BY CORBIN ADKINS
BUILD THE
A few months ago, I saw an article on an
Arduino blog site that showed a project
that included a GPS sensor, an LCD display,
a servo, and an Arduino board. It had a
button on top that when pressed would
turn on the circuit, which would then check
the GPS signal and display the results on
an LCD. It was concealed in a nice wooden
box that locked itself with a servo motor.
The premise was that it would only unlock if you reached a certain global location,
making it what the builder called a “reverse geo-cache” box. I saw that and got inspired to
make my own self-locking box, just not activated by GPS. I only wanted to use what I had
lying around — and I didn’t have a GPS module at the time. I did have the Parallax RFID
reader sitting in my parts box and realized it would be perfect to use. So, I built my own
RFID-controlled self-locking box. I’ll show you how to do the same. First, here’s an
introduction to RFID and an overview of the project.
Introduction to RFID
(For a more in-depth study of RFID, see the links
provided in the Resources box.) RFID stands for “Radio
Frequency Identification” and the technology is used in
more places than you may think. It is used in
supermarkets, toll booths, farms, and airports.
There are three types of RFID tags: battery, passive,
and battery aided. The battery type is powered by a
battery, providing good range at the cost of battery life.
This one isn’t used a lot in the consumer market. The
passive type gets its power from a signal, but has a short
range. This is the most common type, since it is useful, as
well as cheap. Battery aided tags use a battery for power,
providing greater range than the passive type. Power from
the signal is used to “wake up” the device so the battery
power is not constantly being consumed. (This is the kind
used in toll booths for “drive-through” payments.)
This project will use the passive type. We don’t need
(or want) long range, so it will work great. The RFID
reader located inside the device (reading the tags) sends
out signals set to the frequency of the tags, and then
receives an ID number assigned to each tag. The ID
number is stored in ROM inside the RFID tag and is
transmitted after receiving the signal. The signal powers
the RFID tag with an electromagnet. In the same sense,
power is turned into a magnetic field; a magnetic field can
then be turned into power.
Hardware Overview
Parallax is my choice for microcontrollers, so I used a
Propeller. I also used a Parallax serial LCD for a display
and a Futuba continuous rotation servo to control the
lock, which is a pistol type with a long metal “rope.” The
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