IDX-PRO+VDI

[scrungy_doolittle]

The whites 5900 schematic has a good one.
Basically the same as the 6000di

[jlsilicon]

Ok, here it is then :

Understanding and Building Metal Detector CIRCUITS - Opamps, Oscillators, Transmitters, Receivers

http://www.geotech1.com/forums/showthread.php?21935-Understanding-and-Building-Metal-Detector-CIRCUITS-Opamps-Oscillators-Transmitters-Receivers

[golfnut]

this is a good place to learn some basics, I would recommend getting a DL of pspice and modeling your oscillator. This way you can see the output without the hassle of a build. As Im sure you know. It is good = that drift seemed extreme on any level, dont think it is the cap drift, gut feel..

[Koala]

Currently, I am trying to build and learn Metal Detector Circuits bottom up.
I am a Digital EE practiced, but never could understand analog circuits - beyond theory.
I can never Tweak the Analog circuit to get it to work.
So, I am trying to learn the analog, op-amps, radio - and so metal detector circuits - by hands-on practice and design.

Best to start a new thread. Post your circuit with your results. Ask what you need to change to achieve your goals. You get more views than in a dedicated IDX thread

Best of luck

[jlsilicon]

Currently, I am trying to build and learn Metal Detector Circuits bottom up.
I am a Digital EE practiced, but never could understand analog circuits - beyond theory.
I can never Tweak the Analog circuit to get it to work.
So, I am trying to learn the analog, op-amps, radio - and so metal detector circuits - by hands-on practice and design.

[scrungy_doolittle]

did you use a linear taper or audio taper pot. It makes a big difference. Our ears hear sounds on a log scale. an A15 taper (also known as an audio taper is what you probably need)

[Koala]

Ok,
What should I replace the 2n3904 then ?

We can only guess what is happening you need to post what you are doing

2N2222 has 3x the current If it's required we have no idea because we don't know what you are doing.

more info here

http://www.sentex.ca/~mec1995/tutorial/xtor/xtor7/xtor7.html

[jlsilicon]

Think you need to post what it is you are trying to do. Maybe in a new thread. Even though the colpitts oscillator works very well for our application it is subject to drift. 2n3904 will also very low current. More than about 10 mA and it will heat up and drift.

Ok,
What should I replace the 2n3904 then ?

[Koala]

Thanks - Interesting Article.

But, my Metal Detector Projects seem to have a drifting pitch up or down over the span of 1 or 2 minutes - even with the Polyester caps.

For example, one Oscillator project that I was testing last night:
- Just a 2n3904 , a 10K resistor , and (2) 0.1uf polyester caps (little red squares 1/2cm across - mica I believe).
On my Datascope, it showed a drift from 60khz up to 100khz over about 1 minute time
- until I powered it down for a few minutes to repeat the same results again ... and again.

Think you need to post what it is you are trying to do. Maybe in a new thread. Even though the colpitts oscillator works very well for our application it is subject to drift. 2n3904 will also very low current. More than about 10 mA and it will heat up and drift.

[der_fisherman]

Thanks - Interesting Article.

But, my Metal Detector Projects seem to have a drifting pitch up or down over the span of 1 or 2 minutes - even with the Polyester caps.

For example, one Oscillator project that I was testing last night:
- Just a 2n3904 , a 10K resistor , and (2) 0.1uf polyester caps (little red squares 1/2cm across - mica I believe).
On my Datascope, it showed a drift from 60khz up to 100khz over about 1 minute time
- until I powered it down for a few minutes to repeat the same results again ... and again.

That could be simple temperature instability.
regards
Andy

[Koala]

Just as I thought. 11.8 x 8.93mm is a lot larger than 5mm square. 5mm square = ceramic.

As I said that's for a 100v look at the size for 25v or 50v I don't need one so can't be bothered to look myself. But I have a 63V from RS and it measures 6.5 x 5

polycarbonate/polyester are the way to go for best stability you can put whatever you want in your build.

A surface mount board there is much less choice so some ceramic might be unavoidable.

If you are building from the silverdog kit it's will work as is.

[ampirate]

Let me correct that :
Red Square 1cm x 6mm x 3mm - 0.1uF

I ordered a bunch of 150nF (0.15uF) np0 caps.
I hope this should solve the drift problems.

[Gwil]

Just as I thought. 11.8 x 8.93mm is a lot larger than 5mm square. 5mm square = ceramic.

[Koala]

Hi jlsilicon

Something strange here - "(2) 0.1uf polyester caps (little red squares 1/2cm across - mica I believe)". Are you sure you mean polyester?
A 0.1uF capacitor 5mm square is almost certainly a ceramic type, I honestly doubt if a polyester one could be made that small. And definitely not mica, a mica 0.1uF would be the size of a brick.

Could you post the schematic of your oscillator? I'm intrigued.

Gwil

original board as supplied by whites C2,C20 and C24 were the only ones that were ceramic all others were polyester and a few polarized . I did not use any ceramic for small values I used radial leads they fitted in twisted around a bit. Some photos of my board are further back on this thread somewhere.

http://www.taydaelectronics.com/capacitors/polyester-mylar-film-capacitors/0-1uf-100v-5-mylar-film-capacitors.html

5mm lead spacing and that's for a 100v version

[Gwil]

Hi jlsilicon

Something strange here - "(2) 0.1uf polyester caps (little red squares 1/2cm across - mica I believe)". Are you sure you mean polyester?
A 0.1uF capacitor 5mm square is almost certainly a ceramic type, I honestly doubt if a polyester one could be made that small. And definitely not mica, a mica 0.1uF would be the size of a brick.

Could you post the schematic of your oscillator? I'm intrigued.

Gwil