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This circuit
produces a sound similar to the police siren.
It makes use of two 555 timer ICs used as astable multivibrators.
The frequency is controlled by the pin 5 of the IC.
The first IC (left) is wired to work around 1Hz. The 47uF
capacitor is charged and discharged periodically and the voltage
across it gradually increases and decreases periodically.
This varying voltage modulates the frequency of the 2nd IC. This
process repeats and what you hear is the sound remarkably similar
to the police siren.
Two presets VR1 and VR2 are provided to vary the siren period of
repetition and the tone of the siren.
By varying VR1 you can set how fast the siren changes from high
freq. to low freq.
VR2 sets the siren frequency. Adjust VR1 and VR2 to suit your
taste.
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