Very Small Panel Meters

teomannaskali · 4th December 2007, 18:18

"high resistance isolation system will not load cells" -> sartek

Does this mean: "the lascar displays that we use have a high impedence."
(Which means that there is such a high resistance going in to the display that it does not draw current which is good for the cell)

They use the 10k right? and read the voltage with a lascar.

Edvin M · 18th December 2007, 09:55

I borrowed a book at the library last week and started to read about op/amps and have come up with a new circuit together with one of my teachers that I think should work better than my last one. Does it work and could the op/amp EL2276cn be used? http://www.datasheetarchive.com/pdf/1320319.pdf

I have also seen that various people use the op-amop max949, would it be a better choice and why?

Stephane · 18th December 2007, 10:17

Hi Edvin,

No it won't work mainly because you don't have any ground. The op-amp is powered from a single 9V battery but where is the 0V (GND) ??? You could use a virtual ground (putting the + pin at 4.5V) but there is no need here.
Also, the Élantec op-amp is a race car, not much needed here. You'd better use a low-power, single supply, rail-to-rail op-amp and much much more better, a precision op-amp (like the LT1006 from Linear Technologies).
Use a non-inverter amplifier, the gain will be more than 1 and this is what we want. And feed it with the cell in the right polarity (e.g. - goes to GND, + goes to the op-amp). Also, don't forget the 10k load resistor in parallel of the cell.

**jmurba** · 21st December 2007, 22:19

Quote: (Originally Posted by Stephane)

Hi Edvin,

No it won't work mainly because you don't have any ground. The op-amp is powered from a single 9V battery but where is the 0V (GND) ??? You could use a virtual ground (putting the + pin at 4.5V) but there is no need here.
Also, the Élantec op-amp is a race car, not much needed here. You'd better use a low-power, single supply, rail-to-rail op-amp and much much more better, a precision op-amp (like the LT1006 from Linear Technologies).
Use a non-inverter amplifier, the gain will be more than 1 and this is what we want. And feed it with the cell in the right polarity (e.g. - goes to GND, + goes to the op-amp). Also, don't forget the 10k load resistor in parallel of the cell.

Hi stephan.
I totally agree with you.
It won't work,
if you use a floating voltmeter and connect the battery groung to the sensor ground the voltmeter will read "- " out of range.
if you amplify the signal you will either have to use a virtual ground, a dc/dc converter, to isolate the grounds, or to use a battery ground referenced volt meter.

I've designed oxymeter circuits for many years now, and learnt one thing : the simpler the better.

there are Too ways imho:

the analog way with no alarms, and there no op amp is necessary, using op amps only add complexity
the digital way (using a microcontroller), where the op amps are indispensable but that allows audibles or visible alarms multiple sensor that make it possible for the information to be treated (voting logic, averaging ...), and above all that makes it possible to calibrate the sensors without opennig the houssing , without unscrewing any port and without using any tool to mecanicaly calibrate

analog way is really simple, any sensor, a 10K to divide the signal and any lcd...ok if one does not use a high output sensor, only 2 decimals will be displayed...: " 1.23 1.24 1.20 " instead of "1.234 1.243 1.208"
but were is the problem ? 2 decimals are far enough to pilot the rebreather.

I took me 2 days to make my first analog pcb and I've dived it of years ... after that It took me years to properly designed, built, program a descent digital circuit using op-amps, a microcontroller etc etc....

even now, even if I've dive it for a couple of years I would not publish the circuit to prevent anyone to have any problem because of an error I would have done.

however I can say I successfully use a non inverter op amp amplification amplifier stage.
the op amp I use is an OP90 analog devices, it is a single supply , precision and low volt op amp.

I use a chip containing 4 op amps (0P 490).
I'd say that one have to filter the amplifier stage with a couple of capacitor wisely chosen to avoid any noise the affect the signal.

I'd also say that the program must be carefully written to avoid bug that can froze the microcontroller which is very dangerouse cause the lcd's continue to display the last information sent to them by the controller even if the microcontroler stops running.

unless one is really involved in his digital Gauge project, and unless one has time ahead to finish the projets, it is better to use an simple analog owymeter with no op amps.

the good solution IMHO is to start with a simple analog to b able to dive, and then taking the time to fully study the problem, make loads of tests, ask friends having do the same before etc etc.

regards

Tom Rose · 21st December 2007, 22:46

Using a 90, 290, or 490 is a good choice but not to drive a LED.

It is a precision low power device with almost no offset.

They are available from Digikey...

One of the best uses for an op amp is to use it to buffer the output of your cell. As directed use the correct resistor across the cell pins...that can range from 10k to 100k....download and read the data sheet for the sensor.

The set up the first stage of your op amp as a unity gain amplifier. This simply reproduces the output of the cell with enough current available to drive several display devices. (Read multiple handsets etc)

Then go from there.

Circuits for unity gain output are available on the data sheets for the 90, 290, 490 op amp series.

Have fun

Tom Rose

Edvin M · 1st January 2008, 15:13

Quote: (Originally Posted by Stephane)

Hi Edvin,

No it won't work mainly because you don't have any ground. The op-amp is powered from a single 9V battery but where is the 0V (GND) ??? You could use a virtual ground (putting the + pin at 4.5V) but there is no need here.
Also, the Élantec op-amp is a race car, not much needed here. You'd better use a low-power, single supply, rail-to-rail op-amp and much much more better, a precision op-amp (like the LT1006 from Linear Technologies).
Use a non-inverter amplifier, the gain will be more than 1 and this is what we want. And feed it with the cell in the right polarity (e.g. - goes to GND, + goes to the op-amp). Also, don't forget the 10k load resistor in parallel of the cell.

ok, I have come up with a new cicuit. I left out the 10k load on the other drawing to make it less complex but I added it this time. Does it work?

teomannaskali · 2nd January 2008, 23:44

OK! I did it and it seems to work.

All you need is
1 lascar.
1 trimpot 2.2K that you will mount externally
and your standard 10 K load for the Cell

Damn my nikon camera. There is something severly wrong with it! And the batteries dont even last a single picture when fully charged. So no pics. unfortunateyl. But there isnt much to take a picture of. A power supply connected to V+ and GND. A hole in the side of the lascar with cable sticking out at the end of which there is a pot. And The cell Connected to the INL and INH with a 10K in between.

Stephane. Would you be so kind as to slightly bend the back light and take a detailed photo of the trimpot and the surrounding circuitry so that I might explain how to do this?

It's a happy day for me! Now I can go to bed with a smile.

Teoman

teomannaskali · 3rd January 2008, 00:02

This is what I have in mind for the final ppo2 meter. It will all reside in a plexiglass tube.

The pcb will be printed somewhere. Battery can be 9V or li ion. You have to change a resistance in the bottom for the backlight. And with li ion the backlight wont be so bright.

Who is interested. So I can order a few...

Sorry for the last blurry picture.

rchrds · 4th January 2008, 15:01

Now that is starting to look familiar.

teomannaskali · 4th January 2008, 15:52

Hmm
Wish you had posted earlier....

4th December 2007, 18:18	#51 (permalink)
teomannaskali RBW Member Current Rebreather/s: Home Build Other Rebreather/s: Not Bought Yet Home Build Join Date: Feb 2006 Location: Istanbul TR Posts: 421	Re: Very Small Panel Meters "high resistance isolation system will not load cells" -> sartek Does this mean: "the lascar displays that we use have a high impedence." (Which means that there is such a high resistance going in to the display that it does not draw current which is good for the cell) They use the 10k right? and read the voltage with a lascar.
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18th December 2007, 10:17	#53 (permalink)
Stephane Stéphane Acounis Current Rebreather/s: rEvo Other Rebreather/s: rEvo Join Date: Nov 2005 Location: Nantes - France Posts: 788	Re: Very Small Panel Meters Hi Edvin, No it won't work mainly because you don't have any ground. The op-amp is powered from a single 9V battery but where is the 0V (GND) ??? You could use a virtual ground (putting the + pin at 4.5V) but there is no need here. Also, the Élantec op-amp is a race car, not much needed here. You'd better use a low-power, single supply, rail-to-rail op-amp and much much more better, a precision op-amp (like the LT1006 from Linear Technologies). Use a non-inverter amplifier, the gain will be more than 1 and this is what we want. And feed it with the cell in the right polarity (e.g. - goes to GND, + goes to the op-amp). Also, don't forget the 10k load resistor in parallel of the cell. __________________ Stéphane
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21st December 2007, 22:19	#54 (permalink)
jmurba wet bear Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: May 2006 Location: cannes france Posts: 920	Re: Very Small Panel Meters Quote: (Originally Posted by Stephane) Hi Edvin, No it won't work mainly because you don't have any ground. The op-amp is powered from a single 9V battery but where is the 0V (GND) ??? You could use a virtual ground (putting the + pin at 4.5V) but there is no need here. Also, the Élantec op-amp is a race car, not much needed here. You'd better use a low-power, single supply, rail-to-rail op-amp and much much more better, a precision op-amp (like the LT1006 from Linear Technologies). Use a non-inverter amplifier, the gain will be more than 1 and this is what we want. And feed it with the cell in the right polarity (e.g. - goes to GND, + goes to the op-amp). Also, don't forget the 10k load resistor in parallel of the cell. Hi stephan. I totally agree with you. It won't work, if you use a floating voltmeter and connect the battery groung to the sensor ground the voltmeter will read "- " out of range. if you amplify the signal you will either have to use a virtual ground, a dc/dc converter, to isolate the grounds, or to use a battery ground referenced volt meter. I've designed oxymeter circuits for many years now, and learnt one thing : the simpler the better. there are Too ways imho: the analog way with no alarms, and there no op amp is necessary, using op amps only add complexity the digital way (using a microcontroller), where the op amps are indispensable but that allows audibles or visible alarms multiple sensor that make it possible for the information to be treated (voting logic, averaging ...), and above all that makes it possible to calibrate the sensors without opennig the houssing , without unscrewing any port and without using any tool to mecanicaly calibrate analog way is really simple, any sensor, a 10K to divide the signal and any lcd...ok if one does not use a high output sensor, only 2 decimals will be displayed...: " 1.23 1.24 1.20 " instead of "1.234 1.243 1.208" but were is the problem ? 2 decimals are far enough to pilot the rebreather. I took me 2 days to make my first analog pcb and I've dived it of years ... after that It took me years to properly designed, built, program a descent digital circuit using op-amps, a microcontroller etc etc.... even now, even if I've dive it for a couple of years I would not publish the circuit to prevent anyone to have any problem because of an error I would have done. however I can say I successfully use a non inverter op amp amplification amplifier stage. the op amp I use is an OP90 analog devices, it is a single supply , precision and low volt op amp. I use a chip containing 4 op amps (0P 490). I'd say that one have to filter the amplifier stage with a couple of capacitor wisely chosen to avoid any noise the affect the signal. I'd also say that the program must be carefully written to avoid bug that can froze the microcontroller which is very dangerouse cause the lcd's continue to display the last information sent to them by the controller even if the microcontroler stops running. unless one is really involved in his digital Gauge project, and unless one has time ahead to finish the projets, it is better to use an simple analog owymeter with no op amps. the good solution IMHO is to start with a simple analog to b able to dive, and then taking the time to fully study the problem, make loads of tests, ask friends having do the same before etc etc. regards __________________ when will I be able to think about something else then spending hours underwater, when will I be normal ? http://web.mac.com/jmurba/Site_2/home.html http://web.mac.com/jmurba/Site_3/home.html
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21st December 2007, 22:46	#55 (permalink)
Tom Rose Still Learning Current Rebreather/s: Other CCR Home Build Other Rebreather/s: Prism Topaz Other CCR Home Build Join Date: Oct 2005 Location: USA,North Carolina Posts: 331	Re: Very Small Panel Meters Using a 90, 290, or 490 is a good choice but not to drive a LED. It is a precision low power device with almost no offset. They are available from Digikey... One of the best uses for an op amp is to use it to buffer the output of your cell. As directed use the correct resistor across the cell pins...that can range from 10k to 100k....download and read the data sheet for the sensor. The set up the first stage of your op amp as a unity gain amplifier. This simply reproduces the output of the cell with enough current available to drive several display devices. (Read multiple handsets etc) Then go from there. Circuits for unity gain output are available on the data sheets for the 90, 290, 490 op amp series. Have fun Tom Rose __________________ 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
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2nd January 2008, 23:44	#57 (permalink)
teomannaskali RBW Member Current Rebreather/s: Home Build Other Rebreather/s: Not Bought Yet Home Build Join Date: Feb 2006 Location: Istanbul TR Posts: 421	Re: Very Small Panel Meters OK! I did it and it seems to work. All you need is 1 lascar. 1 trimpot 2.2K that you will mount externally and your standard 10 K load for the Cell Damn my nikon camera. There is something severly wrong with it! And the batteries dont even last a single picture when fully charged. So no pics. unfortunateyl. But there isnt much to take a picture of. A power supply connected to V+ and GND. A hole in the side of the lascar with cable sticking out at the end of which there is a pot. And The cell Connected to the INL and INH with a 10K in between. Stephane. Would you be so kind as to slightly bend the back light and take a detailed photo of the trimpot and the surrounding circuitry so that I might explain how to do this? It's a happy day for me! Now I can go to bed with a smile. Teoman Last edited by teomannaskali : 3rd January 2008 at 00:21.
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4th January 2008, 15:01	#59 (permalink)
rchrds New Member Current Rebreather/s: Other CCR RB80 / Clone Home Build Other Rebreather/s: Other CCR RB80 / Clone Home Build Join Date: Feb 2005 Location: Clarksville, TN Posts: 42	Re: Very Small Panel Meters Now that is starting to look familiar.
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4th January 2008, 15:52	#60 (permalink)
teomannaskali RBW Member Current Rebreather/s: Home Build Other Rebreather/s: Not Bought Yet Home Build Join Date: Feb 2006 Location: Istanbul TR Posts: 421	Re: Very Small Panel Meters Hmm Wish you had posted earlier....
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