A few possibilities that I'm considering:
1. Bore some 3/4" holes in the soffit under the eaves and mount inobtrusive, simple $3 PIR sensors there and have an Arduino monitor the lot of them and use a double or triple trigger across different sensors to mitigate false positives. I'd have to use tape or paint on the little Fresnel domes to mask out detecting anything at the sidewalk or beyond. Anyhow, easier said than done: it would require a lot of up-and-down on a ladder to tweak it right. It may also be that the 3/8" thick Hardieboard soffit material would overshoot on masking the detection range.
or for $7 surface mount a two-wire version of it:
or possibly a $9 surface mount ceiling PIR with a dual detector:
That's just a first approximation. Probably isn't for outdoor use. Maybe there's something better? For $70, there's the Visionic:
but I don't think it's for outdoor use either. Notionally, a detecting strip like this would be less likely to pickup street traffic:
The next image below a $70 outdoor one. Probably meant to be installed on a wall, but maybe it would work facing down from a soffit under the eaves for the intended purpose?
2. Mount retroreflectors on the fence and tree and use active IR. Never tried anything like that before. This would be better for catching things heading toward the sides than catching regular activity at the front. On the other hand, I guess maybe I could shoot a long beam from one side of the yard to the other across the entire front yard. Not very convenient, as I'd have to run wire to power the thing. For just a regular electric eye (no retroreflector), and around $300 for a 250 foot range, there's this outdoor unit:
I'd have to trench two wire runs under the turf, though. Unless I can hijack wiring already buried for irrigation control, running the wire would entail the greatest amount of labor.
3. Find regular motion detecting light fixtures. Rather than modify them to get their sensor input directly, a ghetto approach would be to let them do their thing and then just notice when they come on, using either current detection or a photodetector. The trick will be finding ones with good detection but reasonably discreet detectors. This is probably the least effort. I'm probably leaning toward something like this as an initial step and see how far it takes me. Trouble is, I'm not sure if any have different modes, where they can be simply turned on and left on for certain circumstances, or whether they just do their lights-on-after-motion thing. Judging from photos, the PIR detector isn't too obvious, but maybe in real life it is:
Hard to find an appealing design, but maybe I can find something with LED in a more modern style than coach lights. However, not finding anything like that. So, it's a strong argument for keeping the light fixture separate from the detection.
