Few realize how close an optical coupler resembles a relay. It is so close ... Note that the optocouplers must have a CTR (current transfer ratio) greater than 100% or else the circuit cannot latch.

In this quick Arduino tutorial ... to introduce a delay in your circuit. The variable “stayON” is used to delay() the program execution with the desired amount of time. In our case after the button is ...

1. The well-established relay-driving circuit uses an optocoupler to isolate the grounds and noise coupling between the low-voltage digital-control signal circuit (left) and the relay as load (right).