You can easily attend a Telephone call in darkness using this White LED lamp. When the Telephone rings, the White LED flashes in sympathy with the rings. So sufficient light will be available to locate the phone to attend the call. This circuit is provided to explain the working of Optocoupler.
The circuit uses an Optocoupler IC MCT2E as the inter phase between the Telephone line and the White LED lamp circuit. Optocoupler is a combination of an LED and Phototransistor enclosed in a light proof case. So when the LED inside the Optocoupler lights, Phototransistor conducts.
Normally in the ON hook position , the telephone lines have 48 volts which drops to 12 volts in Off hook position. Normal voltage will not pass into the Optocoupler due to the presence of C1. When the Telephone rings, AC signals passes through C1 and the LED inside the Optocoupler flashes. Phototransistor inside the Optocoupler then conducts and triggers T1. White LED then flashes according to the rings. When the phone is in Off hook, White LED turns off.