The replacement sensors project an light beam across the inside of the garage door to prevent accidents and are required for safe use of all garage door openers. Whenever there is an obstruction, the garage door will automatically reverse before causing any damage. these sensors replace defective or damaged sensors for every garage door opener manufactured after1997. Compatible with sensor 820cb For use with every Cham garage door Use: replacement for damaged sensors Easy installation Enhanced trouble , most intuitive reversing sensors Low profile reduces risk of being knocked out of alignment Whenever there is an obstruction, the garage door will automatically reverse Due to different product batches, the appearance , which does not affect the use Colour:As Shown Material:plastic Package Contents: 1 x transmitter 1 x receiver Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.