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A smart doorbell is an internet-connected doorbell that notifies the home owner on his or her device (smartphone or any other gadget) when a visitor arrives at the door. It activates when the visitor presses the button of the doorbell, or alternatively, when the doorbell senses a visitor with its built-in motion sensors. The smart doorbell lets ...
High-frequency direction finding, usually known by its abbreviation HF/DF or nickname huff-duff, is a type of radio direction finder (RDF) introduced in World War II. High frequency (HF) refers to a radio band that can effectively communicate over long distances; for example, between U-boats and their land-based headquarters.
The Ring Doorbell Pro interoperates with Amazon Alexa to play prerecorded greetings to visitors and allow visitors to leave a message. [45] The "Ring Chime" accessory is a unit plugged into a power outlet to play the doorbell's chime. The "Chime Pro" is an extended version that also doubles as a wireless repeater for Wi-Fi networks. [46]
Doorbell at the entrance of Chetham's Library, Manchester, England Sound of a two-tone mechanical doorbell. A doorbell is a signaling device typically placed near a door to a building's entrance. When a visitor presses a button, the bell rings inside the building, alerting the occupant to the presence of the visitor. Although the first ...
AN/FLR-9 in Elmendorf, Alaska c. 1964. The AN/FLR-9 is a type of very large circularly disposed antenna array, built at eight locations during the Cold War for HF/DF direction finding of high priority targets.
Maxwell K. Goldstein (January 15, 1908 – February 18, 1980) was a first generation Jewish-American scientist and engineer who was instrumental in the development and deployment of high-frequency direction finding by the United States Navy during the Second World War.