MPP build stage 4 headphone test failed - seek advice

[Fatboy]

Dear all,

I am building a Minipulse Plus but stuck at stage 4, cannot hear anything from headphone. My coil is a basket coil based on the Chance PI thread.

I think the problem is my coil, but I don't have an oscilloscope or any device to verify the coil's inductance value.

I am considering buying an old oscilloscope to trouble shoot but not very sure what to do even with the device.

This is my first metal detector project, so still trying to understand what am I exactly doing.

Some help will be appreciated.

Regards,

[Qiaozhi]

Dear all,

I am building a Minipulse Plus but stuck at stage 4, cannot hear anything from headphone. My coil is a basket coil based on the Chance PI thread.

I think the problem is my coil, but I don't have an oscilloscope or any device to verify the coil's inductance value.

I am considering buying an old oscilloscope to trouble shoot but not very sure what to do even with the device.

This is my first metal detector project, so still trying to understand what am I exactly doing.

Some help will be appreciated.

Regards,

Discard the basket coil for the moment, and simply jumble wind 20 turns of wire on a 10" (254mm) diameter former. Then secure the loop with electrical tape. This will give you a coil with an inductance of approximately 300uH.

[Fatboy]

Thanks Qiaozhi,

I made normal coil and it worked finally.
However the problem was not the coils but the how I connected them. I did not realized that I need to short TX and RX although it is drawn on the schematics.
Now I am can proceed to next step. I found the R3-2w (3r3) and the IRF740 very hot during the headphone test, is it normal?

[Qiaozhi]

Thanks Qiaozhi,

I made normal coil and it worked finally.FE

I made normal coil and it worked finally.
However the problem was not the coils but the how I connected them. I did not realized that I need to short TX and RX although it is drawn on the schematics.
Now I am can proceed to next step. I found the R3-2w (3r3) and the IRF740 very hot during the headphone test, is it normal?

You don't need to have the TX and RX pins shorted together for the TX oscillator to function on its own, unless of course you connected the coil to the RX pin.

It's possible that the MOSFET and series resistor can get hot if your coil's inductance and resistance are very low, and the TX pulse width is wide compared to the pulse period. However, with a 300uH coil (1R0 resistance) and a TX pulse of 58us (at 1000pps), the coil current should peak at around 1.5A, but will have an average value of less than 80mA. Hence the power dissipated in the series resistor (with a 12V supply) should be no more than 20mW.

But ... if (for example) you increase the supply voltage to 14V, and increase the TX pulse width to 100us, the peak coil current shoots up to 2.2A. This results in an average dissipation in the series resistor of around 500mW.

And ... if you haven't bound the coil wiring very tightly, so that your 300uH coil is actually 100uH (for example) the coil current (even with a 58us TX pulse width) will give similar results.

[Borisimo]

Hi All,

I am piggybacking on Fatboy’s thread as it is the same problem.
I am building the MPP and following the build document (ref. 2015022. I got slightly carried away and built to step 7, but I’m having second thoughts about the test for step 4.

All tests ok until end of step 3, but no headphone noise at test 4. Since then I have managed to borrow a scope so am posting some pictures that hopefully an experienced eye can have a look at (I am a beginner).

I have made a coil in accordance (I think) with the Making a Fast Pulse Induction Mono Coil By: Joseph J. Rogowski advice. It was some time ago so I cannot remember the exact number of turns, it was covered with insulation tape and an aluminium tape as a shield, gap around 2cm.

At step 5 test I get a voltage at TP3 but no drop when a target is presented.
Hopefully the attached photos will help.










So any advice most welcome, many thanks in advance.

[Borisimo]

Any advice on the previous post would be most appreciated.

I am trying to learn some of the basics but I'm a beginner so not really sure what the tests are telling me.

Thanks, B.

[Impulse]

Hey Borisimo i am new to this electrickery too! but i can see that for c21 and C22 you have 2 different values? as per the build doc c21 and 22 are the same value (100nF) (i am new to this too so please correct me if i am wrong) also c6 i have a 473j100 not a 47nj100? i had no tone in the headphones because i had connected the tx rx wrong they must be together then 0v on the other side. also in position U1 what is that? i have a 79L05 as per the build doc, it looks like you have a mosfet there? also i have a rev D board 'kit' did you get the board on its own then order the parts?

heres a pic of mine at step4

i have read so many posts and learnt so much on this forum and they all say DO NOT move on untill you have confirmed that the stage you are on is correct and i have learnt SO much from the kind people here on this forum that it is invaluable knowlege (the barracuda is ongoing IT WILL LIVE! )

[ODM]

473 may mean 47*10^3 pF eg. 47 nF. Also not all TO-220 housed parts are mosfets.

[Borisimo]

Thanks for your replies.

I got the board and parts from Silver Dog as a kit, and I realise that I have committed the cardinal sin of going too far before completing the headphone test….sorry. Anyway…I won't do it again.

Ref. C21 and C22. My (limited) understanding is that although the capacitor codes are different the values can be the same. This decision was informed by this answer on another forum at http://www.electroschematics.com/qa/9194/

'Hi guys, is the.1K63 capacitor value equal to 0.1µF (100nF) or not? As I can see here http://www.electroschematics.com/qa/...apacitor-value the .1J63 code is 100nF (0.1µF) so basically has the same value.'

Ref. C6. I think this is the same case 473j100 is 47nF according to http://www.electroncomponents.com/47...-Box-Capacitor
Ref. U1. It is a LM7905CT which I think is an equivalent to the 79L05 (but happy to take advice on it).

I think the problem is my coil / coil connection, which I sense is a common beginners error.
So, (Impulse) when you say…
‘i had connected the tx rx wrong they must be together then 0v on the other side’.
Does that mean the both ends of the coil connect to the Tx pin on PL2 and the shield connects to pin 3 on PL2?

Do my scope photos throw any light on this ??

Anyway thanks very much for taking the time to reply.
Boris.

[godigit1]

Check out figure #7 in the build document. It looks like by your pictures you do not have the tx and rx connected or I just cant see it.
Good Luck,

[Borisimo]

Thanks for your reply Godigit1,

You are right they were not connected, presumably a simple jumper across the plugs should do it...

Stupid question no.43...do the headphones need to be special in any way? presumably switched on and in close proximity to the coil ?

Cheers

[Qiaozhi]

Stupid question no.43...do the headphones need to be special in any way? presumably switched on and in close proximity to the coil ?

Cheers

No ... the headphones do not need to be switched on. The electromagnetic field from the detector coil will interact with the coils in the headphones, and you should be able to hear the TX oscillator as an audio tone.

[Borisimo]

I tried again with the Tx and Rx connected (see photo) but no sound in the headphones. Maybe I have gone deaf...

I also tried with another very simple coil but could hear nothing, I'm going to check the connections on the board.



Cheers

[geoscash1]

I tried again with the Tx and Rx connected (see photo) but no sound in the headphones. Maybe I have gone deaf.

It will be a very quiet tone, barely audible, sort of like sound at a hearing test, but it will be there if you listen carefully if the transmit oscillator is working correctly.

coil connections for mono coil


Pic of my completed board for reference,


Also there are numerous scope shots that you can use for reference in this thread.
http://www.geotech1.com/forums/showt...uild-2-Started

Hope that helps

[Qiaozhi]

I tried again with the Tx and Rx connected (see photo) but no sound in the headphones. Maybe I have gone deaf...

I also tried with another very simple coil but could hear nothing, I'm going to check the connections on the board.
[ATTACH]35745[/ATTACH]
[ATTACH]35747[/ATTACH]

Cheers

Are my eyes deceiving me, or do you not have the coil connected in the first attachment?
It just looks like you have a loop of wire connecting the TX and RX pins.