This solution is not mine, I have borrowed with pride and figured I would share for the benefit of the group.
Might Mule (FM231) makes a highly reliable driveway alarm that senses the presences of a vehicle based on a electromagnetic sensor that buried next to your driveway by detecting masses of metal moving within a 3-12 ft. This eliminates false alarms based on animals, wind, sun, and etc. This unit has a wireless range of 400 feet which is great if you have a long driveway. The unfortunate part is the alarm receiver does not have signal outputs to connect directly to dry contact sensor (Aeotec Dry Contact Sensor Gen5; ZW097-A). To solve this I soldered leads to the visitor LED contacts and added a RED LED (Radio Shack LED 5mm; 2760330/NTE3144) to the leads outside of the receiver enclosure. Now when a vehicle breaks the electromagnetic field the visitor LED flashes and the new RED LED also flashes. Then I picked up a photocell sensor from Home depot (Hampton Bay Photocell Sensor Wire @ Home Depot; ZB2-262468-1500 SKU# 1001543162). Using a drinking straw with a white interior and cut to about 1 inch lenght I placed the added RED LED into the straw and placed the photocell sensor at the other end. Secured the RED LED and the photocell sensor to the straw. Now when the mighty mule visitor GREEN LED flashes the RED LED will flash as well, which is registered by the photocell and the signal is sent to the Aeotec Dry contact sensor. It took several LEDs to determine the correct LED for the entire setup. The green LED wave length is not compatible with the photocell sensor and white LEDs require more voltage than is available from the existing visitor LED leads.
1.) Driveway Alarm, Might Mule FM231 ~ $65
2.) Red LED, Radio Shack LED 5mm; 2760330/NTE3144 ~ $3
3.) Photocell Sensor, Hampton Bay Photocell Sensor Wire @ Home Depot; ZB2-262468-1500 SKU# 1001543162 ~ $15
4.) Dry Contact Sensor, Aeotec Dry Contact Sensor Gen5; ZW097-A ~$35
5.) Drinking Straw