Friday, February 08, 2008

Solenoid Lock -- Overview

First let me preface this by saying if anyone stumbles upon this who isn't friends or family, this was my first attempt at a real hardware hack, so be gentle ;)

  • Basically it locks a door using a pull type solenoid. Pretty simple, the only caveat is that I wanted to use the doorknob as a combo lock. So it works by turning the knob until you see the appropriate number, holding it on that number(The period next to it lights up when it's considered inputted), and keep doing this for all numbers in the combo. Once it's done correctly the lock opens.
  • I did make a much simpler version as an intermediary, that I think anyone could do in a couple hours.
  • Electronics -- This consists mainly of the display, relay, power, solenoid, and a PIC to drive the whole thing.
  • Hardware -- This is a modification to the door handle to have it turn a trim-pot and an eye bolt used to catch the solenoid's piston.
The Good:
  • It works without any problems. Doorknob is responsive, locks solidly, and will allow you to unlock it twice in succession with a single correct password(So it can be shut again)
  • Minimal damage to the door, only required 2 small screws into the frame to hold up the solenoid.
The Bad:
  • Three power supplies! It could easily be done with 2, I just didn't find one lying around that matched what I needed. I'm sure there is a way to do just 1, but I didn't spend enough time get it.
  • The trim-pot is held on with hot glue, which I don't trust but hasn't given me trouble so far
The Ugly:
  • The wiring is horrendous, since it has wires sticking out from the door, to the wall, and too the door handle from the inside. I used a phone jack even to plug the PIC into the trim-pot, adding to the mess further. It's all tucked away fairly neatly now(Except the phone wire), but is still horrible.
Future Improvements:
  • Hook up the trim pot by using something that clamps to the sides, instead of keying a dowel and gluing it on.
  • Put everything inside the door itself so it looks much neater. This would mean swapping the positions of the solenoid and the eye screw. I didn't want to harm the door itself is why I didn't do this, since it is the 'correct' solution.
  • Use 2 solenoids so it could be defeated by simple removing the hinges
  • Use one power supply
There is a video of it running below, the password being used is 1B50. It is hard to tell what's going on unfortunately since the control box is on the ground :(

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