The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 12
Some IREDs are manufactured with integral lenses that focus the output into a beam. For
example, Harris’ F5D1 and F5E1 IREDs are identical, except that the F5D1 has a lens that
aims the energy in a narrow beam, while the F5E1 has a flat window and wider beam angle.
An IRED with an integral lens is an easy, low-cost option, if it can do the job. A flat-window
package is useful if you want to add an external lens, or if you want a wider beam, to reach
multiple receivers around a room, for example.
If you’re interested in experimenting with lenses, Edmund Scientific has a huge selection,
including inexpensive educational-grade lenses, lens mounts, optical benches, and books on
optics.
Although infrared links are most often thought of as line-of-sight paths—for transmitting
across a room, for example—optics can also overcome this limitation. For example, with
mirrors, you can transmit around corners.
Radio Links
Another possibility for wireless links is to use radio frequencies. Radio transmissions consist
of high-frequency electromagnetic waves that travel through the atmosphere. Most radio
waves will also pass through windows, walls, and other solid objects. This makes radio
useful where a direct line-of-sight between transmitter and receiver isn’t available. Radio
can also be a good choice for outdoor links, where daylight may interfere with infrared
transmissions and wired links are inconvenient.
Radio circuits require special construction techniques, and radio transmissions must not
violate regulations of agencies such as the Federal Communications Commission (FCC).
For these reasons, the easiest option is to buy the transmitters and receivers for your link,
rather than build them yourself.
One source of low-cost radio-frequency transmitter and receiver circuits is Electronics 123.
The links transmit in the range 300-315 Megahertz, a frequency band used by many
garage-door openers and alarm systems. The boards come with complete schematics and
instructions for use.
The circuits are similar to the infrared-transmitting circuits described earlier. The transmitter
sends 4-bit codes to a receiver identified by an 8-bit address. The transmitter and receiver
use Holtek’s HT-12E encoder and HT-12D or HT-12F decoder chips, also available separately
from Electronics 123 and Digi-Key. The chips are similar in operation to Motorola’s
146026/7 encoder and decoder, and you can in fact use them in infrared links as well. The
encoder and decoder each require just one resistor to set the oscillator frequency.
216 The Microcontroller Idea Book