original position. I found that for my servo, 4/20 of a
second was the perfect time for a twitch correction.
Testing
Turn on your box with the outside switch. You should
hear the servo, and the screen should light up. If this
doesn’t happen, open the box and check the power light.
If the power light is not on, check your wiring around the
switch circuit and (I know this sounds crazy, but it could
happen) check to make sure the batteries are inserted
correctly. If the light is NOT on, then turn the power off
and check for a short circuit. If the light is on, then check
the servo/LCD connections. If it starts up with no
problems, let the second counter run down to zero. You
should have the box closed and you should hear the
servo. If you hear the servo but the box is not locked,
check the position of the lock and servo, and make sure
they are well secured. The problem
may be that the notched bar on top
is not going all the way into the
Tips and Mods
• If the servo doesn’t fit
underneath the lock, you will
need a taller box. I didn’t have
this option and my lock stuck up
above the edge about 3/4 inches.
I took the board I removed from
off the top (to insert the controls)
and used it to raise the edge up
so the lock was hidden. I used
sheet metal on the inside to cover
up the sides so that someone
couldn’t reach in and take the
contents. The box didn’t close
exactly right, but it worked.
• If you are planning to put
the circuits in the bottom of the
box and connect the RFID reader
to the top, you‘ll need to take the
inductance of the wires into
consideration. I tried to use a four
wire, 12 inch cable from a
computer to connect the RFID
reader, but it didn’t work. The
middle cables were twisted,
causing unwanted inductance and
cross talk among the serial lines. If
you want to extend the distance
of the wires, you will have to use
a serial line driver/extender (like
the Maxim line of serial drivers —
the MAX2323).
• During construction, you
should keep the RFID reader’s
header as close to the chip’s pins
as possible for minimum
inductance and noise. You will
also need a three-pin wire to go
from the serial LCD to your
microcontroller unless you plan
32
July 2010
