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Battery life indicator for 1.5V battery source Answered

Hi guys,

I want to make a simple battery life indicator for a device running from a 1.5V battery source.

I would just need a red/green LED to light when the Voltage is above a certain limit (say 1.2 V). and to turn off once the limit is reached..

Is there any simple circuit that enables me to do this?


Not a simple one.

To light a red LED (and even more a green one), you'll need a kind of joule-thief to increase the voltage. Red LEDs need 1.6V, green ones about 2.2V.

How big is your battery? A small AA cell, or a big D one? For a small cell, the indicator circuit will drain your battery quickly.

Why do you need the indicator? As long as your device is running, voltage is fine, when it stops, you'll need a fresh battery.

It is a AA battery.

The problem is that the device is not running continuously.. It only rotates a container at intervals using a timer. So there is no indication as to whether the battery is dead, unless I turn a knob manually to see if the container rotates.

That is why I want to do the indiacator..

Any way it can be done?

First, you'll need something (like a joule thief) to increase the voltage, if there's just 1.5V around.

Second, if the LED is attached permanently to the battery, there will be no need to check. The battery will be empty sooner than you'll like it.

So, the easiest thing would be a joule-thief (there are some around on this site) that is operated by a push button. So you'll still have to press a button to check the battery, but you will not have to rotate the container.

An automatic option would be a astabile multivibrator with a very low duty cycle, like 99% off and 1% on. Think of something like the indicator in a smoke detector - a short flash every minute. Can be done discrete with transistors or a NE555 (or two). te trick is to make it really low current. I'd go with the push button. 

Oh.. What if I add another 1.5V battery to my device and make it run using that.
So, now my battery source is 3V and is there any simple battery life circuit I could come up with now?