I recently recalled an interesting security key for a building car park at a block of units we stayed at a few years ago. It was a small block of units with maybe 10 – 20 units in the whole complex, probably built in the 1970s. Underneath was a carpark with a roller door to prevent unauthorised access. A small box stood near the entrance with a receptacle for the key.
In other places we have visited we were usually given a remote control to gain access or sometimes a card to insert. This place had a 6.5mm audio plug attached to a key ring to insert in a socket near the door.
I found it particularly curious as the plug only had two connections, that is it was a mono plug. I wondered how they had implemented security with it. I unscrewed it to reveal a deceptively simple circuit… a wire.
I have to give the creators full marks for simplicity, but not so many for security.
Garry's blog 
Ha, love it! Security by obscurity is generally frowned upon, but it sometimes does have its place. But now that you’ve exposed the secret, I’m going to keep my eye out for 6.5mm jacks near doors 😉
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Hi Ken, I would like to know the story about how it came to be there. Was it installed by a professional that was expected to do it cheaply or was it the owner that had some knowledge of electronics. I guess it’s a simple solution 🙂
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There is a mobility device that I’m leaving nameless that uses quarter-inch jacks and provides the owner with a removable key. One airline transferred the scooter but not the key. I was trying to remember if it was mono or stereo and thinking about what electronics could have been baked into the fob and preparing to go full 007 “Q” on the problem to try to rescue a vacation. I made a trip to the office and tried a longshot.
Jamming a pencil into the hole worked.
They didn’t pass any electrons through the key at all. They used the insertion detection (the one that normally routes ground to the amp/speaker until a plug is inserted and then directs it to headphones once the plug is fully inserted) as the key. Simply mechanically separating the connectors is all it took. Vacation rescued!
Sometimes your “opponent” in security isn’t about stopping the person that can just pick the device up and swoop it into a truck; it’s about stopping a fifth-grader from joyriding your needed wheels.
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