Bests & Worsts Reviews from Amazon

This is a good product, but you have to understand its specific limitations and the limitations of all driveway alert systems. The product does work, and if installed carefully in the right environment, may work well. It may in fact work better than several competing units, including Mighty Mules' FM131, which I'd previously bought and used for a couple of years. First, most driveway alarms do NOT detect "vehicles." They're not video cameras (though for some people that may be another option). Some respond to infrared motion - a motion detector - caused by the movement of nearby objects, while others detect local changes in magnetic flux caused by the movement of a nearby metallic object. In most single home applications, motion detecting driveway alert systems will respond to the movement of nearly anything - birds, pets, people, deer, windblown trash. So motion detectors are not a good idea for settings where those variables are likely, but may work well in more controlled environments. Magnetic field detection units - like the Mighty Mule FM231 (also sold as the GTO 4450)and the FM131 work differently. They will only respond to changes in magnetic fields - but since most deer, pets, birds, trash, and people aren't terribly magnetic they're fine in situations where motion detectors won't be suitable. (It's the same concept used in anti-submarine warfare, where sub hunters try to find subs in the ocean by monitoring the changes in the magnetic field a moving steel sub makes underwater. Pretty cool for under a hundred bucks.) I purchased this specific unit, the FM231/GTO 4450, after a couple of years of living with the FM131. The 131 worked, but its sensor and transmitter unit shared the same small green plastic housing that was mounted on the end of a pvc tube about 30" off the ground. This meant that while the 131 did a fairly good job of picking up cars, strong winds regularly caused the pvc pipe to sway enough so that the sensor thought something metallic was moving past IT - rather than that IT was simply moving slightly in the larger magnetic field. As Einstein said - all motion is relative. A sensor can't tell whether something is moving near it or it is the something that's moving. The upshot is that in many situations the above ground sensor of units like the FM131 will give many false positives, making you think a vehicle is approaching when it isn't. It should be said that the worst failing of a driveway alert systems is not a false positive but a false negative - making you think nothing is coming down the driveway when something really is. Most units are fairly good on this score. A false positive is annoying. A false negative can be serious, like if it's muggers coming to assault you or worse. Another reviewer mentions possible burglars coming down his driveway. The FM231 solves this by burying the sensor in the ground where it CAN'T move. So it never tends to think something is coming by when it isn't. At least not when it's properly installed in the right environment. But bury it near a deposit of iron ore, or next to a junkyard or someone's old jalopy or washer covered by vines that you don't even realize is there and it will drive you nuts. And most of us don't know where our mineral deposits on our land are, or where Uncle Wally deep sixed the old refrigerator. All of which is to say the unit may need installation tweaking. You may get it spot on the first time but I didn't. It took a good full day of digging holes and testing and then digging new holes and testing and fiddling to get it sited and working properly. Also, the reviewer who warned about putting in the batteries after test placement is right. The unit will remeasure the magnetic field around it every time the batteries are reinstalled. I didn't realize that and it caused me to think the unit wasn't working when the reality was I'd been recalibrating it by reinstalling the batteries in the transmitter with my steel shovel and pick and tamping rod lying in the dirt next to the sensor. When you install it correctly you can test it simply by waving your shovel or pick near the buried sensor and listening for the receiver's chime if you have the windows open and aren't too far away. But move the steel tools before the final battery insertion. All in all it works far better than the old FM131 -- no false positives so far - and we live in a very windy area - and it's spotted everything that we know to have come down or up the drive. And despite some reviewers' comments, it also transmits a stronger signal than the FM131 did. Both are advertised at 400' line of sight but the 231 is much stronger. We can place the system forty feet farther up the drive than with the 131. Be aware too that trees or hills or just the structure of your house - metal siding or brick, for instance - will reduce transmission strength. And the volume of the receiver chime has been fine for us - I consistently hear it from my downstairs study even though the receiver is upstairs and in a different part of the house. Still, noisy environments or hearing loss may be problematic, as others note. Bottom line - if you need a driveway sensor this may be a very good bet. But not for a short driveway near a road or by a metal working plant. And be prepared to spend time installing and testing and even perhaps to send it back if the environment isn't right. But don't blame the product if it isn't. You'll just need to find a different solution. And you could still try to hunt for subs.