I'm prototyping a device that allows me to electronically open my front door, which it accomplishes by using a relay to close an existing circuit in my house. All I know about this circuit is that it measures around 12V AC.
Now, I'm far from keeping this device permanently installed, if I ever will, but thinking ahead, my biggest fear is that it would in any way fail with the relay closed while nobody is home, making noise all day (opening the door makes a loud buzzing sound) and letting all the unwanted people in.
I've thought of all kinds of methods to prevent this, like putting the timing of the relay under control of a really simple 555 or ATTiny circuit. I'll do that regardless of any other measures, since security is best applied in layers.
Ultimately, however, I would only leave the house comfortably if there's a super simple and reliable safety measure I can install in series with the relay in the 12V circuit, that would cut off the connection if it's closed for more than a few seconds. Something like a timed fuse. Something that, once triggered, can not be restored without physical intervention. It may even be sacrificial, since it will only be activated if all earlier measures fail.
What kind of protection would you advise me here?