described as follows.
Resistor R9, R10, and R11
form a 2Vs/3 and Vs/2 divider
reference to U1A and U1B.
Therefore, the output of U1B is
2Vs/3. Since U1A references the
Vs/2 point, has a gain of -1, and is
connected to the output of U1B
(U1B is Vs/6 above Vs/2), the
output of U1A is Vs/3.
When a signal is fed through
C2 to U1B and the audio gets
more positive, the output of U1B
decreases and the output of U1A
Since the reference points for
oscillator U1C are closer together, the frequency
Likewise, when the signal to U1B goes negative, the
output of U1B increases and the output of U1A
decreases. Since the reference points for oscillator U1C
are farther apart, the frequency decreases. This is clearly
seen in Figure 6A. The output from U1C is shown in
The amplitude is always rail-to-rail/on and off, but the
frequency changes based on the audio input signal.
Diodes D1 and D2 make sure that the opposite polarity
reference is used each time the U1C output switches from
low to high or high to low.
The U1C output is fed to Q1-Q2 which forms a
current mirror. R13 limits the current through Q2 to
approximately 25-30 mA to drive the IR LED LD1. The
current mirror assures that the LED output will be constant
regardless of temperature or battery voltage (Vs).
The IR receiver shown in Figure 7 is used to recover
the original audio signal. The received FM pulses are fed
to a PLL (phase locked loop) such as a CD4046. The PLL
tries to match its frequency to the frequency of the
incoming IR pulses. The phase error correction signal
changes the local oscillator to match the incoming
The error correction signal is the same as the original
audio signal which can be amplified and sent to a speaker.
Another use for the IR transmitter-receiver pair is for
remote listening. To demonstrate this technique, get a
small speaker and place a layer of aluminum foil over the
frame of the speaker. Connect the speaker to an amplifier
and play some music or a voice recording. Point the IR
transmitter at the speaker. Do not connect any audio to
the transmitter input. This gives a constant output
frequency. Point the IR receiver at the speaker.
As the speaker diaphragm vibrates back and forth, so
will the aluminum foil. As the IR waves reflect off of the
aluminum foil, they will be frequency modulated by the
vibrations due to the Doppler effect. The IR receiver will
try to match the local PLL oscillator frequency to that of
the incoming IR pulses, and you will hear the audio via
the reflected IR beam.
Spies have used this technique to eavesdrop on
conversations through windows which vibrate when
sounds are made inside a room. They can remotely hear
the conversation because of the vibrations of the window
glass. (This would make a great science fair project.)
Now, with this information, you should be able to
make light dimmers, PWM motor speed controllers, model
train controllers, AM and FM modulated transmitters and
receivers, heater controls, and a host of other applications.
FIGURE 7. IR FM receiver using
FIGURE 6B. Signal at U1C output.
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