Inside a door entry phone

This brand of door entry system is probably the most popular in the UK. It's very simple, but is often installed badly by security companies, so it's useful to understand how it works to fix the legacy problems they create.

I'll cover the main door voice unit and button panel in a separate video. In summary, it's basically a simple power supply and a dual amplifier to amplify the local microphone to the common phone speaker buss and also to amplify the incoming common microphone buss for the local speaker.

This comes across as a well evolved legacy design that is refreshingly simple, but with lots of clever features. The use of a microcontroller to create the ring-tone was a surprise, but a nice one. The PIC microcontroller literally has just three connections. 0V, 5V and audio signal out.

I specifically avoid using intrusive mid-run video adverts for a more relaxing viewing experience.
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