There are tons of different user submitted scripts, but the most basic (and helpful to ballooning) is the ultra intervalometer. This script allows us to tell the camera to automatically take a picture every X number of seconds. Once CHDK is installed on the camera, and the ultra intervalometer is setup, the camera will start taking pictures as soon as the camera is turned on. In the case of my payload, I setup the SD400 to take a photo every 20 seconds. It's really freaky to have a camera just sitting there snapping away photos. But that's what we want for the balloon!
Anyone remember BASIC? I found a script that is basically an interval timer (set to take a picture) but also turns off the LCD. So simple, but reasonably hard to find. Thanks Vicente Piorno! You can download my entire CHDK zip (it's better to get the right CHDK for your camera from their website), or you can grab just the script."