[HSP] got tired of locking his door with a key, so he decided to upgrade to a keypad system which he’s designed himself.
It uses an Arduino Mega with the standard 44780 display, a standard keypad, and the “key override” (shown above) for fun. The locking mechanism is a standard 12V actuator based lock which was modified to run off of only 7.5V, by softening up the spring inside and running it upside down (as to let gravity help do the work). The whole system draws less than half a watt on standby, and engaging the lock peaks at only 4-7W.
What’s really clever about this design is how he locks it from inside the room. He’s programmed the Arduino to write 1 to address 128 of the EEPROM — at power on it will increment this by 1, and after 5 seconds, it will reset to 1. This means it can detect a quick power cycle, so you can lock the door by turning it off, turning it on for a few seconds, and turning it off and on again — he did this so he didn’t have to make a button or console, or any kind of wireless control on the inside.
Now we know that kind of adds a huge flaw to the overall security of the system… but [HSP] learned his lesson last time he built something “too” secure.
The relay board is inside the box on the wall on the outside, and the lock is only locked with power. This is insecurity by design. This is to keep the casual people out. The door itself is thin wood with cardboard in between. I previously had a lock which was locked on power failure, and the machine (Windows) running it, crashed. I got to climb through the roof window, 7 meters up to get inside without trashing the door, so now I have a little respect for the possibility of failure, and design my systems for the expected threat-level.