First of all, being new here, I'm not sure if it applies to share here this script.
I needed a simple hdr script with the flexibility of setting the number of steps, the step range, the hability to turn off auto focus and take the same number of underexposed and overexposed shots, as well as the capability of running on a A3100 IS camera.
I started from the shipped hdr.lua and added two new parameters: number of steps and an autofocus switch. I also used HDRlapse.lua as a guide to Lua commands I needed.
So, in regards to the way the script interfaces with hardware, this one has the same approach of the shipped hdr.lua.
Because of that, I believe this one should run on most cameras.
UPDATE: Added script version 1 (hdr1_v1).
Change log:
1 - Added First Shot Delay parameter
2 - Added Raw Delay parameter - In some cases (like mine), if you enable raw saving (outside the script, in CHDK standard menu), the script crashes. Adding a 2s delay (in my case) before each shot, solved the issue (Thanks to waterwingz). As noted below by philmoz, running the script as it was in its first version under chdk version 1.3 and raw enabled also solved the issue. (In this case, leave this parameter with the default setting of 0s)