Honeywell Ademco 5818MNL Recessed Door Transmitter The 5818MNL is a magnetic contact sensor that provides concealed protection for doors or windows. The 5800 Series of door and window transmitters provide the most reliable, convenient and cost effective solutions for security protection. These devices are powerful and versatile enough to help solve even the toughest, most labor intensive installa With a demonstrated outdoor range of over a mile, the 5800 Series has the best performance of any transmitters available today. Includes self-contained transmitter and reed sensor Virtually disappears once installed Easy replaceable AAA Lithium battery 0.5 gap (transmitter to magnet) \Features a tapered cylinder for easy and flush fit Dimensions: 0.75" diameter x 3.0" length