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Low battery indicator Answered

Can someone please provide me with a schematic to turn on an LED when a 9v PP3 gets low (~<20%), no microcontrollers, the simpler the better. I am hoping you can do this with a few diodes, transistors, resistors, and maybe some capacitors, but I can't think how this would work.

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andy70707Goodhart

Reply 8 years ago

A 3-legged IC like that would be good, as I previously said, an 8-pin DIP would be too large for my application and would probably have to use flying wires as I am using stripboard, although it only does 4.6v. By then, the voltage regulator would have stopped drawing any power and the LED probably wouldn't even be on. The voltage regulator stops working at about 7v, so I want the low battery indicator to trigger around 7.8v. A 9v battery is considered 'dead' at around 7.2v anyway. As I said, I have already solved the problem using a transistor, 3 resistors and a potentiometer.

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Goodhartandy70707

Reply 8 years ago

Oops. sorry for the late response then.

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guyfrom7up

8 years ago

This can be accomplished by using a basic opamp and a zener diode, such as:

http://www.aaroncake.net/circuits/lowvolt.asp

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Goodhartguyfrom7up

Reply 8 years ago

Hey (waving) haven't seen you around for awhile :-)

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guyfrom7upGoodhart

Reply 8 years ago

haha hey! yeah I've been busy and stuff and I just stop by once in a while.

AP Classes suck, lol

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Goodhartguyfrom7up

Reply 8 years ago

Well, it's certainly good to know you are still around :-)

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kelseymh

8 years ago

Conceptually, I'd start with a joule thief circuit, so that the LED will work when the battery is low. Then, you add in a small relay in the normally closed (NC) configuration. You want a relay where the "minimum trigger voltage" is either adjustable, or is fixed at the value you want as a cutoff (e.g., 7.2V).

The idea is that as long as the battery is strong enough, it holds the relay open with some small power drain. Once the battery drops too low, the relay closes, and the joule thief kicks in to illuminate the LED.

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andy70707kelseymh

Reply 8 years ago

Thanks, I never really thought about that. Is there a way to do this with semiconductors instead as it is going to be part of a phone charger circuit and relays are large and a bit primitive.

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kelseymhandy70707

Reply 8 years ago

There are solid-state relays, which are neither large nor primitive :-) Charging cell-phone and other "smart" batteries is more complicated, as they have some built-in electronics of their own. You may need to do a bit more research (have you tried searching for "battery charger circuits" online?) to get up to speed on some of the details.

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andy70707kelseymh

Reply 8 years ago

I had a look at at solid state relays, but the trigger voltage is too high and they are quite expensive. I was hoping to do it with transistors. As fir the charger circuit, I will not be charging it directly, but I have a simple voltage regulator to go from 9v to 5v (USB).