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Old 2nd February 2012, 01:40 AM   #1
Skree is offline Skree  United Kingdom
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Default simple NiMH discharge level indicator??

i'm trying to design a simple discharge indicator for NiMH cells. What i need the circuit to do, is to turn a little low current LED like a Kingbright L-7104L off (or on?) ( http://www.rapidonline.com/pdf/56-0400.pdf ) when the batteries reach a certain level of discharge..

I have two circuits, one using 2x AA NiMH, and one using 3x AA NiMH and want to stop discharging in the circuit when the 2x pack gets to 2.0v and the 3x pack to 3.0v under load (1.0v/cell) - so maxmium voltage the LED will see is 4.2v - any circuit losses

seems to me like it should be two very simple circuits but all i can find online is complex circuits and usually for big 12v SLAs, I need something simple and compact

any help much appreciated
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Old 2nd February 2012, 04:37 AM   #2
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TL431?
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Old 2nd February 2012, 05:58 AM   #3
Skree is offline Skree  United Kingdom
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that looks nice but unfortunately my skills do not extend to building that into a circuit that does what i need..

i was thinking more along the lines of a resistor (or diodes in series) that stops the LED from blowing at full volts but is of the correct value that when the battery reaches approx. 1.0v per cell (2v or 3v) the voltage cutoff of the led is reached and it goes out
no go on that?
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Old 2nd February 2012, 07:07 AM   #4
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A precision voltage reference is a good idea. I Googled "low voltage indicator" and found this Maxim app note. Its downside is that it's designed for a Low Battery Output (LBO) signal. That signal could be "DIY'd", but it quickly becomes impractical IMO.
This page shows some simple discrete designs. They could be adapted for your use.
What you really require is a comparator circuit that changes state when it senses the 1V/cell (or equivalent).
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Old 2nd February 2012, 12:45 PM   #5
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Quote:
Originally Posted by Skree View Post
that looks nice but unfortunately my skills do not extend to building that into a circuit that does what i need..

i was thinking more along the lines of a resistor (or diodes in series) that stops the LED from blowing at full volts but is of the correct value that when the battery reaches approx. 1.0v per cell (2v or 3v) the voltage cutoff of the led is reached and it goes out
no go on that?
Try this one:
Circuit - 3v Low Battery Voltage Flasher Circuit- Hobby Circuit
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