Very Small Panel Meters

teomannaskali

While you are at it you might want to consider making one of the resistors on the op-amp a trim pot so that you do not have to migle with the xxxsmall pot inside the lascar.

Teoman

Edvin M

hmm, perhaps I have missunderstood an english word here, I thought an op-amp was an trimpot. Very well, then the round thing in the middle is a trimpot instead :P What is an OP-Amp then?

btw. I have been looking at the desig and the data sheet and I think I should change the 220k resitor to a 2200k resitor to get it 10 times higher, correct?

Stephane

Excuse me Edvin, but do you understand what you are doing or do you just get the drawing somewhere on the net and trying to adapt it for the SP400?

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Stéphane

teomannaskali

Hmm. In that case I fear that you cannot get more gain from it to the best of my knowledge. because with resistances you can only divide the voltage. Not amplify it.

Trimpot -> trim the potential
O-Amp -> operational amplifier

Teoman

Art Soppa!

Edvin M

I dont know very much about electronics and I surtenly dont know all the english word but I know some and when I have to learn something a do it.

In this case I mailed Lascar and got the reply to use a 820k resistor as Ra and a 220k resistor as Rebreather in the circuit found in the data sheet. As this did'nt work as planned Ichecked the data sheet, under scaling there is a tabel which says to set Rb to 100k if you want to use it to measure 2v, 10k to measure 20v, 1k to measure 200v etc.

So I draw the conclusion that I now "measure" 0.2v but I want to measure 0.02v so I should use a resistor ten times higher. Perhaps not so electronical, more logical explenasion.

Stephane

No Edvin, it does not work like this, sorry.
What you want to do is called amplification as Teoman suggested you. In this case, and Op-amp (operational amplifier) is needed.

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Stéphane

teomannaskali

What the lascar is is this. It is a volt meter that measures 0 to 0.2 Volts.

So, if you have 0-200V you put the resistors to devide it by 1000 and you scale it to 0 - 0.2 V.

So. Our system is slightly different. You do not even have 0.2 v to begin with. YOu have something like 0.008 - 0.014 V max in air.

At his point you have 2 options.
1. Use an op amp to amplify it: transofrm the 0.014 V to 0.14V or so...
2. Modify the lascar so that it reads smaller voltages. This is what I am trying to do. Change the internal resistors i.e the smaller pot of the lascar so taht it can read smaller voltages.


If I succeed you can mail me your lascar and I can modify it.

Teoman

Edvin M

hmm, just realized how wrong I was thinking. Perhaps I should sleep the night before I go into a project like this and now be out all night to drive some of my drunk friends home.

I read your previous post on how to modify the internal. Is it enogh to just change the 100k resistor to a 203k? Could a 220k workI have some lying in around?

ANd do you just use the internal trimpot then?

teomannaskali

Yes that will work, but you have to re calculate everything. And when it works I do not think that it will be usable. The pot on the back has 270 degrees turning. And it will be too hard to do it by hand.

What I will be doing is removing the trimpot and the 3k resistor and adding an external 22k high turn trim pot (I think it had something like 20 turns to reach from 0 to 22k).

Remember all my calculations are for the R22D and using a 3.7V li-ion battery.


I should have the first prototype ready soon. I will post then.

Teoman

PS. The op amp route is not difficult either.

Edvin M

So the value of the resistor changen because of what battery you use? Then why not use a single aa i e 1.5v battery?