122-27400 Parallax Inc, 122-27400 Datasheet - Page 46

• Manufacturer Part Number: 122-27400
• Description: MANUAL FOR SUMOBOT
• Manufacturer: Parallax Inc

Specifications of 122-27400

• Accessory Type: Manual
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: SumoBot®
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

Page 36 · SumoBot – Mini Sumo Robotics
The IR detectors have built-in optical filters that allow very little light except the 980 nm
infrared that we want to detect onto its internal photodiode sensor. The infrared detector
also has an electronic filter that only allows signals around 38.5 kHz to pass through. In
other words, the detector is only looking for infrared flashed on and off at 38,500 times
per second. This prevents interference from common IR interference sources such as
sunlight and indoor lighting. Sunlight is DC interference (0 Hz), and house lighting tends
to flash on and off at either 100 or 120 Hz, depending on the main power source in the
country where you reside. Since 120 Hz is way outside the electronic filter’s 38.5 kHz
band pass frequency, it is, for all practical purposes, completely ignored by the IR
detectors.
THE FREQOUT TRICK
Since the IR detectors only see IR signals in the neighborhood of 38.5 kHz, the IR LEDs
have to be flashed on and off at that frequency. A common 555 timer can be used for this
purpose, but the 555 timer circuit is more complex and less functional than the circuit we
will use in this and the next chapter. For example, the method of IR detection introduced
here can be used for distance detection; whereas, the 555 timer would need additional
hardware to do distance detection.
A pair of BASIC Stamp enthusiasts found an interesting trick that made the 555 timer
scheme unnecessary. This scheme uses the FREQOUT command without the RC filter
that’s normally used to smooth the signal into a sine-wave. Even though the highest
frequency FREQOUT is designed to transmit is 32,768 Hz, the unfiltered FREQOUT output
contains a harmonic with useful properties for a 38.5 kHz IR detector. More useful still
is the fact that you can use a command such as
FREQOUT Pin, Duration, 38500
to send a 38.5 kHz harmonic that the IR detector can detect.
Tuned electronic receivers, such as the IR detectors we’ll be using, can detect
components of this signal that are called harmonics. These harmonics are actually
components of the unfiltered FREQOUT pulses. The third harmonic of 38.5 kHz can be
controlled directly by entering commands such as FREQOUT Pin, Duration , or
FREQOUT Pin, Duration, 40000 for 40 kHz (as might be required by different
detectors), etc.