JohnRob:
Try changing R1 to 1100 ohms or so.
Eh?
He originally specified 220 Ohms. 
Why would increasing the value improve things?
I do agree that a Darlington is a bad choice at 5 V. A common eBay (opto-isolated) relay module would do the job perfectly well - if you trust the clearance at the contact terminals - the ones with the slot in the PCB.
People sometimes unfortunately stumble across the old, obsolete 4N25 opto-isolators. The ones in the eBay modules are modern versions with much more appropriate CTR.