# Low Battery Voltage Indicator

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In this instructuble you'll learn how to make a low battery voltage indicator. There are many ways to make one, you can use a 741 IC, 555 IC, 8211 IC or a SCR, but here we will learn how to make one using a transistor. The basic principle is that we use the transistor as a switch i.e the transistor flips between the ON and OFF position according to the battery voltage level.

The advantage of using a transistor is because of its low current consumption compared to IC's. A IC 555 would consume around 5mA, a IC741 around 3 mA, while the present circuit would just consume around 1 mA current.

Another advantage of this circuit is it's ability to work even at voltages around 1.5V which gives it a clear edge over the IC based circuits.

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## Step 1: Components Required

My design takes in 2 inputs. One is a constant 5V to power the two LED's and the other input is the battery whose voltage you want to check. This way the intensity of the indicator LED does not change with change in the battery voltage you want to check.

The components required are:

1. BC 547 transistor
2. Resistors (x2)
3. LED (Red,Green)

The 2 resistors are used as a potential divider. The aim is to create 0.7V at the base of the BC 547.

Following is the calculation required to find the values of the 2 resistors:

If V is the voltage at which you want the LED to indicate low battery, then,

1.42*V=1+(R1/R2)

Choose appropriate values of R1 and R2.

## Step 2: Circuit Diagram

Once you have calculated the values of the two resistors you can go ahead and set up the circuit. I designed this indicator for a robot i'm working on so the 5V comes from the microcontroller(Arduino) output.

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## Discussions

I would like to make a compact and simple circuit which either turns a small LED on or off at 0.7v. It's for a single AA electric toothbrush as a battery change indicator and would get power only when the brush is on. Because the motor slows gradually, I can't easily tell when it's time to change batteries due to the sound. Thank you.