Timelapse+ is a small intervolometer timelapseplus.com/ that I use with some great capability for a small and inexpensive package including USB camera control and a built in light sensor. A recent upgrade in firmware brought these capabilities into play with a auto-bulb ramping capability.
I'm relatively new to timelapsing but with a little trial and user error I was able to get the function working and took it out for a test run in my back yard. The run was a 3 hour 10 sec interval. I started when the setting light enabled me to get a beginning shutter of about 1/10. I had programed in a 4 hour run but pulled it after about 3.
I processed in LRTtimelapse and After Effects and was pleasantly surprised with the outcome I had achieved for such a small and relatively easy to use tool for a newbie like myself in changing light conditions that included sunset, clouds and back yard lighting.