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Modify this circuit to make it lower voltage? Answered

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.



change the 336 for another zener of lower voltage

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...

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


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...

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.

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

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.


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


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.

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


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.

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 ;-)

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?