Introduction: Multiple LEDs on a Single Resistor

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This Instructable will show you how to calculate the required values to connect multiple LEDs with a single resistor vice an individual resistor per LED.

Step 1: Using Ohm's Law

Ohm's Law dictates the relationship between voltage, current, and resistance. It is defined as the following equation:

Current (I) = Voltage (E) / Resistance (R)

In a circuit, all voltages gets used up, and each component offers some resistance that lowers the voltage. Knowing this, the above equation comes in handy for things like figuring out what resistor value to match up with an LED.

Step 2: Understanding Parameters of LEDs

LEDS have predefined voltage drops across them and are designed to operate at a particular current value.

Typical Maximum Current Value of an LED : 20mA (0.02 A)

Predefined Voltages for LEDs:

Red and Yellow: 2 Volts

Green and Blue: 2.9 Volts

White: 3.3 Volts

Step 3: Determining Values to Create Circuit

Remember Ohm's Law? We'll be using it to help determine how many LEDs we can get connect in Series (CONNECTING LEDS IN PARALLEL WILL NOT WORK). This is dependent on the voltage source. Determining resistance helps us ensure we do not exceed 20mA.


You have a 9 V Source and a handful of RED LEDs. Determine how many LEDs you can pair in series and determine the resistance value required.

So here's the information we do know:

The maximum current allowed is 20mA.

Red LEDs have a voltage drop of 2V.

Our supply voltage is 9V.

First let us determine the resistance required.


I = V / R => 20 mA = 9 V / (R) => R = 9 / (0.02) = 450 Ohms

Therefore you need a minimum resistance of 450 Ohms to prevent damage to the LEDs. You should however, pick a slightly higher resistance value to ensure you do not exceed 20mA.

You should also note that the higher the current the brighter the LED will be.

Second, let us determine the number of LEDs we can connect in Series. This is dependent on the voltage source.

The first rule we must apply is that the voltage drop across the LEDs combined should not exceed 80% of the voltage source. Therefore since we have a 9V source, we can only have 7.2V combined voltage drop of the LEDs in series.

We do know that each red LED has a 2V drop. Therefore, we divided are maximum allowable voltage drop by the voltage drop of a single LED.

# of red LEDs = 7.2V / 2V = 3.6. YOU WILL ALWAYS ROUND DOWN!!!!

Therefore, you can connect (3) red LEDs in Series.

Step 4: Build Your Circuit

Now, you determined the resistance value of the resistor and how many LEDs you can connect.

Build your circuit in accordance with circuit diagram above.

Step 5: CHEAT SHEET!!!!

If you want to skip the reading, here's a cheat sheet.

You can also combined different color LEDS as well.Ensure the calculations are made prior to building in accordance with Step 3.


Thanks again for reading!!!!!