27220 Parallax Inc, 27220 Datasheet - Page 131

• Manufacturer Part Number: 27220
• Description: BOOK STAMPWORKS
• Manufacturer: Parallax Inc

Specifications of 27220

• Accessory Type: Booklet
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• For Use With/related Products: StampWorks
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Not applicable / Not applicable

Moving Forward · Page 121
Behind the Scenes
As explained in Experiment #18, the raw output of FREQOUT is designed to be
filtered (producing a very clean sine wave) before application to an audio amplifier.
If we remove the filter, the raw output will be filled with a lot of harmonic content
above the specified output frequency. The harmonic content happens to be strong
enough to modulate an IR LED.
By specifying 38.5 kHz for FREQOUT – which is actually above the legal value for a
frequency – what we get is a fundamental plus a harmonic at 38.5 kHz; this
harmonic is used to modulate the IR LED. Why modulate? Because the environment
is filled with natural and man-made IR signals, and by modulating and demodulating
at a specific frequency we are able to detect our source. The figure below shows the
harmonic spike created by FREQOUT; note that it is near enough to the target of
38.5 kHz to be useful.
The IR detector is special as well. Note that the FREQOUT instruction must end
before we can sample the output from the detector. The detector in use holds its
output long enough to allow the pin scan to occur.