This page describes the construction of an RFID reader using only an Arduino (Nano 3.0 was tested, but others may work), a hand-wound wire coil, and some assorted low cost common components.
RFID readers are devices sold by companies such as Parallax to read RFID tags with embedded identification circuits (we focus here on passive tags, activated by the reader's transmitted RF energy). The design presented here shows how to wind a simple wire loop by hand (or create an equivalent printed circuit spiral version), connect it to an Arduino (or its chip), add a few low cost common components and create your own RFID reader. To make it more interesting (i.e. challenging), we will focus on the FSK class of RFID tags, which are fairly common among the 125kHz devices, but for some reason are not supported by the Parallax kits.
Micah Dowty has shown a design for an FSK/ASK RFID reader built around a Parallax Propeller device. His code, which is in assembly language, implements an ingenious (but complex) algorithm to create a dynamically variable analog bias voltage, which is used to pull the weak RFID signal into range, so it can be discriminated into binary signals by the Propeller's digital input circuitry. He also dynamically tweaks the transmit/receive RF frequency to keep the antenna's tank circuit in peak resonance for optimal signal to noise. There are three problems with his approach: first, the passive detection circuit lacks amplification, which makes it very sensitive to noise and therefore raises reliability issues. Second, the design is based on the Propeller chip, and if you are a fan of the Arduino and/or associated Atmel AVR chips, it leaves you out. And third, the dynamic slewing of frequencies and bias voltage is overly complicated, making it hard to debug. His general concept is attractive, however: use a microcontroller chip and wind your own wire loop to create, with some simple components and appropriate code, a complete DIY RFID reader.
Asher Glick has presented a solution for reading and decoding FSK RFID tags using the Arduino/AVR family (which he calls AVRFID), which is good except it apparently requires obtaining and modifying an existing Parallax RFID reader device (which natively only supports ASK).
Our goal here is to present a simple solution for reading FSK tags which addresses the above shortcomings: make it robust and reliable for real-world noise environments, base it on the Arduino, and build the RFID reader ourselves using a few simple low-cost parts, rather than buying and/or modifying one.
The circuit diagram above was derived from the "World's Simplest RFID Reader" design posted by Micah Dowty. Based on the Parallax Propeller, Micah's approach was to use passive components only, without amplification, in order to achieve the ultimate in simplicity. The lack of amplification, however, results in a weak signal, potentially less than 2V PTP. This signal is then biased by an analog level produced by the Propeller, to try to maintain the signal's DC level near the discrimination point of the Propeller's binary-digital input circuitry. His code attempts to dynamically calculate that optimal midpoint level, and feed it into the circuit using a filtered PWM DAC output. Since the signal is weak, it can be distorted by interference and noise, which results in reduced reliability. The circuit presented here includes (as Micah suggests in his documentation) one active component: a common low-cost LM234 quad-opamp IC (or equivalent). This addition provides several significant advantages, at a negligible cost. First, the signal is amplified (using one of the four opamps on the IC package) to a more noise-immune level (of 2-3 volts PTP). Second, the DC level of the signal is maintained at exactly Vcc/2 using another opamp on the IC, which eliminates the need for the DC propping code in the Arduino. Third, having the signal amplifier in place allows another low-pass RC filter stage (another capacitor and resistor), which makes the final discriminated digital signal cleaner and more reliable. The end result is a more robust detected signal with improved noise immunity.
As a quick review of the circuit, the loop is made of a toroidally-wound #22-30 magnet wire (we used an empty roll of Scotch 3.25" I.D. packing tape as former), and can be remoted from the circuit if needed, via coaxial cable. The inductor L1 and capacitance C1 should be matched to resonate at around 125 kHz. When driven at its resonant frequency by the Arduino's 0-5 volt square wave signal, the center point of the resonator (which connects to D1's cathode) will have a fairly pure voltage sine wave, of about 30V PTP. When coupled to an RFID tag, the pure sine wave RF will fluctuate visibly as the tag opens and closes its own loop antenna to repeatedly transmit its code. This modulation is then detected from the RF envelope by D1, C2 and R1, which produce a negative bias voltage with the small detected coded signal, e.g. about 11 RF cycles per coded cycle. The coded cycles are of two different wave lengths (or frequencies), which represent streams of logic ones and zeros, and they need to arrive at the Arduino chip as binary levels which can be timed reasonably accurately so as to reliably tell the difference between the two distinct frequencies.
The relatively large capacitor C3 decouples the negative bias voltage from the signal, and is followed by a low-pass RC filter stage (R2 and C4) which attenuates some of the residual RF spikes from the lower frequency coded RFID signal. Capacitor C5 decouples the resulting signal and presents it to the amplification stage, implemented by the LM324 opamp, IC1. The latter amplifies the weak signal from about .15V to about 3V PTP (depending of the ratio of R4 to R3), and places it on top of a Vcc/2 bias voltage, about 2.5V in the arduino's case. This signal is then fed into one of the digital input ports on the Arduino (which also includes some helpful hysteresis), and is discriminated by the internal comparator into a square wave of ones and zeroes.
The Arduino sketch, derived from the code posted by Asher Glick, uses a single timer channel in the Arduino (using the Timer1 library) for both RF signal generation as well as timing clock to count the width of each input signal wave. There are two distinct cycle lengths in the detected input signal, "long" and "short", corresponding to logical ones and zeroes, respectively. A binary stream of stretches of repeated ones and zeroes is assembled, and then decimated into the original coded bits on the RFID tag, after decoding the Manchester encoding.
Here is the actual code:
The device and transceiver antenna have been built and tested on multiple FSK RFID tags of various kinds, in breadboard and soldered perfboard versions, connected to remote and local probes. When the probe is properly tuned, the device can reliably detect FSK RFID tags within a range of 0 to at least 2 inches from the coil, although it may be possible that this can be extended with larger coil sizes and/or other optimizations. The circuit has also been simulated on Spice, as described below.
As seen in the LTspiceIV screenshot above, the circuit (with a passive virtual ground reference - see note below) was simulated on a computer, and the results confirmed the essential design, closely replicating the waveforms actually seen on the oscilloscope. The RFID transponder tag was simulated as a coupled transformer winding with a resonantly tuned capacitor, shunted to ground by a square-wave signal. The RFID tag's ground is connected to the main circuit's ground for simulation purposes. The inductive coupling between the two "transformer windings" is a variable which can be changed in LTspice, and was varied for testing between 1 and 0.01 (0.015 is shown in the waveforms above), equivalent to having the RFID tag positioned at different distances from the reader coil.