Hello,
I need to sense the voltage of batteries(6 AA's in series), and to know when they are at 3vdc or so (maybe 2vdc) to drive a circuit that will flash a LED at about 1 Hz. Maybe with 555 timer and comparator. I am not sure where to start, or the most effecient way. Thanks for the help.

 

Combine one of these:
http://ww1.microchip.com/downloads/e...Doc/21434h.pdf
with a RED flashing LED and you should be good to go. You may need a transistor to boost the current output (5-10ma) to drive the LED.

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That's a cool part. Available a Digi-key too

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2VDC, that's just 0.333V per cell, alkalines will probably leak and rechargables will die if discharged to that voltage.

If you go for the old fashioned approach an LM311 comparator bandgap referance, potential divider and flashing LED is the way to go.

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Number 1: Would 3 vdc be better or should it be higher?

Number 2: Could you explain that a little better(differtent)

Thanks

 

Where do I go from here?

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lowvoltage.jpg (34.1 KB, 12 views)

 

Take a look here. Explains comparators.

http://home.cogeco.ca/~rpaisley4/Comparators.html

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I don't know what to do from here. The sites help a little, but not much for someone who doesn't know what they are doing. lol This is all I have now. Can someone steer me in the right direction?

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Ok, here is the thing. You want to detect when your batts drop to say 3V. You are also using batt voltage to power your 311. Problem is that the 311 needs +5v to operate, so when you batt dies so will your circuit. The other problem is that your also using your batt as your vref. When the batt dies so does Vref. The nice thing about the part Kchriste suggested is that it has built in Vref so the drop in batt voltage will not cause a problem. And the part is only 3 pins.
I think the LM139 operates down to 2vdc.

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Would this work? Maybe it draws too much power.

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I would normally say about 0.8V per cell giving a total voltage of 4.8V for six cells. Rechargeables are more sensitive, ideally you need a coulomb counter but more practically speaking cutting the load at between 0.9V and 1V will do.

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Looks like your getting the idea, but you still have a few problems with the circuit. I recommend the following changes:

• Your LED is in backwards
• Move Vref to + pin of amp
• Vsense to - pin
• For Vsense just use a pot instead of divider, then you can adjust your set point.

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One more thing. Your schematic symbol for the 139 is all wrong, Pin assignments. I assume you know that however.

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Are you trying to flash an LED when your 6 cells in series drops to 6V?

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If he does the things that I have highlighted in red, the LED will be on until the voltage drops below Vref. I don't think that's what he wants. Of course, he really wants a flasher anyway, so an LED on the comparator output ain't gonna work anyway.

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