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I have bought and used two different driveway alarms.

Above is the DRIVEWAY PATROL-INFRARED ALERT SYSTEM.    It is worthless.   It goes off repeatably for no reason and then doesn't sound when a car pulls in view of the infrared beam.  It runs about $20.00.

I took it back and got a refund after trying it for three days.


 

Above is a  RWA-300R REPORTER WIRELESS DRIVEWAY ALARM.  It also uses an infrared beam but this one is really good.  It runs about $70.00.  I have used mine for over three years and am very satisfied with it. 

The sending unit (the green part in the photo) mounts on a post or wall near your driveway or walkway.  The receiver (the black part) sits on a tabletop or desktop. The receiver is powered by a 110 volt / 9volt transformer and has a 9 volt emergency backup battery.

It does not make a very loud beep when it sounds, but after you get accustomed to it, you can hear it for 25 feet or so.  If that is not loud enough, there is a provision for plugging in a easy to make 9v powered buzzer made from Radio Shack parts.    

The only thing negative is it’s a little difficult to change the batteries (4 - AA's) in the sending unit, the green part in the photo above.  The positive is this battery lasts about a year.  You also must keep spider webs out of the infrared eye (the rectangle on the green piece). They block the infrared signal.

9-28-2009..The RWA-300R stopped working today.  I changed batteries, cleaned the lens opening with a Q-Tip and it went back to working.  I also always seal up the top and the screw holes to keep rain out, but I don't really know if that is necessary.  It must be eight or ten years old by now.

I'm sure that there are some costing much more that may be better, but for the price, I think this one is a bargain.   

If this has helped you, take a second and punch the YES button below and maybe we can help someone else.  Thanks.