Digital Signal Processing Reference
In-Depth Information
As seen in Figure 13.6, the circuit on the IR proximity module utilizes a small
feedback oscillator to set up a transmit frequency that can be easily detected by
the detector module. This module utilizes a band-pass filter that essentially
filters out ambient light. Some older first generation electronic ballasts used in
commercial fluorescent lights can interfere with the IR sensors since they
operate at the same frequency as the filter. Newer ballasts now operate at a
higher frequency since they also caused problems with IR TV remote control
signals.
38 KHz
180
Oh m
1 of 2
IR LEDs
LED Enable. H
5v
Signal Detect.L
IR Detector
Figure 13.6
Circuit layout of one LED and the receiver module on the infrared detector.
In Figure 13.6, when the Left_LED Enable signal is High, the Low side of the
IR LED is pulled to ground. This forces a voltage drop across the LED at the
frequency of the 5v to ground oscillating signal. In other words, the LED
produces IR light pulses at 38 kHz. Using a 38 kHz signal helps reduce noise
from other ambient light sources.
Since the IR detector has an internal band-pass filter centered at 38 kHz, the
detector is most sensitive to the transmitted oscillating light. The 5v pull-up
resistor allows the IR Detector's open collector output to pull up the SOUT
signal to High when no IR output is sensed. To detect right and left differences,
the right and left LEDs are alternately switched so that the detected signals are
not ambiguous. If both the left and right LEDs detect an object at the same
time, the object is in front of the sensor.
If the IR sensor was built from component parts, a hardware timer implemented
on the UP3 board could be used to supply the 38-40 kHz signal. Similar IR
LEDs and IR detector modules are available from Radio Shack, #276-137B,
and Digikey, #160-1060. Assuming two FPGA-bots are equipped with IR
sensor modules, it is also possible to use this module as a serial communication
link between the robots. One FPGA-bot transmits using its IR LED and the
other FPGA-bot receives it using its IR sensor. To prevent interference, the IR
LEDs are turned off on the FPGA-bot acting as a receiver. Just like an IR TV
