Always Misplacing Your Keys? You Can Fix That With Some Logic Chips

Every time he came home, it was the same thing. As soon as he crossed the threshold, his keys just disappeared. There was no other logical explanation for it. And whenever it was time to leave again, he had to turn the house upside down to find them.

One day, [out-of-the-box] decided he’d had enough and built a door-activated alarm system out of stuff he had on hand—a decade counter, a cheapo reed switch-based door alarm, and some transistors. When the door is closed, the decade counter’s output is set to light up a green LED. When he comes home and opens the door, the reed switch closes, triggering the decade counter to shift its output to the next pin. The red LED comes on, and NPN transistor grounds the piezo, sounding the alarm. The only way to stop it is by inserting a shorted 1/4″ phone plug conveniently attached to his key ring into a jack on the circuit board until he hears that satisfying click of safe key-ping.

For those times when immediately plugging the keys into the wall isn’t feasible, or if his keys should disappear before he has the chance, there’s a momentary on the board that will stop the symphony of robotic cicadas blasting out from the piezo. It’s also good for family members who don’t want to play along or haven’t yet earned their 1/4″ plug.

Be sure to check out the build video after the break, which is just through that door there. And keep an eye on your keys, eh? Hackaday is not responsible for lost or stolen personal articles. Should you lose them, we can only suggest making a new car key from the spare and printing replacements for any standard keys.

19 thoughts on “Always Misplacing Your Keys? You Can Fix That With Some Logic Chips

  1. This is quite clever. I like how simplistic it is. When I clicked on the article, I was expecting some kind of ESP32 or RFID nonsense that would have been a good POC, but this just takes the cake for functionality!

  2. He should just hang the main key from a string round his neck like kids used to do.
    Oddly enough kids/parents suddenly don’t seem to do that anymore for unknown reasons, causing kids to be locked out a lot or losing their keys, kids being kids and all. The ancient string technology was lost as humanity lost its mental acuity, instead we have president trump and smartphones.

  3. I put my keys in my pocket. Kind of a crazy concept, but it works for me.

    Also, somewhat related. Please don’t leave your keys right inside the front door in a bowl or on a hook, thieves love that.

    1. I’ve saw the same type of thing in a hotel in Paris a decade ago: when you came in the door, you had to put your key card in a slot on the wall that turned on electricity in the room. When you left, electricity turned off again. (And then you handed the key card to the desk clerk…)

  4. Just learn some self discipline for goodness sake! Have a place for your keys and always put them there when you get in, once you have done it enough times it becomes a habit and you do it without thinking.

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