|The Triangle Wave Astable produces a wave that ramps up and down evenly.
The output is fed to the Frequency Modulation input of the Astable which causes the output frequency to vary in proportion with the voltage at any one point. The more voltage, the more deviation from the original frequency creating a rapidly rising and falling tone.
The output from the Astable is connected to the Darlington Driver whihch boosts current for the speaker.
By turning VR1 on the Triangle Wave Astable gradully clockwise, the speed of the warbling will increase to a point where it matches the frequnecy of the second astable. This can produce some very unusual sounds. For a standard siren, turn VR1 fully anti-clockwise