The Microcontroller Idea Book - Jan Axelson's Lakeview Research

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Because there are three possible states, the information is trinary (as opposed to binary, with
just two states). With five address inputs and three possible states for each, you can have as
many as 243 receivers, each with its own address. Although data inputs D6-D9 also transmit
in trinary form, the receiver decodes open inputs as logic 1’s, so in effect the data bits are
binary. For testing, you can use jumpers or switches to +5V or ground, or leave the pins
open, to set the data and address inputs.
Transmit Enable (TE) has an internal pullup, which turns off the transmitter when pin 14 is
not connected. To enable transmitting, TE must pulse low for at least 65 nanoseconds. For
manual operation, you can use a jumper or switch (S1) to bring TE low.
Components R1, R2, and C2 set the frequency of the on-chip oscillator. This in turn controls
the width of the transmitted pulses. The figure shows the data sheet’s formulas and
recommendations for selecting values for these components. For best performance, use
components with 5% or tighter tolerance.
With the values shown, the oscillator’s frequency is 1 kilohertz, which is at the low end of
Motorola’s recommended range for the chip. With this frequency, the narrowest transmitted
pulses are 500 microseconds wide. I chose this pulse width to be compatible with the
requirements of the infrared module in the receiver circuit, which may not respond reliably
at higher frequencies. If you have an oscilloscope or frequency counter, you can monitor
the oscillator frequency at U1’s pin 12.
For each transmission, the encoder sends all nine address and data bits in sequence, waits
three data-bit times (24 milliseconds at 1 kHz), and then repeats the entire transmission. A
complete transmission requires 182 milliseconds from the time that TE goes low. If you hold
TE low, the encoder will transmit continuously. Otherwise, the transmission ends after
sending the information twice.
The encoder’s output drives infrared-emitting diode IRED1. Instead of directly driving the
IRED with the encoder’s output (DATA OUT), NAND gate U2B combines DATA OUT with a
40-kHz oscillator. The result is that the encoder’s pulses transmit as bursts of 40-kHz pulses.
As we’ll see, the infrared receiver is designed to reject stray signals that don’t pulse at 40
kilohertz. Pulsing the IRED also saves power, since the IRED is never on constantly.
Oscillator alternatives. I’ve included a choice of two designs for the 40-kHz oscillator. One
has a stable, accurate output but requires a special timing crystal, while the other uses more
common components but requires a constant power-supply voltage and accurate resistor and
capacitor values for best stability and accuracy. You can choose whichever you prefer, and
connect the output to pin 5 of U2.
The crystal-controlled oscillator uses a 40-kHz quartz crystal and an HCT132 Schmitt-trigger
NAND gate (U2A) operated as an amplifier. If you substitute a different inverter, you
The Microcontroller Idea Book 203