Turning off circuit when battery is low

[Bach On]

NEWB alert:

The LM3914 is designed to drive LEDs to indicate the voltage in a circuit. As an example, a rechargeable battery's voltage can be shown using the LM3914 and some LEDs. Many have done this circuit.

But the datasheet for this device indicates it can power more than an LED. So here is my question.

If I wanted a device to automatically turn OFF a light when the battery voltage drops below 12 volts, could I determine the LM3914 pin that would drive the appropriate LED. But instead of having the voltage go to a LED, could I have it go to a relay (or a circuit that serves the same purpose) to turn off the light?

I have some home brew emergency lights (12 one watt LEDs) that turn on when we have power failure. It uses a simple 120 volt relay to connect the lights to a 12 volt battery when the power goes off. (The battery has it's own smart charger.) The lights go off when the the power comes back on. It isn't complicated. And it prevents my having to stumble around in the dark looking for flashlights and/or candles.

I don't want to shorten the life of the battery by draining it completely. So an auto shutoff to the light is needed. (Yes, I can do in manually - but why not make it automatic?). In an ideal world, I could also have a warning beeper or blinking LED to signal that the battery voltage had dropped to a certain point. This would prepare me for the imminent dark that will come when this circuit kicks in so I can get flashlights or candles ready.

I'm sure there are several devices that could accomplish my primary purpose, but the LM3914 provides the capability of indicator lights on the battery's voltage AND I believe could turn my LED lights off. (I'm not overly concerned about the drain of this circuit on my battery.)

I doubt a mechanical or solid state relay could be used because of the low current provided by the LM3914. But perhaps some sort of simple IC or a transistor or two could be used to turn off the LEDs (It draws right at one amp at 12 volts nominal). I'm open to any practical ideas. But I don't want a circuit with 1,396 parts to solder together.

Any input on this idea would be appreciated. I'm into easy.

Bach On

 

[k7elp60]

You can put an opto coupler on the output pin of the 3914. Connect the input to the opto coupler in series with the pin selected to Vcc. When the output pin goes low the opto coupler will turn on and you can use the output transistor of the opto coupler to switch a relay or better yet a power mosfet as you on/off switch. Be sure and use a current limiting resistor on the input of the opto coupler if necessary.

 

[MikeMl]

Here is my favorite "Disconnect below x Volts" circuit. The x-axis is the voltage of the discharging battery. The Y-axis is the voltage across the load as a function of various settings of the pot ranging from .3 to .7

 

[Bach On]

Thanks, guys. Some good approaches here. I'll explore both.

BO

 

[ethelknox]

How to Turn Off the Smoke Alarm Beep Read more: How to Turn Off the Smoke Alarm Beep

Although a smoke alarm is designed to beep as soon as it detects smoke, the beeping doesn't always detect an emergency situation. Sometimes the beeping results from cooking smoke or a problem with the alarm from user error or malfunction. An incorrectly fitted cover, poor battery installation, low battery, alarm malfunction or issues with the backup electrical source can cause the alarm to beep repeatedly. To turn off the beeping in any of these situations, you simply need to disconnect the power source.

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[Bach On]

Uhmmm. Maybe I've missed your point. But I'm not certain what your posting has to do with what I'm doing. Are you saying I should not automate cutting off power in the system I've setup and should, instead, do it manually?

BO