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Battery Level Indicator – Easy/Moderate

Overview / how it works:

This circuit uses a LM3914N LED controller to give a visual indication of the level of a battery. Although the LM3914N is a fairly complex chip the principle of how it works is simple, it reads in a voltage and uses that voltage to determine how many LED’s to light. In this way we can use it as a voltage level indicator!

Building the circuit:

The parts list is as follows: A pack with all these parts is here

Ω      One LM3914N integrated circuit

Ω      Five LEDs (two green, two yellow and one red)

Ω      One 120Ω resistor

Ω      One tactile switch

Ω      One 82Ω resistor

Ω      One 5.6KΩ resistor

Ω      One 82KΩ resistor

Ω      One 470KΩ resistor

Ω      One 9V battery clip

Ω      One 9V battery

Circuit Diagram: Above is the schematic for our circuit! You can see the LM3914N connected to 5 LEDs and the 82Ω resistor from power to ground (via a switch). You can also see the voltage divider into pin 5 consisting of the 470KΩ resistor and the 82KΩ resistor. The 5.6KΩ resistor and the 120Ω resistor pull pins 4, 6 and 7 to ground. The 9V battery is what we are getting the level reading from.

Further Detail:

The LM3914 has18 pins on it, of which 10 can drive LED’s (in our circuit we only use 5 but you can add in more if you want!). The other 8 are for determining what to do with the LED’s.

Pin 2 is the V- pin (or the ground pin). This is what connects to the negative terminal on the battery.

Pin 3 is the V+ pin (or the power pin), we need this for providing power for the chip to work.

Pins 4 and 6 are used to make a voltage divider with pin 5! However we connect both of these to ground through resistors in our circuit and use an external voltage divider on pin 5.

Pin 5 is the pin we use to determine the number of LED’s we want on! We provide this with a voltage (through a voltage divider like in our transistor night light) and it uses that voltage to decide how many LED’s should come on.

Pin 7 provides a reference voltage. Although this is not used in our circuit, in some it is used along with 4 and 6 to determine the input signal voltage.

Pin 8 is used to adjust the voltage pin 7 outputs at, in our circuit this is just connected to ground.

Pin 9 is used to set which mode the chip is running in. There are two modes: single LED mode or cumulative (building up) LED mode. In singe LED mode only one LED is lit up at a time and that ine is changed with the input voltage, whereas cumulative LED mode (the one we use) has LED’s come on in sequence. To set it to single mode you connect it to ground, and cumulative mode you connect it to power.

So this chip can give us an idea of the battery level, but in operation a circuit will draw current from the battery and its level will change, so how do we find out what its level is like when running? Well because the chip doesn’t draw much current at all we have to add in a part that will, for this we use an 82Ω resistor connected between power and ground.

As we know from ohm’s law I (current) = V (voltage) ÷ R (resistance), so if we have 82Ω of resistance, and a 9V battery the current draw can be calculated.

I (current draw) = 9 (from the 9V battery) ÷ 82 (from the 82Ω resistor) = 0.110A or 110mA.

This is fairly normal current draw for a small circuit so this gives us a fairly realistic idea of the battery’s level when in use.

Circuit Build Images:        