Handy tool for checking the working distance the reed switch is from the magnet or just making sure the parts are positioned and working correctly once installed.
Plug in the reed switch and switch on. Bring the magnet in close and the green light will switch off indicating a door is closed. As the magnet is moved away from the reed switch the green light will come on indicating a door opening. Batteries included. To assist, download the Reed Switch and Positioning Sheet in Brochures here.