The problem
A person alone at home had no way to see who was at the door, or to summon help quickly.
The innovation
A motorised camera scanning a set of peepholes, feeding a bedroom monitor, with two-way voice, remote door release and a panic alarm.
How it worked
The camera slides between peepholes at different heights so the resident can view a caller of any height without opening the door.
Why it mattered
Described, in 1969, every functional element of a modern video doorbell.
The legacy
The patent has been cited by dozens of subsequent security and surveillance patents.
You use this today
Video doorbells and home CCTV.
Sources
US Patent 3,482,037
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