A lot of electronic circuits using NE555 timer IC are already revealed here and this can be simply another one.Here is that the circuit diagram of a police siren primarily based on NE55 timer IC. The circuit uses t NE555 timers ICs and every of them are wired as astable multivibrators.The circuit will be powered from something between 6 to 15V DC and is fairly loud.By connecting an extra power amplifier at the output you'll be able to further increase the loudness. IC1 is wired as a slow astable multivibrator operating at around 20Hz @ 500th duty cycle and IC2 is wired as quick astable multivibrator operating at around 600Hz.The output of 1st astable mutivibrator is connected to the control voltage input (pin5) of IC2. This makes the output of IC2 modulated by the output frequency of IC1, giving a siren result. In easy words, the output frequency of IC2 is controlled by the output of IC1. Notes. The circuit can be assembled on a Perf board. I used 12V DC for powering the circuit. Instead of using two NE55 timer ICs, you can also use a single NE556 timer. NE556 is nothing but two NE555 ICs in one package. Refer the datasheets of NE555 and NE556 to have a clear idea. Speaker can be a 64ohm, 500mW one.