Very Small Panel Meters

teomannaskali

The internal working of the meter is like this:

There is a reference voltage inside the meter against which the output display value is determined.

That voltage depends on 1. the battery voltage, 2 the value at the pot.

On one of my previous posts I puth the schematic. And on another document that I posted the internal workings are explained.

As you know all batteries have different voltages. And their voltage changes as they get depleted. My 3.7V lithium ion battery starts with 4.2 V and ends with 3 V. SO you have to take that in to account too.

You can not use 1.5V because you need at least 3V to operate the display.

Teoman

Silver-Diver

Hi everyone!I'm not really a rebreatherdiver, today I only dive OC but I have tested an Inspration in ap pool and I liked it. I just bumped donw here because I and a friend is trying to build oiurselves a nitrox analyser since we just completed our nitrox class. :PI been trying to understand the formulas on how to get the right value on the resistor that you posted teomannaskali but we hav'nt been able to understand them. Perhaps you could help us a bit?We have already built one model using a much bigger DPM where we simply used this formula to know how to change the resistor and it worked:You have a cell that gives 10mV in air and you want it to show 21%, that is a gain of 21/10=2.1 We take the existing resistor and multiply it with 2.1 and get the right result. Shouldnt that work here to?

Stephane

Hello,

The simplest way to build an oxygen analyzer is to get a high output cell (R22DHO from Teledyne - 25mv in air), put a 10k ohms multi-turn trimpot and feed that to an unmodified panel meter.
That's it.

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Stéphane

teomannaskali

If you are planning on using a lascar SP400.

1 What batteries are you planning on using?
2 Can you solder surface mounted devices?
3 Do you want to use the internal potentiometer or add an external one. I do not think that the internal one will be precise enough.

If you can acheive nr 2, then let me know I will further help you.

Teoman

Silver-Diver

Hi again!

The reaso nwhy I wnt to use the Sp400 is that it's much smaller than the one I have right now and as we both want one nitrox analyzer each, I might ass well built my own one smaller.

Your questions:

1. Either I'll just use 3 or 4 aaa batteries or if the Lithium batteries you were planning to use is smaller I might as well go with those. I guess that they are a bit more expensive but it isnt an battery eating device so...

2 Yes, I can solder SMD's, they wont be the best looking in the world but they will do. I have solderd some before and with a bit of practising before they should be quite good.

3 Well, if the internal one wont do I can just add an external one. Just one question here, cant I connect the external one while keeping the internal? Or is it much better to just add a better external one?


Stephane I can get some pretty cheaphigh output cells but I can get free low out put from a rebreatehrdiving friend of mine who just can give me hes old ones. AS far as I have understood they should still be good enogh to measure max 40% mixes.

teomannaskali

1. Either I'll just use 3 or 4 aaa batteries or if the Lithium batteries you were planning to use is smaller I might as well go with those. I guess that they are a bit more expensive but it isnt an battery eating device so...

>>> you only need 3 batteries. or even 2 might do. But if you are going to make it a hand held do not go with the lascars. In my opinion they are not worth the trouble. plus I payed about 28usd for one (I have 6) which will make 2 doupble and 2 single ppo2 meters. (One set will probably be surplus)
With the housing and all it ends up being just as big...


2 Yes, I can solder SMD's, they wont be the best looking in the world but they will do. I have solderd some before and with a bit of practising before they should be quite good.
>>> then you will not have any problems doing the work

3 Well, if the internal one wont do I can just add an external one. Just one question here, cant I connect the external one while keeping the internal? Or is it much better to just add a better external one?
>>> I havent tried any of this. Hopefully tomorrow night I will have the pcb ready. Then I will be able to tell you how it went. I must admint not having tried this I am slightly afraid of giving any advice. From what I calculate and estimate the internal trimpot will bee too sensitive. so you might have to use an external one.


Stephane I can get some pretty cheaphigh output cells but I can get free low out put from a rebreatehrdiving friend of mine who just can give me hes old ones. AS far as I have understood they should still be good enogh to measure max 40% mixes.
>>> I think that you should have %100 o2 calibration gas in that case...


If you want to try something immediately then open up the lascar display and fint the trimpot and the resistors on it. (They are very easy to locate) then short the 3K resistor temporarily with a paper clip or something. The 3k one is next to the trimpot. (Not the 100K one) you should be able to read them.

->> hmm you cant do that because the display doesnt work without its plastic case :@

When you do that you should be able to read the cell on the display. However it might be very hard to calibrate....
This is why I am adding a high turn pot externally.

If you have patience then wait for a bit and I will have more experience on this matter.

Teoman

Tom Rose

How to do a really good job surface mount soldering.

Try this link and download the video.

Surface Mount

It really helps

Tom

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The 50-50-90 rule: Anytime I have a 50-50 chance of getting something right, there's a 90% probability I'll get it wrong the first time.

www.atlimp.com

jmurba

750 dollars, for 3 pots, 3 lascars type dpm's, a plastic cable gland, a piece of pvc tube machined in 5 minutes, one o ring and a bit of plexiglass, and 3 screws..
no alarms
for me it is far too expensive fr wha it is..I did the same 10 years ago for a couple tens of euros....
regards
jean mi

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when will I be able to think about something else then spending hours underwater, when will I be normal ?

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http://web.mac.com/jmurba/Site_3/home.html

teomannaskali

Thank you very much Tom.

I was hoping that you would post something when I saw that you were reading the thread.
Teoman

teomannaskali

Hmm. Now that I have the electronics almost sorted out. What kind of display should I aim for? All in a tube? Or should I try something like those ones at sarek?

They both seem equally easy to manufacture. What do you advise?

And does anyone have the battery type on those? How do they swithc them on and off. They claim a 9V standard battery then what is the black thingie seen on the double display?

Teoman