I built one for a pro photographer friend of mine. He used it to get pictures of the space shuttle lift-off in Florida, using a Nikon with a motor drive and a 36 exposure roll of Kodak EktaChrome... It was mounted near the Gantry where no humans could be, so it had to be triggered by the sound of the engines firing.... We also included a few seconds of delay before the first exposure to give the engines time to come up to full power for the actual liftoff...
No PIC, just a couple of transistors to amplify a microphone, a couple of diodes to rectify the audio, and a small relay to fire the motor drive. It also had a clock to wake it up at the earliest scheduled launch time, so that trucks and other human movement prior to launch didn't set it off. It had to be placed about 12 hours prior to the scheduled launch.