OpenHAB connected doorbell

I wish to connect my doorbell to my OpenHAB server, without interfering with the current installation. I will be using an optocoupler for this purpose.

What I have:

• A Honeywell D780 transformer that converts 230-240V AC @ 50Hz to 8V to a maximum of 1A.
• A couple of opto-couplers marked 'NEC 2561 WK722'