I always wanted to be able to control things such as air-conditioners, lights, etc, remotely…but I never thought about putting the front door of my house under remote control until I saw this “knock to open” door hack on Hackaday. It’s a door that will unlock when the correct knock pattern is performed. After being inspired by the mechanical door opening system in that hack, I decided to do something similar to allow me to open my door with a cell phone! I mean c’mon, door locks with keys are obsolete..everyone uses them..they’re so old fashioned! They’ve been around for over 4,000 years. Yikes! It’s time for a little change.
This project would have been much simpler if I had an electronic door lock…but I didn’t! They’re expensive, so I didn’t want to buy a new electronic door lock. I wanted to open the front door of my house without “seriously” modifying/replacing any door components. I thought about using a servo/stepper motor to twist the door knob on the door, but their control circuitry is slightly complicated if you’re not using a microcontroller. So, I decided to use a simple, 24VDC, “pull solenoid”. They don’t have any complicated control circuitry and you just have to supply the required voltage to make them work. I simply fastened the solenoid to the door knob with a metal wire. When the solenoid is switched on, its linear motion twists the door knob and opens the door. Positioning the solenoid properly was probably the only mechanical challenge in building this door opener because the force applied by a solenoid, is theoretically inversely proportional the square of the length of the air gap. Thus, strongest force is generated when the air gap is smallest.
Now I’m thinking about building a password protected door by placing a keypad outside my house which will generate DTMF tones. A circuit on the other side of the door will check the numbers entered, and if the password is correct, it will open the door. However, there’s one little snag. Any nasty programmer would just love to use a computer to generate DTMF tones and crack the password using brute force…I would. :)