Thanks for responses.
AWOL, CHDK could be part of a solution, and could even do the motion detection, but with a lot of time spent learning/hacking; here's a link on that: http://chdk.wikia.com/wiki/UBASIC/Scripts:_Multipurpose_Motion_Detection
zoomkat; thanks, but I've never been able to get any sort of off the shelf PIR light to work around my place, they trigger every few minutes, and they stay on constantly on windy nights. We also have lots of bugs, maybe crawling over the sensors? Happens even in winter, though. We're in s. Illinois, or lot is wild and wooly.
Chagrin, thanks, I suspected that was too long a distance for little IR LED.
Pauly, Triggertrap might do part of the job, but apparently still pre-production.
Much thanks for replies. Maybe I will figure all this out; looks like a challenge!
At the very least I'd like to have a count of entry into our drive, I'm sure I could figure that out.