The main function of doorbells is to inform people inside the building about visitors. Doorbells are installed near the door or inside the door itself, depending on the type and design. All modern-day doorbells are actuated by an electric relay. Door viewers are supplementary visual aids that allow people inside the building to see and identify visitors. They are considered to have a bigger impact on safety because with the aid of viewers occupants are able to assess possible threats posed by unintended visitors. A door viewer can be installed in the door (a peephole), or in the doorbell panel, which may also contain a microphone. Such set-ups are commonly used by reception or guard staff in large industrial and commercial facilities. Mega Depot offers a great variety of door viewers and doorbells at the best prices.