I'm running a IR dual beam from Optex across my driveway about 50'. The unit I have is rated for 120' outdoors.
I tried microwave and ultrasonic with a large number of false triggers. The IR beam has been almost 99% reliable. It only falses during the few days of the year when the rising Sun shines directly into the receiver.
The only installation problem with this type of detector was trenching the power wire to the transmitter through my gravel driveway. My initial installation had both transmitter and receiver side by side with a passive reflector on the other side of the driveway. It worked fairly well except the received signal strength was boarder line and created false alarms. A more efficient reflector might have solved that problem.
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