LED circuit need help...

[Keenox]

I have 2 rechargable batteries at 1400 mAh each. I want to make a circuit with 3 LEDs that will show me the charge of the batteries i.e.: when they are almost discharged only the 1st LED will light, when they are medium charged the 1st and the 2nd LED will light and when they are at almost full charge all 3 LEDs will light. Need some ideas and help :? .

Thenks a lot

 

[stevez]

It sounds like you want to indicate the battery voltage and use that as an indicator of charge. This may not be a very precise method of determining charge but it's relatively simple and it's what lots of people do in the absence of better methods. Just a note of caution - in reviewing information on batteries I'd conclude that the voltage vs % charge curve, in the useful part of the curve, is not that steep suggesting that small changes in voltage imply larger changes in charge level. Battery temperature also affects the voltage to the extent that a change in temp could lead one to an incorrect conclusion on battery level. I've seen projects that use a simple LM339 comparator. The battery voltage is input then compared to reference levels. The comparator changes output state when the difference between the reference and input voltage goes from - to + or the other way around.

 

[Uman]

Consider a simple IC Voltage Detector like a Fairchild KA75xxx. The IC's come in various voltage threshold levels and produce an output when that level is exceeded. Add one VC for each LED. Check them out here:
https://www.fairchildsemi.com/ds/KA/KA75290Z.pdf

Jeff in Florida

 

[audioguru]

If your two battery cells are Ni-Cad or Ni-MH then there probably isn't enough voltage to light an LED, especially when the batteries are low.
So you would need a DC to DC stepup converter circuit or a charge-pump IC.
The batteries aren't perfect so their voltage will change with load current. You don't say but if you are drawing an amp or more then they will probably indicate Low, even if they are fully charged.
Look at www.energizer.com and click on "technical info" in the upper right corner. Select your battery and you'll see that the voltage barely changes with amount of charge.

 

[timothyjackson]

Hallo Keenox,

Will your circuit have it's own power supply? If yes, it might make things easier. You could use a coin cell (v small/v cheap/easy to load). This could be powering the led read outs.

You could use small flashing leds to save on power. Put them with a "hood" to make them brighter.

As audioguru pointed out, the discharged battery may not drive any illumination.... but wouldn't this be an indication in itself?

I like this project, whats it for?

Timbo

 

[Keenox]

8) Hello every body,

10x for all answers 'till now... You are right, when the batteries are almost discharged, the leds will not light so it will be a useful sign.
Answer to the last question: I saw the latest Logitech mouse (and the first one that uses laser), and i liked the ideea of a battery indicator (it always bothers me when the mouse stops working suddenly, so i decided to "upgrade" it :wink: and mount a battery indicator :!: .

I tried to make this circuit with 3 transistors, but it didn't entirely worked... Any suggestions to use them???

Thanks ppl

 

[williB]

8) Hello every body,

10x for all answers 'till now... You are right, when the batteries are almost discharged, the leds will not light so it will be a useful sign.
Answer to the last question: I saw the latest Logitech mouse (and the first one that uses laser), and i liked the ideea of a battery indicator (it always bothers me when the mouse stops working suddenly, so i decided to "upgrade" it :wink: and mount a battery indicator :!: .

I tried to make this circuit with 3 transistors, but it didn't entirely worked... Any suggestions to use them???

Thanks ppl

this is an IR optical mouse..or something new..?
by IR i mean transmitting the signal to the pc..

 

[Keenox]

I have an wireless optical mouse....:>