This month I am making 3 different types of siren circuits based on the 555
timer. The first circuit simulates the siren of a British police car. 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 a 1 Hz astable multivibrater.
The output of the left timer is to frequency modulate the right timer. This
causes the right timers frequency to alternate between 440Hz and 550Hz at a 1 Hz
cyclic rate. The transistor is used to help strengthen the signal to the
speaker.
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