Rotary Phone Desk Lock
This was a fun little project I put together one afternoon over my Christmas holiday. As a proof of concept, I built an electronic desk lock out of an old rotary phone I had sitting about. I pick up the phone, dial the combination, and the latch for my desk cabinet's door turns. Simple right? Turns out, yeah, it's very simple.
As the dial spins, a switch is opened and closed repeatedly a number of times indicating which digit you dialed. Using an Arduino microcontroller I keep track of the last few digits that have been dialed and the telephone hook state. When the digits being tracked match a combination, the associated instructions execute. In this case turning a simple latch.
While the latch configuration I use can easily be broken, the phone takes up a fair amount of desk space, and dialing takes an impractical amount of time, I am now one step closer to the dream of having secret passages in my home.