Connecting a 3.3V powered arduino to a 5V signal could blow the pin in question, or even fry the
whole chip. Adding a current limiting resistor can reduce/eliminate the risk, but here the circuit you
are controlling probably needs a certain minimum current. An opto-isolator is an easy way to
do things if the current is small anyway, then you don't need common grounds. Or an NPN as
an open-collector inverter. For low current a 1k resistor from arduino pin to transistor base,
emitter to ground, collector to your door opener signal.