This is Electronic Siren circuit diagram. The sound produced imitates the rise and fall of an American police siren. When first switched on the 10u capacitors is discharged and both transistors are off. When the push button switch is pressed to 10u capacitor will charge via the 22k resistor. This voltage is applied to the base of the BC108B which will turn on slowly. When the switch is released the capacitor will discharge via the 100k and 47k base resistors and the transistor will slowly turn off. The change in voltage alters the frequency of the siren.
Current drain is fairly high in this circuit so a suitable power supply is required. The duration the tone takes to rise and fall is determined by the 10u and 22k resistor. These values may be varied for different effects.