Door entry
Voice intercom and remote sounder system
A typical block entry system for flats. It has four functions :
- Entrance door button to sound a buzzer in one individual flat
- Entrance door-lock release. A common line so the lock can be opened from any flat
- Speech from a microphone in the entrance panel that can be heard in any handset speaker
- Speech from any handset microphone heard from a speaker in the entrance panel.
1980's-style typical system description
The enrty-phone is a white plastic wall-hung handset like a telephone. Lifting the handset releases an off-hook switch which makes the microphone and speaker live and you can then talk to anyone outside. You can also speak to anyone else with an off-hook handset. There may be an amplifier in the system or it may be just a signal voltage generated by a carbon microphone and heard in a sensitive speaker. All that would need to work is a small direct current (DC) voltage to energise the microphone.
The buzzer and lock circuit is separate and runs on alternating current (AC) low voltage and it's the 50Hertz buzz you can hear at both.
Often the entry panel has back-lighting for the call buttons, this is also run from low-voltage AC.
The handset is hard-wired through a multicore telephone-type cable having 5-6-8 strands, usually in colour coded pairs e.g. blue/white + white/blue, same for orange, green, brown.
Circuit diagrams
... to be continued. Any info gratefully received!