Detect when door is closed with 100 percent precision

Thread Starter


Joined Jan 8, 2020
3' X 4' implies a rather large dog so 2" laser spacing over bottom 1/2 of door should work. Might use light dependent resistors for receivers. Drill tapered holes for laser modules, tight fit on exit,
ring of fast set epoxy on back; aim & hold till set.
Any circuits or ready made recommendations


Joined Aug 7, 2008
Ready made light curtains seem pretty expensive so if I really needed it I'd give it a try. For a test
I used 7 layer plywood X 3/4 " X 2" strip. Started 1" from bottom & 3" spacing for remainder but only drilled for 2 . For laser drilled 15/ 64 ths. thru, enlarged to 1/4" .3" deep for wiggle room.
'Used a shinny Al foil cone consentrater with CdS photo cell which allows about 1/2 " target for laser beam. Drawing only a suggestion based on CdS cell that I have. Comparator might be a
quad LM339. Used a LED in series with R4 for testing.