I need a low battery warning circuit that turns on an LED when two 9V batteries need replacing.

If anyone has a circuit like this or knows any websites which cover this kind of stuff please help.

I am limited for space so the smaller and simpler the better.

Thanks
John

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Have a look at my webpage http://www.lpilsley.co.uk/cybot/driver.htm, which includes a circuit for lighting an LED when a 9V battery requires replacing. I even include an explanation of how it works :lol:

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Thanks again Nigel, it looks quite simple.

I am using two 9V cells which drive a motor at 15v, I want to provide a warning when the battery is beggining to die.

I see yours provides a warning once a 9V cell gives 6.5V, that seems a little low to me.

What voltage would you suggest to turn on the led as the battery is just starting to run out?
I would rather warn people too early than too late.

Also should I just wire the battery warning circuit to one cell or both?

As the whole battery will be replaced at once.

Thanks

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The first rule of mstechca club is do not talk about mstechca club.
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It's not actually my circuit, it's from the magazine robot Cybot, and it is nice and simple isn't it. As you say, for your purpose it would be best to monitor both batteries - you could do that by altering the resistor values. For a start increase the LED series resistor to keep the LED current the same, and increase the top base resistor to give the switching piont you require.

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It seems it would be possible to use a opamp and a bargraph led to make it a more precise battery indicator correct? Also, I was under the impression that nine volts batteries held 9 volts of voltage until the very end where there was a dramatic voltage drop. It would seem that the circuit there measures a drop in voltage.

 

Yes, an opamp and bargraph would be better, but the requirement was for small and simple.

The battery voltage gradually falls during use, and then drops more suddenly at the end (at least for Nicad or Alkaline) - it's somewhere along the gradual fall you have to detect.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk