Q. Explain the working principle of an opto coupler?

When it is necessary to block the voltage between one electronic circuit and another, and transfer the signal at the same time, an amplifier coupling capacitor is often used as shown in figure given below. Although this method of coupling does block dc between the circuits, voltage isolation is not complete.

A newer method, making use of optoelectronic devices to achieve electrical isolation, is the optical coupler, shown in figure given below. The coupler is composed of an LED and a photodiode contained in a light-conducting medium. As the polarity signs in figure show, the LED is forward biased, while the photodiode is reverse biased. When the input signal causes current through the LED to increase, the light produce by the LED increases. This increased light intensity causes current flow through the photodiode to increase. In this way, changes in input current produce proportional changes in the output, even though the two circuits are electrically isolated.