Nuts and Volts - July 2010

servo is simply glued to the key. The wooden box is approximately 10 x 14 inches at the base. The top is a “treasure chest” style and slants on the front and back sides. I mounted the display where I took a board out so it faces you; the power switch is located beside the LCD and controls power to the Propeller protoboard. This is shown in the opening photo.

The removed board is replaced by sheet metal that has holes drilled in it for the components. The servo is secured at the bottom by two strips of sheet metal and hot glue. The lock is secured in the same way, with the upper strip of sheet metal coming out of the edge of the box. This may not be the most secure fashion, but it worked for me. The protoboard is secured with screws and the RFID module is secured with hot glue.

I like to constantly modify and upgrade my projects, so the circuits are all built on a breadboard that is fastened to the protoboard. The protoboard has female headers soldered to the pins for easy access. The RFID reader is plugged into the breadboard and a serial connection made with the Propeller chip. The servo and serial LCD are connected with headers straight to the protoboard, as well.

Software Overview

The software is super simple so it can be adapted and modified easily. It is written entirely in SPIN — the native high level language of the Propeller (as opposed to the native low level language PASM) — and takes advantage of

How to Build It

1. Find a wooden or plastic box that has a hinged lid. If you use a metal box, you will need to either mount the RFID reader on the outside or cut a hole so that the metal is not in the path of the RFID signal. You will need some sheet metal, Plexiglas, or acrylic to secure the lock and controls to.

2. Get a “rope” type pistol lock. Since the circuits on the top of the box make it heavy, the rope on the lock helps keep it from tipping over.

3. With the key in the lock, place it in the box to make sure it fits. The top of the lock should be level with the edge of the box. I taped everything down while I secured the servo to the key (see Figure A). I then screwed a small “loop” screw into the servo and ran a screw through the loop in the lock and the loop screw. (Look at Figure A again to see what I mean.) I glued down the servo and the key and servo joint. Figure B shows the completed locking mechanism. I did use a strip of metal over the top of the lock to prevent the lid from being strained so hard that it pulls the lock right off the servo.

4. Solder or assemble the circuit on a breadboard. On the RFID reader, connect VDD to 3.3V and VSS to ground (marked VSS on the board). Connect the “\EN” (Enable) to P1 and “SOUT” to P0. Connect the “5V” to the 5V line on the LCD. (This can be found right below the power regulator to the far right.) Connect the ground to VSS and the signal line to P8. Connect the red wire to 5V on the servo, and the black wire to VSS. You should place a resistor between the servo control pin and P4. Since the Propeller is a 3.3V device, there could be certain voltage spikes that might damage the microcontroller. (This is a precaution that can be omitted if you are using a chip with a rating of 5V or more.) Connect the pushbutton with one end to VDD and the other to P16.

■ FIGURE A.

■ FIGURE B.

6. Remove the top from the hinges if possible. If not, then be sure to measure carefully. Line up the end of the rope (from the lock) with the newly secured lock. Make sure the notched side lines up with the hole in the lock. Fasten this down with whatever you like. (I went with “U” nails used in roofing and bent nails to hold it in place.) Make sure it fits properly before moving on. (See the top right corner of Figure C.)