Our dorms offer the unique benefit of having full length mirrors on the doors so I figured that suction cups would be the best way to go as they were easy to apply, remove and are non marking.
Being that I already had a Basic Stamp, Parallax's RFID reader seemed to be the obvious choice as it was the cheapest reader I could find and already had example code written for it.
My initial plan involved just a servo, standoffs, and suction cups...
Modeled in Autodesk Inventor...
After some of the parts came in I set up a quick prototype to make sure the servo was even strong enough to flip the latch and that I could read the tag through the door. To attach the servo horn to the latch I tied a piece of string to the horn and taped it to the top of the latch, this is how it done in the final version as well. The reason for using string is so that you could still easily unlock the door with the key, it would just put slack in the string. The only fault with this method is that you cannot use the RFID tag to lock the door.
I quickly realized that I had given no thought as to how I was going to mount all the electronics. As a result the servo remained like that on my door for weeks. As I was cleaning my desk one day I found this box and instantly realized that it was perfect for the door unlocker.
Another quick mockup in Invetor to see if everything fit, and it did perectly..
Ordered some 6-32 screws and nuts from McMaster and started assembly.
The servo fits perfectly height wise, it sits against the bottom of the base when closed. Now that I had the servo and RFID reader mounted I needed to get some holes drilled for the suction cups and basic stamp. Due to my
I finally got around to drilling the holes and mounting the whole thing...
I used paper under the basic stamp to insulate it from the metal box.
The assembly did not go without problems, however.
The first major problem was when I went to reassemble the box after I put on the suction cups. I didn't take into consideration that the nuts would get in the way of the box closing. As you can see in the pictures I had to snip spaces in the box where the nuts were so the box would be able to close all the way.
The second major problem was a weird one. The RFID reader was originally going to be mounted on the inside of the box. This box was perfect because the front was plastic and didn't interfere with the reader. The problem was, when I closed everything up in the box the range of the reader was almost cut in half and I couldn't read the tag through the door. However, if I pulled the box apart (like it is being shown in the above pictures) the tag could be read through the door. My best guess is electrical noise is causing this but I honestly did not want to spend any longer on this project, and there seemed little alternatives. I ended up just mounting the reader on the outside of the box. It does not really make any difference other then looks.Here is how it looks when it is all up and running.
The basic stamp program can be downloaded here