The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 12
may have to experiment with different component values. Some combinations might cause
the oscillator to run at harmonics of two or more times the crystal frequency. Digi-Key is
one source for the sometimes hard-to-find 40-kHz crystals. A ceramic resonator is another
option for a 40-kHz source.
The other option is U3, which is a TLC555 timer configured as a 40-kHz oscillator.
Components R7, R8 and C5 determine the output frequency, according to the formula
shown. For accuracy and stability, use 5% or 1% tolerance values for these components.
The timing error of the 555 can also add a few percent error to the output frequency.
For best accuracy, use a CMOS timer like the TLC555, rather than the bipolar 555. For an
adjustable frequency, substitute a 50K potentiometer for R8. Connect the center tap and one
other lead of the potentiometer in place of R8, and adjust the wiper for a 40-kHz output. If
you have no way to monitor U3’s frequency, you can adjust R8 later, by watching the
receiver’s response as you transmit.
The two inputs to NAND gate U2B are the 40-kHz oscillator and U1’s pin 15 (DATA OUT).
When DATA OUT is high, pin 6 of U2B pulses at 40 kilohertz. When DATA OUT is low, pin 6
of U2B is high. The result is a form of modulation, with the presence or absence of the
40-kHz signal representing the logic levels at the encoder’s output.
When pin 6 of U2B is low, PNP transistor Q1 switches on, and current through IRED1 causes
it to emit infrared energy. When pin 6 of U2B is high, Q1 and IRED1 are off. The result is
that IRED1 pulses at 40 kilohertz when pin 15 of U1 is high, and IRED1 is off when DATA
OUT is low. Resistor R3 limits Q1’s base current. You can use any general-purpose or
switching PNP transistor for Q1. Resistor R4 limits the current through IRED1 to about 50
milliamperes, which is high enough for basic testing. If necessary, you can increase the
IRED’s current later for increased range.
For best results, use an IRED with a high-power output. Radio Shack carries high-output
IREDs. Digi-Key also has a selection, including Harris’ F5D1QT and F5E1QT. Devices
with outputs at 880 or 940 nanometers are acceptable. Look for a maximum continuous
forward current of at least 100 milliamperes.
Receiver Circuits
The IRED transmits the encoded address and data. On the other end, you need to detect the
transmitted signal, find out if the address matches, and if so, convert the received data into
a usable format. Figure 12-2 shows a circuit that does these, using an infrared-receiver
module and an MC145027 decoder that complements Figure 12-1’s encoder.
The infrared-receiver module. MOD1 is a Sharp GP1U52X infrared-receiver module. Its
circuits are enclosed in a metal cube about half an inch on each side. The module has just
204 The Microcontroller Idea Book