The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 12
accomplished, you can replace the manual switches with computer control of the transmitter,
receiver, or both.
The circuit uses some specialized components that do a lot of the work of detecting, filtering,
encoding, decoding, and error-checking of the transmissions. Two of these are Motorola’s
MC145026 and MC145027 encoder/decoder pair, which are low-cost chips intended for
remote-control applications.
Transmitter Circuits
Figure 12-1 is the infrared transmitter, which has three main functions. It converts four bits
of parallel data into a serial stream. It then modulates the resulting signal by chopping it at
40 kilohertz. And, it causes one or more infrared-emitting diodes to transmit the encoded,
modulated data to a receiver that is tuned to respond to 40-kilohertz signals.
The encoder chip. The encoder (U1) has five address inputs (A1-A5) and four data inputs
(D6-D9). The logic states of these inputs determine the transmitted address and data. The
encoder outputs a different code for each of three states that the inputs may have: Logic 0
(1.5V or less), Logic 1 (3.5V or greater), or Open (no connection). Figure 12-3 shows the
transmissions that result for each of these states.
Figure 12-3. Pulse patterns at DATA OUT of the MC145026 encoder.
202 The Microcontroller Idea Book