This project came to life when i was looking for a siren for a police motorcycle, one of the most recognizable in law enforcement: the kawasaki KZ1000P Police.
There are official units (from whelen and federal signal for instance), but those units are very hard to find, are only really available in the USA (so you have to ship them) and if you can find them, they are still sold for a lot of money!

With a bit of digging around on the internet, i managed to find the complete manual of the federal signal PA300, still in use in a lot of cars. This a simple unit, no drivers for lights or anything, just a selector switch for the signal you want, a swith (for airhorn and manual sound) plus it has a PA function.
For our use, we will be changing some items on the siren so it can be used on the motorcycle. The following alterations are made:

All switches and switch logic is removed (the AND and OR gates etc.)
I am not using the PA circuit (at least, not now)
I am also not happy with the peak circuit, so that will be replaced and the original removed
The entire end amp will not be used, and is moved to a seperate housing.

So effectively, we are left with:

The wave generator
Alternate switch logic

(click for larger version in a new window) The main generator circuit is made up of several blocks:

rate generator
rate switching
input selection (for the VCO)
blockwave / siren waveform generator (VCO circuit)

To protect the amplifier, the VCO is also controlled by the low frequency detection + inhibit.

As noted, the peak circuit is replaced to something more sounding like in the movies. This is marked as alternate peak on the circuit. The airhorn circuit is also modified a little to sound better.
Each of these blocks is marked on the circuit diagram.

As noted, there is no amplifier for the horn on-board. For this, you can use any available 100Watt amplifier available. Even a cheap chinese horn with the amp built in will do.

All that needs to be done: get the parts, get a soldering iron and start building it!

Provided below are several file for you to download to get started

description	File
The full circuit diagram in pdf	circuit.pdf
List of components	bom.csv

To be complete, i checked the actual rules and regulations on sirens in the USA so i can be fairly sure the siren does actually sound correct!
The documents on this topic are from the National Institute of Justice NIJ guide 500-00 and National Institute of Justice NIJ, technology assessment program NIJ standard 501.00

In a nuttshell these documents contain information on how the sirens are tested, but more importantly: what the signals should look like on a scope and what the frequencies etc. should be.