I designed the Parallax RFID Reader Module (Figure 1, Module initiates all communication and can connect
previous page) as a low-cost solution to read passive, directly to any T TL-compatible UART.
low frequency (125kHz) RFID tags from up to 3 inch- If you plan to connect the RFID Reader Module
es away. The module requires a single +5VDC supply directly to a PC’s serial port, you’ll need a common
and it interfaces to a PC, BASIC Stamp, Microchip RS232 line driver (also known as a “level shifter”),
PIC, or other processor via a 1-wire, 2400 baud serial such as the Maxim MAX232 or Linear Technology
TTL interface. It costs $39 from Parallax (parallax. LT1780, to bring the module’s TTL-level outputs to
com, part #28140) and can be integrated into any the PC’s expected range (Figure 2).
design using only four connections (VCC, /ENABLE,
SOU T, GND). It just can’t get much easier. Making Something Fun
An LED shows the state of the reader module. Armed with our knowledge of RFID and the Parallax
When the module is successfully powered up and RFID Reader Module, let’s make something. Because
is in an idle state, the LED will be green. When the the RFID Reader Module simply sends a 12-byte
module is in an active state and the antenna is ASCII string containing the tag’s unique ID, we can
transmitting, the LED will be red. connect the output of the reader to the input of a
The RFID Reader Module is controlled with a serial LCD module, such as Parallax’s 2x16 Serial
single TTL-level active-low /ENABLE pin. When the LCD regular (part #27976) or backlit model (part
/ENABLE pin is pulled low, the module will enter its #27977), which will display whatever ASCII characters
active state and enable the antenna to interrogate it receives from its input pin onto the LCD.
for tags. Keep in mind that when the antenna is Combining the RFID Reader Module and the serial
active, the device is broadcasting and consuming LCD will give us an instant, portable RFID scanner
about 100 to 200mA from the power supply. If that you can use as a “bug finder” to snoop through
you’re going to build a project that involves batteries, your house and scan yourself and your friends, fam-you may want to add a physical button to activate ily, and pets to check for RFID tags. The best thing
the reader only when a card is actually present, or about this project is that no software is required!
use a timeout in your controlling software/firmware The RFID Reader Module itself handles all of the
to disable the reader periodically and reduce the underlying RFID-reading functionality, and the serial
load on the power supply. LCD is already expecting to receive serial data.
This particular product is designed to detect To build our project, simply hook up the two piec-common, passive read-only tags within the EM es as shown in Figure 3. If you’re using a 9V battery
Microelectronics-Marin SA EM4100 family. When or any other power source above 5V, you’ll need to
a valid RFID tag is placed within range of the reader, use a voltage regulator (like the standard 7805) to
the tag’s unique ID will be transmitted as a 12-byte bring the voltage down to 5V. Don’t even think about
ASCII string via the TTL-level SOUT (Serial Output) connecting an unregulated power source directly to
pin in the format shown in Table 1. the RFID Reader Module or LCD!
The start byte and stop byte are used to easily Your final product will look something like Figure
identify that a correct string has been received from 4. I built mine on a simple breadboard to show off
the reader (they correspond to a line feed and car- at the Make Fest at O’Reilly’s Emerging Technology
riage return characters, respectively). The middle Conference in San Diego (March 2005) and still
ten bytes are the actual tag’s unique ID. haven’t taken it apart. The text string shown on the
All serial communication is non-inverted at 8 data LCD, “04129C1A89,” corresponds to the unique ID
bits, no parity, 1 stop bit (8N1). The baud rate is of the most recent tag held up to the reader.
configured for 2400bps, a standard communications With your RFID scanner ready to go, you can be-speed supported by almost any microprocessor gin your hunt for RFID tags — hidden or otherwise.