LBO help - Low battery indicator

[thedude8888]

Hi everyone, first of all thanks for a great community. I love all the information being inserted into my brain everytime i visit this site!

I have a question i hope i can get some help with.

I am making a raspberry pi project, where the powersource is going to be 4x Nimh 1.2V 2400mAh batteries in series. Giving a total of 1.2V 9600mAh to play with.

My plan is to feed this power to a DC-DC boost converter like this one, which takes inputs from 0.8V and boosts it to 5V
http://www.ebay.com/itm/DC-DC-Step-Up-Boost-Module-Voltage-Conversion-Input-0-8V-to-3-3-3-3-6-4-5-6V-/332182189317?var=&hash=item4d5799f505:m:mbRHQya7_cObL187KktlmDQ

Then this 5V will be fed to the Raspberry pi.

I just wanted to also make a LBO (Low battery indicator), for protecting the batteries.
My plan was to make a red LED light up, when batt voltage would hit 1V.

I have looked at different schematics of how to make one, and found this one:



I found it to be a very simple sketch, but then my problem was i couldnt find a 1V zener diode. Since they apperently dont get made.

So i was looking into maybe using a TLV431, but it has a voltage reference of 1.24V, or 2.5V other model. And it seems to be a bit more complicated than my first shown sketch.

So i have made a blokdiagram of what my plan for the PSU + LBO would look like, and i was hoping you could help me decide what LBO circuit is the most simple to build for this project.

The LBO does not need to cut off power when low voltage is detected, it should just power ON an LED. That is most suitable for my project.

I was hoping an alternative, where resistors and other simple components could be used, instead of using an IC.

Attatched is my blokdiagram.

I hope i have been succesfull in pulling my question out of my head and writing it down on the world wide web, so another human being can understand it

Thank you very much and have a wonderful day

[thedude8888]

Okay i think the blokdiagram might be wrong. Here is a updated version 

[Rbastler]

Maybe this idea of mine could help: Its a zener diode in series of the base from a transistor. Going from the collector there is a led parallel to the transistor. Collector and led resisotr are the same. That way if the voltage is too low, the transistor will close, and therefor allowing the flow current through the led.

[tautech]

Welcome to the forum.

There's a good # of ways to do this and it depends on how much battery capacity overhead and how low current consumption you want for it to be efficient.
Think "micropower" if you want low quiescent current consumption as it's mostly monitoring and you want/need the power for your other applications.

I've used ICL7665, these are just for this type of application and there's plenty of others too.
Datasheet attached for your study.

[thedude8888]

Thank you for your quick answers! 

tautech, the ICL7665 looks like a good candidate. As far as i can see in the datasheet, the operating voltage is min. 1.6v and max 16v.

But in the "absolute maximum ratings", the supply voltage is -0.3V to 18V.

So this microprocessor would be able to run at 1V to 1.2V right?

[thedude8888]

I have found a schematics using the ICL7665 here:
http://archive.siliconchip.com.au/cms/A_103514/article.html

Here is the schematics:


It is a battery tester, but as far as i can see, it can also be used to indicate battery voltage on a Nimh pack, giving power to a Rpi.
Is is using a 9v battery as power source. And there are 3 leds for indicating battery state.

I plan on using a 2-color red/green 3mm LED. So the LED is green when voltage is above 1V and then it goes red when voltage is 1V or below.

Can someone point me in the direction of what resistor sizes and trimpots i should use? I am quesing it should be other values since the schematics uses a 9V battery, where i am going to use a 1.2V battery source.

[tautech]

Thank you for your quick answers! 

tautech, the ICL7665 looks like a good candidate. As far as i can see in the datasheet, the operating voltage is min. 1.6v and max 16v.

But in the "absolute maximum ratings", the supply voltage is -0.3V to 18V.

So this microprocessor would be able to run at 1V to 1.2V right?

Not according to Operating Supply voltage listed Minimum in Electrical Characteristics.

There's an "S" model too that has about the same operating range but what this is all about is sending you off with some little knowledge to the Maxim site to find current and/or better devices for your needs.
Mostly we don't know about this sort of stuff until we're wised up and then go looking.

Remember you also have to operate an LED too so leave some voltage headroom to allow that and current limit the LED with a series resistor. Check the activated output has enough current drive/sink capability too to switch an LED or use a small Mosfet to do the job.

[thedude8888]

Alright i understand. I will begin researching. Thanks for the input tautech

[tautech]

Alright i understand. I will begin researching. Thanks for the input tautech

You're welcome. There'll no doubt be someone else with a simple and elegant solution, the pickle being the 1.2V you want to operate from. Red LED's have the lowest Vf and any other colours need more voltage to turn on but you could be close to the threshold of getting something to working as you want.
You might need to boost up a little just for this monitoring circuit, consider that.

[2N3055]

I am making a raspberry pi project, where the powersource is going to be 4x Nimh 1.2V 2400mAh batteries in series. Giving a total of 1.2V 9600mAh to play with.

Well, no, that would give 4.8V 2400mAh in total. When connected in series....

[sokoloff]

4 1.2V batteries in series in 4.8V nominal, not 1.2V. Your problem might be much easier than you think. (it gets even easier if you can go to 5 NiMH in series and use an LDO linear regulator to get a stable 5V).