I was looking at my little clown car and wagon that I recently posted and thinking how cool it would be to have sound. I have this little speaker from a laptop that's amazingly loud with my MP3 module, so that started the wheels turning...
So, using the MP3 module I use for other sound projects, I cranked out this little board.
Since the couplers are rarely used on small units, I figured they'd be a good remote controlled item that I could use to trigger some sounds. The first two inputs go through optocouplers to isolate the triggers as the MP3 module takes enough power that I wanted full-wave rectification for the P/S, so it doesn't have a common ground.
Since I had some additional inputs, I figured I might as well have access to them. The additional three triggers come out to a terminal block so I can use discrete isolated switches for them. So now, how to use them. The thought hit me that having a sound be able to be triggered anywhere on the track would be cool, so I added a pair of reed switches right at the bottom of the skirt of the little wagon. Laying a magnet next to the rail triggers these as the rig comes by, this will allow me to have two automatically triggered sounds. I can move the magnet anywhere I like to trigger the sound. Having them on both sides triggering two different sounds gives me lots of possibilities.
I still have one more input, maybe I can find a place to hide a pushbutton... 
