Need some help with a LOW battery indicator

[WildRice]

Good'ay all. Since I started my job a few months ago, my free time has severly shrunk. I am finishing up some DD (well, with a resistor) LUX III lights. Since the LUX will be dim by the time the unprotected cell (Li-Ion) gets to 3.2v, undervoltage isnt really a problem. However, I would like to put in a LED that will trun on when the voltage gets to about 3.4(ish) volts. The circuit would only be on when the light is on, so current draw isn't really an issue.
Here is where I need your help....
I would like to build this as cheap as possible, no PICs or custom IC's. I just basically need a small comparitor circuit that will run on 3(ish) volts. I am pretty sure something could be built with a couple low voltage transistors, a zener diode, and some extra parts. If anyone has a circuit that can do this, your help would be GREATLY appreciated. Also, space is NOT an issue, as this would be going into a MAG D size space.

Thanks in Advance

Jeff

 

[MrAl]

Hi there,


For a circuit like this you probably want to use a voltage reference diode rather
than a zener for more stable operation. National makes the LM339 comparator
chip that would function as your sense circuit (add a few resistors and the
reference diode) and drive a high brightness LED as an indicator.
If this sounds interesting to you let me know.

The voltage ref diodes are available everywhere too, not an exotic part and
not expensive.


Take care,
Al

 

[WildRice]

Thanks AL, That is exactially what I was getting at. I have some LM339's lying around, I guess I will cross reference them to check the operating levels. I was originally going to put the overvoltage/undervoltage board in the light itself and put a charger jack on the side. But I thought it safer to have the battery charge EXTERNALLY for safety reasons ('D' size GB). I have worked with comparitor circuits and I did think it could be done descretely.


Jeff

 

[Doug Owen]

I'd suggest LM358. It's got two amps (you only need one), but it's half the size of the 339 (which has four).

I agree that you'd be better served with a 2.5 or even 1.25 Volt reference IC (looks like a small transistor, 3 leads) like LM385 or LM336.

And plan on driving a red LED (low Vf).

Doug Owen

 

[WildRice]

Cool, I am making 3 of these lights, 1 for me and 1 each for my dad and uncle. While I would be able to tell when the light level is dropping, I am not sure about them. I am looking for some 3xAA to D size adaptors as an alternate power source. This would cause a problem if I included the 'battery protection' circuit in the light. The LUX's I have are UX1L's and they are pretty dim at 3.1v. I plan to add some resistance too so OTC current will be between 1.0A and 1.2A. Right now off of 3x NiMh OTC with no resistance I get about 475mA, so with .5 - 1.0ohm it should be about 400(ish)mA. I figure a BRIGHT red LED right above the stock switch would be VERY noticable.

THANKS

Jeff