Working emergency lights and siren

[Adeor]

I am planning new MOCs and two of these are police vehicle and fire truck. I want both of them to have working emergency lights and siren. For the police vehicle, I want three siren modes: "woop woop", scrambler and avail. 

My question is how do I pull this off and can I do this using PF lights and BuWizz battery?

[Zerobricks]

You could use the old 9V components, the sirens have 2 types of ringing according to the polarity. You can connect the siren via 8886 PF extension cable and a 9V cable. For lights, the old 9V system also had flashing lights. Or you can use PF/PU lights, but to there is no support for blinking in the BuWizz app... Yet 

[Mirco Hussmann]

I have a lot of experience building emergency service vehicles from Europe and the US, so I've figured out various setups. I've built a Fire Engine with Wail, Yelp and Hi-Lo sirens like this: 

I bought a Beier Modellbau USM-RC-3, which is a high performance sound, light and servo driver that can be used either in analog or digital mode. It has 16 channels for LEDs that can be programmed in sequences, special functions like brake/reverse lights or with a trigger input, as well as custom triggered sounds, engine noise and randomly played sounds. 16 channels should be more than enough for most vehicles. The Sound module is powered by a cut-up Power Functions cable that you place on top of the drive motor's plug - this way you can connect the outer two wires to + and - and the inner cables to the board's engine input, so the board knows when you're driving/reversing and can trigger sound and light effects. I used brake lights, reverse lights and an airbrake sound.

For the sirens, you'll need a compatible 4 Ohm speaker and the right sound files,  and then they can be triggered by connecting one of the 8 input channels to ground. That can be most easily done by wiring a second PF cable to two relays that are alternately switched on when the PF port is activated in either direction, and the relays will then connect ground to the input channels. If you only want 2 separate sounds, that's it, but for my model I used an Arduino Nano in between that I fed 3 relay inputs via PF cables - Primary, Secondary A and Secondary B. The Secondaries are switched by the two channels of one PF cable, the Primary uses one channel of another unused PF cable.

When the Arduino reads the Primary input as HIGH, it will switch one of three output relays depending on the switch state of the Secondary inputs, i.e. Relay 1 High when Secondary A is high, Relay 2 High when both secondaries are Low and Relay3 High when Secondary B is high. Those 3 output relays are then switching the Sound Module inputs. Why all this work? Because, if you plainly connect two channels of a PF cable, you can never seamlessly switch between sirens like the original - there will always be a break when you toggle the button. With this setup, you can program the 3 inputs to trigger for example Wail, Yelp and Hi-Lo siren tones, and when you control the model you can just turn on the siren (Primary relay) and it will instantly start playing the preselected tone (like Wail). By switching the secondary in either direction, you can seamlessly change the tone to yelp or Hi-Lo. The board also allows for a lot of customisation regarding which sounds should auto-loop, play only once, play only while trigger is high etc.

Here's a video where you can see the system in action. I built in two additional, independent relay inputs for Q siren and Air horn, but that doesn't affect the electronic siren setup. Also an additional note - the roof lightbar is powered by a separate Arduino to account for the 13 inputs needed to recreate the specific pattern, but most vehicles will not need that many independent outputs. The board can handle practically unlimited LEDs per output channel in case you have multiple lights that activate at the same time.