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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?

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  • \$\begingroup\$ my biggest fear is that it would in any way fail with the relay closed while nobody is home, [...] letting all the unwanted people in. Wouldn't it be better to post under a pseudoniem instead of an easy to google name (revealing where you live)? \$\endgroup\$ – Huisman Sep 27 '19 at 18:32
  • \$\begingroup\$ @Huisman hah, yes I did think of that, but I figured that if I wouldn't trust the device enough to post about its existence with my real name, I shouldn't employ it after all \$\endgroup\$ – Bart van Heukelom Sep 27 '19 at 19:02
  • \$\begingroup\$ What kind of relay do you have? If it is a spring-loaded SPDT or DPDT relay, you can just wire it so that it locks when the coil is de-energized. \$\endgroup\$ – Caleb Reister Sep 27 '19 at 20:49
  • \$\begingroup\$ Also, note that electronic locking mechanisms tend to give a false sense of security. Unless you are using some kind of rotating code or one-time key, it may be relatively easy to trick the system into opening the door; solenoids and relays can sometimes be manually actuated using a powerful magnet; or you can use this method. \$\endgroup\$ – Caleb Reister Sep 27 '19 at 20:53
  • \$\begingroup\$ The device on that circuit is probably an electric door strike. When powered from a AC supply they buzz loudly, some can instead be powered from DC for more silent operation (it makes about as much noise as a relay), you'd have to find and replace the AC supply. \$\endgroup\$ – Jasen Sep 27 '19 at 20:54
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You might be able to get away with something as simple as a PTC fuse. These increase in resistance as they warm up, and once they reach a certain point the resistance rises significantly, so that the current through them drops, but remains sufficient to keep them warm, so they remain in that state until powered down and cool off.

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image from here

You'd select one that would trip after a desired time with the circuit current. If you don't know what the circuit is doing that may be less easy, it sounds like your circuit is operating a door electromagnetic lock, so that'd be easy to protect this way.

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  • \$\begingroup\$ I think it should not be resettable: Something that, once triggered, can not be restored without physical intervention. I'd suggest a slow blow fuse. But I wonder how temperature dependent it is. \$\endgroup\$ – Huisman Sep 27 '19 at 18:54
  • \$\begingroup\$ They do vary in behavior with temperature, since the transition is at an absolute temperature, so the rise from ambient is variable, but that's usually tolerable in domestic environment. It will effectively be latching if the fail mode to be protected against is the relay contacts getting stuck closed. \$\endgroup\$ – Phil G Sep 27 '19 at 19:01

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