By making the
high (50 kHz to
100 kHz), the
PWM beam of
light can carry the
audio input signal
works like AM
The output wave
form is shown in
Figure 4A. Notice
how the on and
off times vary as
the audio varies.
A receiver (refer to Figure 5) that
detects the IR beam can recover the
information by amplifying the signal and
processing it using a low pass filter
(integrator) — much like an AM radio
signal is recovered.
The on time of the light pulse is
similar to the amplitude of the RF
envelope of an AM radio signal.
By modifying the oscillator circuit as
shown in Figure 6, it is possible to FM
(frequency modulate) the IR oscillator.
The resulting modulation and output
waveforms are shown in Figure 6A and
50 November 2015
FIGURE 4A. Modulated PWM waveform.
FIGURE 5. Optical detector with
8. 6 kHz low pass filter.
FIGURE 6. FM modulated oscillator
circuit with IR output.
FIGURE 6A. Signal at U1C+ input.