30-05-2012, 10:51 AM
RFID Access Control System
[attachment=23448]
RFID Tag Reader
The purpose of the Reader component is to activate and power the tag, demodulate
the response, and prepare the signal for the microcontroller. The components of
this reader are: the antenna, signal generator, peak detector, low pass filter, and
voltage comparator.
Antenna
Many antenna configurations were constructed for testing. Each had limited range
and were difficult to use because the coils would come out of place.
In the end, we settled on a pre-made antenna that consisted of two coils wrapped
around a ferrite coil in a transformer configuration. The inductance of the coils
were measured, and an appropriate capacitor was chosen to tune the antenna to the
resonant frequency using the parallel tank circuit equation:
Signal Generator
A 125 kHz square wave signal generator is required to drive the antenna. We
generated a signal from the MCU for this purpose, but due to time constrictions
we did not have time to build a circuit to make the signal have the necessary
voltage. For now, we are using a function generator as the signal generator. It is
set to output a square wave at 125 kHz, 10 Vpp.
Peak Detector
The peak detector is used to extract the envelop of the AM signal. Figures 3 and 4
show the antenna input without and with the tag in proximity. Figure 5 shows the
signal after the peak detector.
Voltage Comparator
The envelop signal is converted to a square wave in preparation for sending to the
microcontroller. The LM411 comparator was used. Notice the noise in the signal.
This noise greatly affected what the MCU was reading, causing inconsistent
results in our application.
Microcontroller
The PIC16F7X MCU was programmed in assembly language. The MCU is
responsible for decoding the Manchester encoded data, extracting the data,
controlling the LED’s that indicate the ID, and managing the access
control.
Figure
[attachment=23448]
RFID Tag Reader
The purpose of the Reader component is to activate and power the tag, demodulate
the response, and prepare the signal for the microcontroller. The components of
this reader are: the antenna, signal generator, peak detector, low pass filter, and
voltage comparator.
Antenna
Many antenna configurations were constructed for testing. Each had limited range
and were difficult to use because the coils would come out of place.
In the end, we settled on a pre-made antenna that consisted of two coils wrapped
around a ferrite coil in a transformer configuration. The inductance of the coils
were measured, and an appropriate capacitor was chosen to tune the antenna to the
resonant frequency using the parallel tank circuit equation:
Signal Generator
A 125 kHz square wave signal generator is required to drive the antenna. We
generated a signal from the MCU for this purpose, but due to time constrictions
we did not have time to build a circuit to make the signal have the necessary
voltage. For now, we are using a function generator as the signal generator. It is
set to output a square wave at 125 kHz, 10 Vpp.
Peak Detector
The peak detector is used to extract the envelop of the AM signal. Figures 3 and 4
show the antenna input without and with the tag in proximity. Figure 5 shows the
signal after the peak detector.
Voltage Comparator
The envelop signal is converted to a square wave in preparation for sending to the
microcontroller. The LM411 comparator was used. Notice the noise in the signal.
This noise greatly affected what the MCU was reading, causing inconsistent
results in our application.
Microcontroller
The PIC16F7X MCU was programmed in assembly language. The MCU is
responsible for decoding the Manchester encoded data, extracting the data,
controlling the LED’s that indicate the ID, and managing the access
control.
Figure