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,
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
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
7. Cut the positive wire on
the battery holder at midpoint.
The positive has a white stripe;