daftmike Low Battery Indicator

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daftmike Low Battery Indicator

Post by daftmike » Fri Jul 10, 2009 5:40 am

Here is a small circuit I made for a low battery indicator. I posted it on the other site and ended up making a guide of sorts, hopefully some of you guys find it useful.


The green led(led1) is the power indicator, it is on as long as the circuit is powered. The red led(led2) will come on when the battery voltage drops to a level set by the resistors.
For my purpose I'll have it set at ~6.9V, I will use the following values:
R1= 2K
R2= 1K2
R3= 560R
Transistor= BC547

The green led will have around 2V across it when it's lit, the BC547 has a base-emitter voltage of 0.6V. This means that the transistor will need around 2.6V at the base to turn on.
Resistors R1 and R2 form a voltage divider, the point between them will be at a voltage of Vin*[R2/(R1+R2)] when this voltage drops below 2.6V the transistor will turn off and the red led will come on. R3 limits current to the leds.

Using the above values, the circuit will draw under 10mA.

I would recommend using a 2K variable resistor for R1 and make R2= 1K, this will allow the adjustment of the low battery point to values below ~7.8V.

To modify this circuit to use different colour leds or to use a different transistor you will need to know the base-emitter voltage of your transistor (VBE), this will be found in the datasheet and is normally around 0.6-0.7V for most low power npns. You also need to know the voltage across your leds, blue and white leds are normally ~3V.

1.)Add VBE to Vled1, call it Vtrigger.

2.)Choose the voltage you want the low-power led to come on, call it Vlow.

3.)Choose R3 to limit current to the leds, any value ~500-1K is fine.

4.)Choose R1,R2 values such that; Vlow * [R2/(R1+R2)] = Vtrigger, start by making R2= 1K and work out a value for R1, adjusting both if necessary to find common values. They need to be 1K or higher to limit current at the transistor base.

5.)Alternatively make R2= 1K and make R1 a 2K pot in series with a 1K resistor, this will allow you to adjust Vlow to any value ~11-3V.

I've written a little spreadsheet that will work out all the values for you, it's in excel format and is linked below.
http://www.mediafire.com/?sharekey=3c28 ... f6e8ebb871

daftmike's low battery indicator mk2

(the green led is "battery ok" the red led is "battery low")

The transistors can be any npn type but should all be the same, the leds can be any colour, and the potentiometer can be changed for a 2k or 5k etc.
You should adjust the pot until the low battery light comes on when the desired low battery voltage is applied to the circuit.

This circuit has a few improvements over my original:
It only lights one led at a time, a green led for battery ok will turn off when the red led for low battery comes on (or whatever colours you choose),
It can use any colour led without changing the circuit.
It is much more precise and will respond to changes of only a few millivolts.
You can set your own low battery voltage without calculating the resistor values.
It will work down to a 3v supply
Last edited by daftmike on Sun Nov 29, 2009 11:25 am, edited 1 time in total.

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Re: daftmike Low Battery Indicator

Post by eurddrue » Fri Jul 10, 2009 8:13 am

Odd, this looks a bit familiar :lol: Great job, and we have proof that it works :wink:
Banned indefinitely if you desperately need to contact me STOPPHONESPAMPLOX Please dont be a dick and call for something random like "HEY YURDRUE DOO U HAZ SPAM?"
wallydawg wrote:
chainfire95 wrote:220V I believe
I think we should check to see if you can withstand 220 volts

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