Opto-isolated input ground

[AdamSparky]

I have only just started learning to use optocouplers to provide circuit isolation.

I would like to use an optocoupler to protect my microcontroller from the outside world, everything is run from the one power supply as per the attached circuit. I have both sides of the optocoupler attached to a common ground, this feels wrong, but don't know how it should be done.

How is this situation usually handled?

[SeanB]

Ideally you want a separate supply, otherwise all you need is a simple one transistor inverter or a protected voltage divider into a buffer chip. You are negating the use of the optocoupler doing what you did.

[digsys]

There are reasons why people do it - some covered in this thread -
www.eevblog.com/forum/projects/design-microcontroller-circuit-for-industrial-noisy-environment/msg209648/#msg209648
In the simplest case, it offers a 2nd level of protection. Blowing up / then repairing an opto is a heck of a lot easier
than blowing up an MPU - in which case everything is dead.