BRITISH POLICE CAR SIREN USING NE555

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.