[tangent]

I've been thinking about low battery indicator circuits lately.

So far, the only thing I've seen on these forums is ppl's zener/CRD circuit, used in his personal amps and then adopted into the PPA and PIMETA. Just shutting the LED off when the battery is low is kind of weak, though: it isn't an indication, it's a lack of indication. You can infer that the battery is low, or you can infer that there is no power at all.

Last night I came up with this circuit:

It uses a bicolor LED (just two matched LEDs wired back-to-back) set to show green when the supply is over a certain voltage, and red when it drops under that voltage. The comparator (LM311) might oscillate when the midpoint of the voltage divider is right at the zener voltage, in which case you'll get orange or yellow, depending on the LED. There's a low-power version of the comparator (LP311) that would be ideal here; you could probably tune it so the entire circuit draws maybe 2mA more than the LED alone.

Bicolor LEDs are available in all possible combinations of red, yellow and green. Sadly, there are no bicolor blue+X or white+X LEDs yet, probably because the voltage drops are too dissimilar, so matching the two chips for brightness would be a problem.

You could easily modify the comparator circuit to just turn on a low-battery LED, and have a separate power LED. This would open up the full range of LEDs to you, since it doesn't depend on a bicolor type.

Another variation I've seen runs the LED normally while the supply voltage is high, but flashes the LED when it gets low. This one uses just a single LED, but it does need more active parts than any of the above circuits.

A zener plus a CRD is about $2, so all of the above circuits actually are about the same price or cheaper. They all take more board space, so there's little choice in a compact circuit like the PIMETA. But if space isn't an issue, which would you rather have? Are there any other cool low-battery circuits you've seen that you like better?

 

[GWN]

How about this one? Another variation on the theme. The led is off until it hits a threshold low voltage then comes on telling you its time to change batteries. You can adjust the threshold by varying R2.


http://www.designnotes.com/CIRCUITS/lowbat.htm

 

[comabereni]

DELETE: Wrong thread.

 

[Garbz]

a bit off topic but thanks for explaining how the whole 2 colour led thingy works. I was about to build something with one but using a not logic gate to reverse the power, this will save considerable effort

 

[thrice]

Quote:

Originally Posted by tangent Sadly, there are no bicolor blue+X or white+X LEDs yet, probably because the voltage drops are too dissimilar, so matching the two chips for brightness would be a problem.

Never say never:

http://www.lsdiodes.com/multi-color/

 

[flecom]

Quote:

Originally Posted by thrice Never say never: http://www.lsdiodes.com/multi-color/

those are 3 pin LED's

 

[thrice]

oh yeah, I guess you would need two pin leds huh...sorry.

 

[aos]

A comparator shouldn't really oscillate - that's what the histeresis is for. I.e.
there is a difference between low-high and high-low thresholds. I am not
familiar with that particular chip though.

 

[flecom]

what about something like this, recycle the existing circuit and just add a transistor and a second led... this would let you use nice 3 pin leds like white blue, or even better blue red!!

it would light one led like the circuit is supposed to, and then when the led turns off it will light the other... or so i think im not to great at this

 

[aos]

Somehow I think that the bottom LED will always be lit - D1 will always conduct enough current for the transistor to turn on.

 

[flecom]

then put a diode or a resistor or something so that it wont be enough to trigger the transistor

 

[setmenu]

For something to give one a little more info as to battery level, the circuit
I made here is usefull.

http://www6.head-fi.org/forums/showthread.php?t=76412

It could be made smaller with a surface mount chip and smaller resistors.

Setmenu

 

[hadron]

This is what I built a few months back for my DacKit 12V 10 cell NiMH battery.

>12V green
11-12V orange
<11V red

 

[aos]

Hadron, I like your solution. It isn't much more compllicated or expensive or size-consuming than tangent's (except that you'd have to get rid of potentiometers which is trivial) yet it gives a valuable addition of
low battery indicator (not just dead battery indication).

I'm trying to make a much simpler one with two diodes in style that flecom tried but I haven't had much success yet.

 

[aos]

I finally managed to get it to work (I have been testing them on protoboard).
I used two 3mm 1.6V red LEDs but you can use others or even a 3-pin bicolors.
You might need to adjust resistor values in that case though.

Basically you add two transistors and four resistors (and of course a LED) to
the ppl's low battery indicator. The other LED will turn on when the battery
is low and the first one will turn off, as usual.

Here it is. Note: I've updated it since I had LED in a position that would prevent use of 3-pin bi-color LEDs.