Calculate the Value of the Current-Limiting Resistor

• 1). Obtain the specifications for the LED, either from the place of purchase or the manufacturer's datasheet online. LEDs are usually very similar, so if you cannot find the details for your exact model of LED, you can use the data from an LED of the same size and color. The specifications required are the LED's forward voltage, Vf, and the required current, which is usually given in milliamps.
  
  
• 2). Calculate the value required for the current limiting resistor, using "Ohm's Law" that states that voltage equals current multiplied by resistance, or V = IR. For example, to power a red LED with a forward voltage of 2 volts, requiring a current of 20 mA, from a 9-volt battery would require a 7-volt drop across the resistor. Using Ohm's law, this gives: 7 Volts = 0.02 Amps x Resistance, which can be rearranged as: Resistance = 7 volts / 0.02A = 350 ohms. So the required resistance for the current limiting resistor is 350 ohms.
  
  
• 3). Select the nearest available resistor to the calculated value. Resistor values increment in steps, so usually the exact value needed for a resistor cannot be found. As the values aren't critical, you can use the next available resistor that is higher than the calculated value. If the calculated value is 350 ohms, the next available value that can be used is 390 ohms.
  
  

Wire Up the LED

• 1). Insert the LED into the holes on the breadboard. Each of the LED's leads should be on a separate row, as all holes in the same row are connected together.
  
  
• 2). Cut a piece of wire and strip a small amount of insulation from both ends. This will provide the positive connection from the battery and should be inserted into a hole in an unoccupied row on the breadboard.
  
  
• 3). Plug one end of the resistor into one of the holes in the same row as the positive battery wire. Plug the other end of the resistor into a hole in the same row as the positive lead of the LED, called the "anode." The anode lead on the LED is always the longest lead of the two.
  
  
• 4). Cut another piece of wire, and strip some insulation from each end. This will provide the negative connection to the battery. Insert the wire into a hole in the same row as the negative lead of the LED, called the "cathode." The cathode lead is the shortest of the two leads.
  
  
• 5). Attach the wire connected to the resistor to the positive terminal on the battery. Attach the wire connected to the LED cathode to the negative terminal on the battery. This completes the circuit, and the LED will light up.