Working with LEDs
An LED is a special type of diode which emits light when current is passed through it. Like all diodes it is designed to pass current in only one direction. If the LED in this circuit was reversed it would not be damaged but it would also not light. One of the important points to remember about LEDs is how to insert them correctly in the circuit. The short lead of the LED is the cathode and must be oriented so as to be pointed toward ground.
Current Limiting Resistor
The current flowing through most small LEDs must be limited to 20mA (0.020 A). In this circuit a current limiting resistor is used to keep the current flowing in the LED to a safe level. The current limiting resistor is selected using the following equation:
Any value greater than 65 Ohms would limit the current appropriately. A value of 150 Ohms will limit the Current to 8.7mA.