# 9V Low Battery Indicator Circuit

This is a very simple 9V low battery indicator circuit which has 2 LEDs, one green which will light up when the battery voltage is higher than 6.9 volts and one red LED which will light up when the battery voltage is bellow 6.9 volts.
You can use BC547 … BC549.

## Low battery indicator circuit diagram

##### Related Tutorials

• Len Jeffrey

I have built many self-designed 9-volt battery testers using 3-pin bi-colour LEDS. Very few components. If interested, email me at computerlen@hotmail.com and I will send the schematic to you.

• Geoff

FWIW I never did get the above example to function. I found this which has been great though: https://freecircuits.wordpress.com/2011/04/22/low-battery-indicator/
I’ve used it as a battery warning on guitar pedals etc and find it’s doing the job great. The LED stays lit if the voltage is low, but goes out if it’s okay.

When I built the circuit as is could not make it work. Red LED stayed on continually. Green LED went off under 6.2Volts.
I replaced the 47K resistors connected to the gates of both transistors with 10K resistors and the system works perfectly. (I am using BC547 transistors) Maybe there is a misprint and the 47K resistors should be 4.7K?? Hope this helps those who can’t get this project to work

• Jose G

MikeNovo’s, works like a charm!

• MikeNovo

If I may suggest the following. I hope I can explain this. While opamps as comparator are a better option zeners are OK to use as well. I have a different but similar circuit. The zener breakdown voltage together with the forward voltage of the led you use will determine at what voltage it will switch. In my case I used a 5.6v zener and a 2v led giving me 7.6v. Here is how the circuit goes.

From the positive rail, attach a zener followed by a 1k resistor followed bya green led to ground. If the voltage is higher then about 7.6v the green stays lit due to the zener being reverse biased. When it is to low, the zener prevents the flow of current to the led and it is off. This is your power good circuit.

Now on the the red led. From the positive rail, add a 1k resistor followed by a 5.6v zener followed by a 1 k resistor to ground. Attach the base of the npn transistor just below the zener. Attach the emitter to ground. Attach the collector to a 1k resistor which will go to the positive rail and to an led which will go to the negative rail. When the voltage is above 7.6v, current flows through the zener to the base of the npn turning it on preventing the red led to turn on. When voltage is too low the zener is no longer reverse biased, turning off the transistor and current flows through the red led. This is your low power circuit.

Give it a try on the breadboard and feed various voltages via your power supply. It’s very simple.

M~

• Geoff

Hi Garry – glad you got it working. I swapped 4.7k resistors for the 47k ones and see no difference in this circuit with the Zener in either orientation. I am also using BC547.

Is there any other change you made?

• tg

Hi,

Does not work. Waste of time and money..

• Jim

Can this be modified to use a 2 pin bi-color LED to be green when power is on and turn red when battery is low?

• MikeNovo

Bicolor led’s just share the same cathode (negative)

• P. Marian

It’s an interesting request but now I do not have a bicolor LED to test it 🙁 It can be done easily if the LED has 3 pins with a common cathode and 2 separate anodes, it just an LED with 2 leds inside of it.

• Sushant Ranjan

Sir, can you explain how you came up with the values of resistors by calculations ?

• KROKKENOSTER

Just remember guys the circuit’s “Ground is NEGATIVE and the supply is POSITIVE That line at the voltage indicator is wrong it gives the impression that the top is negative and it is positive NPN transistor switches are allways Positive on collector

• Sunil Kumar

dear sirbcan u tell me please why ground is needed here .and if i do not ground it what will be the result?

• Geoff

Did you ever get the chance to revise this circuit? I just constructed it using BC547 transistors and found (similar to other posts above) that the green LED remains lit right down to 3V and the red LED is lit very dimly most of the time the green one is.

• JKPieGuy

Couldn’t you use a Bi-color LED with this project instead of using two different LED’S? I was just wondering because, it doesn’t sound all that difficult to make. Yet again, if it was that easy then I wouldn’t be asking.

• ANUP