Elenco Upgrade Kit SC500 to SC750 - Manual (Page 110)

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You probably noticed that the width of the pulses in the pulse
stream is constantly changing, that is because there is actually a
second type of modulation occurring here. Press the key again and
you hear a siren. A siren is not a stable tone but rather is constantly
changing in frequency. Change the time scale to 0.5ms/div and
observe the range of waveforms:
Time scale
The width of the pulses (or frequency of the signal) is slowly being
changed, at a regular and repetitive rate. This is an example of
Frequency Modulation, or FM. In AM you use a controlling signal
(voice or music) to vary the amplitude of a second signal, in FM you
use the controlling signal to vary the frequency of the other signal.
In this circuit the output frequency from the alarm IC is being
controlled by a signal created inside the alarm IC, but it could have
been controlled by humming like you did for the AM (you don’t have
the parts needed to do this).
Look back at the Light & Sounds project PC4 on page 16. It shows
several different ways of configuring the alarm IC to make different
sounds, all of these are examples of frequency modulation using
different controlling signals created within the alarm IC. It also
shows examples of the frequency spectrum.
Project #PC11
Filtering
With the same circuit as PC10 and the same settings as shown at
the end of PC10, look at the waveform again and then press the
key. Notice how the pulses become more “rounded” when the key
is pressed. The whistle chip (snap part WC) has capacitance that
filters or smoothes the output signal. Now replace the whistle chip
with the 0.02
m
F capacitor (snap part C1) and it should look similar
though you won’t hear any sound. You can also look at the
frequency spectrum in FFT mode like in the other projects.
Typical waveform using
whistle chip
Typical waveform using
0.02
mm
F capacitor