The magnetic sensors detect a disturbance in the ambient magnetic field. People and animals won't trigger them. The easiest way for a car to slip through is to creep past the sensor. A high aluminum content car has a better chance of going undetected. I actually have 2 of the sensors on my long driveway. Occasionally a car will slip through one of them, but I never seen an instance of slipping through both. I get a very occasional false positive when a large trailer, snowplow, or garbage truck goes screaming down the road about 30' from one of the sensors. On the NVR trigger, all I'm doing is an event trigger on a few of the channels. Besides being a backup to tripwire detection, it gets me around the Dahua nvr bug where frames are dropped on the event trigger, which is the absolute worst time to be dropping frames.
Here's one example:
Gate Crafters Outdoor Buried Driveway Exit Sensor CS-202 Series - Gate Hardware - Amazon.com
Find others searching for "driveway exit sensor". This isn't the one I'm using, so I can't say how much work it is to interface it to the NVR. It sounds easy enough, saying "Normally open and normally closed relay connection". Only because I'm a cheapskate, I'm using the Mighty Mule Wireless Driveway Alarm, which costs ~$70, hacked to be a wired sensor. If your not an electronics tinkerer and willing to risk breaking the device, this won't work. I'm happy to share the hack with anybody interested.
