 - 4.82
pin 7 - 1.3
pin 8 - 11.01

C-3 - 4.73V

fixed the small cap to the correct pins 4&8 on the 412, was on 1&8.
The unit powers up but just gives a very quick squeal then goes silent, with or without coil attached.

Double and triple checked everything, all resistors are correct values (xcept damping), caps are correct along with polarity and the wiring/trace layout is correct.
The only thing different in my work up is the damping resistor, the schematic calls for a 330R and I am using a 390R 2 watt, but that shouldn't cause
it to not function would it?, I wouldn't think so but then again, I am a rookie and anything is possible I guess...

Totally stumped...

[Silver Dollar]

It looks like R1 should go to pin 7...

[Silver Dollar]

Here's the new layout;

[geoscash1]

checking everything

Silver Dollar, It looks like R1 goes to pin 7 in a round about way based on the schematic but not to sure if its correct or not.

Since I reversed R1 and R2 and it got hot quickly, as you mentioned it is working since that changed the TX so it
must be something else around the 555 which maybe could be how R1 is connected, just not to sure as my knowledge
isn't up to par with some of you guys.

I have a ton of data and books on the 555 and their circuits, I'll research it a little today while wifey is in the pre op.

Read last night that a signal diode (4148 ) isn't good enough on a 555 circuit like this and that it should be a rectifier
diode 1N4001 so I'll swap that too and see what happens with it.

I am getting a continuous whistle now from the piezo buzzer but no detection whatsoever, no matter what coil is attached
and no matter what uH it is.

I also noticed that on most op amps (detector circuits), there is a diode between pin 7 and the cap before the last transistor on
the audio end, which in this schematic it is missing. Anyone know if that will create issues with it not being installed? I have
to assume it is there for a reason but just not that sure yet so will add one in and see what that does as well....

I'll keep posting updates moving forward with this, it is an interesting little project and seems it could
be very workable if done correctly.

Edit - Just saw that you posted a new pcb image before I posted, will check it out and try it later.. thanks silver dollar for the update and pcb rework...

[Silver Dollar]

I fixed R1 on my board and now have a good TX. The pulses are 125us wide and happen at 7.3 ms intervals for
a 1.7% duty cycle so there should be low power in the FET and coil. I did use a 1.5K for R1.

Try moving R1 to pin 7 and I'll bet it works better. Now to finish wiring the circuit...

[Silver Dollar]

Well here's my build. I substituted a few parts because of availability. Anyone know how much voltage
to expect on the coil (don't want to burn the scope up!). I'll try firing it up tomorrow...

[geoscash1]

That was it silver Dollar, I swapped ends on R1 and attached as you show and BAM, it works.... I knew it was something
simple, just wasn't seeing it.. Good to have fresh eyes and a clear mind check things out, appreciated...

Not a super detector but it works and detects a us quarter at 1-1/2 inches and a standard pair of pliers at 3 inches in air
on the bench. The tests were done with multiple coils, from 300uH up to 550uH with approx same result, the 330uH coil seemed to be best
as far as sensitivity and air test depth at max 3 inches.

I was thinking I will add a couple pots for some adjustability with the timing to see if there is a sweet spot
somewhere. I will be swapping the mpsa13 for a bc547 today to see what if any kind of difference it makes.

Will see what this little board can do with some substitutions and a little thought, I will continue messing with it
and making changes here and there.

Big Thanks to Silver Dollar for the help and with spotting the wrong connection on R1.

My substituted parts are as follows:
390R 2 watt Damping resistor (330R 1 watt original)
LF412 op amp (LM358 original)
MPSA13 transistor (BC547 original)

Added:
.01uf axial across pins 8&4 of op amp

[geoscash1]

Made a Quick Video - Good Results

Ok, been messing with this thing all morning and made some adjustments and a few little changes and ended up with much better results.

(I am still have to swap the mpsa13 for the bc547 later to see if it makes any change)

I added a 5k trimmer in place of R1 and made some minor adjustments, little bit at a time until it showed improvement. Well, it did and quite a bit
too..
So I shot a quick video of my simple pi projects progress so you can see the changes and the difference from my post earlier to now. It is still just bench tests
and in air but the results with just a few adjustments and different coils made a big difference. I'll continue to make changes and adjustments to see if it can be
made even better.

Not a bad little project when put together and adjusted correctly. So far so good....

Thanks to Silver Dollar for the R1 catch.... it was driving me nuts....

[6666]

The Aussie 20 = 20 cents, 100 cents =1 Aussie dollar.

[Harp]

geoscash1,
Here PCB to bite into acid. I fixed also R1 and replaced it with a potentiometer as shown in your video.

[bernte_one]

hello guys

thanks for updating this project

@ harp can you too post the parts placement layer
or is it the same as this
http://www.geotech1.com/forums/attac...5&d=1403103542

thanks