Encodedsignal

The above circuit will produce a police siren effect . Actually I found this circuit online long back(don’t remember the source) and made some changes .
Working :
Both the 555’s are wired to work in astable mode . The first IC555 produces low frequency square waves which is then given to the ” Control ” pin of the second IC, thus when the voltage at this pin we can vary the time electronically which in turn changes the frequency of the output . The output of the second IC555 is given to a capacitor , the capacitor turns the square wave to a triangular wave output (nearly triangular) , this when given to the speaker a “eaeee-uhnnnn” effect .
Notes:
You can change the out put sound (length and so) by changing the output capacitor, ie C3

Note :
The circuit was already available on the net (This is an edited version)

Here is a video