Inside a door entry system voice unit

This is a very standard door entry system voice unit that is often found in the main door button panel of door entry systems in the UK. The Bell System units are the most popular in the UK, and when you look at the circuitry in them it becomes clear why. The design is simple, but clever and has good protection against short circuits and installer error.

This module is the main part that deals with amplifying audio from the built in visitors microphone and also the microphone output from each flat/apartment's phone.

There is a separate video showing the circuitry in the version 5 phone here:-
https://youtu.be/VkBDStN5Nmo

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