Optocouplers or Optoisolators are devices that can pass electrical signals between two isolated circuits using light. Isolation between the two separate circuits implies that there are no direct wire connections linking both circuits. The only connection between the two separate circuits is through light. An optocoupler has a Light Emitting Diode (LED) at it’s input and a phototansistor or a phototriac at the output side; with both in close proximity inside the optocoupler package. When current flows through the LED, it emits light. This light is detected by a phototransistor or phototriac and converted back to an electric current. Current is thus transferred from the input to the output optically. Optocouplers find use mostly in circuit applications requiring the control of AC power mains from a low voltage circuit while protecting or isolating the low voltage circuit from the dangers of AC mains high voltage. Applications iincludes offline Switch-mode Power Supplies (SMPS), Controlling of AC loads or appliances using signals from digital components like a Microcontroller.