Dimming 3 x 40w 12v incandescent bulbs via arduino to audio sequence

Hi everyone,

I'm totally new to Arduino and am trying to make a project happen for an art show with a friend who has some prior knowledge but we are a bit out of our depth. I found this previous article on the board but it is about switching on and off rather than dimming. http://forum.arduino.cc/index.php?topic=201248.0 which I understand is quite a different game?

I have a video file with 3 channel audio sequence (3 speakers) that I would like to have 3 incandescent light bulbs dimming and switching on/off in time to the audio/video file. So each audio track essentially has a light bulb that follows along.

I read that doing dimming with 240v mains (i'm in the UK) was tricky and so I found 12v 40w bulbs used for camper vans and that sort of thing. I bought this: SunFounder L293D Motor Stepper Servo Shield for Arduino Duemilanove Mega UNO R3 thinking it would make the arduino output be able to go up to 12v to handle the low voltage bulbs. But I plugged this 24v DC plug into it with the arduino and it blew up (I thought it could handle up to 25V DC?)

If anyone had some pointers or could point what I'm obviously doing wrong to blow this up and if there is a good way to go about dimming 12v bulbs with arduino.

Thanks and apologies for having such limited knowledge.

I have found more and more articles about dimming different types of bulbs on here, but not 12v incandescent. These kick out 5A per bulb which is really high - would the arduino be able to handle this or would it better to try to dim 240v bulbs on the mains and have lower amps?