The purposes of opto coupler are discussed below.
An opto-isolator, or optocoupler, or photocoupler, is a component that transfers electrical signals between two isolated circuits by using light. This provides complete isolation between automatic object detector and doorbell circuit. The component used is 4N27, which the manufacturer description number.
The above sketch is internal schematic diagram embedded in 4N27 IC. The reasons why we isolated the two circuits are;
1. To provide different supply 6V voltage to doorbell buzzer using 3xAAA batteries.
2. To experiment the use of opto coupler