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Modify this circuit to make it lower voltage? Answered

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What resistors do I change in this circuit to make it go red at <3.2V and green at >3.2V ?
It is a battery monitor that uses op-amp, 2.5v voltage reference diode, and a 2-pin bi-color LED. The trimpot adjusts the voltage at which the colour changes.

http://i48.tinypic.com/28qtb3l.gif

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steveastroukBest Answer (author)2012-11-20

change the 336 for another zener of lower voltage

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jdorne (author)steveastrouk2012-11-20

u sure thats all? i thought I would have to change resistors.

Right now it works fine for two cells. i adjust it so it goes red at 6.4V
I want to make it go red at 3.2 but at about 6V it is red throughout all of the trimpots rotation. hmm...

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steveastrouk (author)jdorne2012-11-20

You can shunt it. I'd forgotten that pin.

Steve

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jdorne (author)steveastrouk2012-11-20

Hmm. now the LED hasnt got enough voltage to light at 3.2. through a darlington it didnt do anything... still only 1.2V across LED...

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steveastrouk (author)jdorne2012-11-21

Ah, the thing is powered by its own rails, and your rails are down at 3 V or less. The opamp's outputs only goto 1.5 V from the rails, so at 3.2 V, the output can only be 3.2-1.5V = 1.7V.

Try a different op-amp, one with "rail to rail" outputs.

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jdorne (author)steveastrouk2012-11-21

what op-amp would you suggest? has to be a dual one.

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steveastrouk (author)jdorne2012-11-21

It needs to be a low supply voltage too.

OPA 2703 springs to mind. The old CA3130 might work too, but you'd need two.

Steve

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jdorne (author)steveastrouk2012-11-21

This is the closest I can get without using eBay. It says CMOS rail-to-rail.

http://www.jaycar.co.nz/productView.asp?ID=ZL3482&keywords=OPA&form=KEYWORD

I can also get the CA3130 from there but as you said, i'd need two which is not ideal as I plan to make the finished design on a tiny 20x20mm SMD board for going inside the 3000mW MagLite Lasers I build.


Thanks for your help btw.

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steveastrouk (author)jdorne2012-11-21

It should work down to 3V supply.Give it a go.

Steve

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jdorne (author)steveastrouk2012-11-22

Thanks for your help! The only "rail-to-rail op-amp I could obtain without eBay was the LMC6482AIN. And this worked perfectly if I changed the potentiometer from 10K to 100K. It actually worked better than expected and I can now make one circuit work for both single cell and double cell battery monitor just by changing the pot.

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steveastrouk (author)jdorne2012-11-21

Ah, now if your going SMD, then look at some of the SC-70 packaged devices from Maxim and TI

You're welcome, and don't forget to vote for Best Answer ;-)

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jdorne (author)steveastrouk2012-11-20

will try it. what zener do you think will be adequate?

I can adjust the reverse breakdown voltage of the LM336 with the adjust pin. that will probably be the same as changing it for another diode won't it?

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