The third circuit simulates the Red Alert siren from the TV show Star Trek.
It uses two 555 timers in the circuit. The 555 on the right is wired as an alarm
tone generator and the second 555 timer on the left is wired as a 1.5 second
non-symmetrical astable that generates a fast rising but slow falling saw tooth
waveform. This waveform is buffered by the transistor and used to frequency
modulate the tone generator and making its frequency rise slowly during the
falling parts of the saw tooth but collapse rapidly during the rising part of
the saw tooth. The output starts as a low frequency, rises for 1.15 seconds to a
high tone, ceases for .35 seconds and then repeats the cycle.
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