This device can send "door open" command to intercom main unit after receiving of incoming call. It only works with coordinate line intercoms. Tested with Cyfral russian intercom.

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This is old deprecated version. Please use new ESPHome version

Parts used

NodeMCU v3 x 1
Double relay module x1 (or two single relay modules)
LED x3 (or one RGB LED)
Button x2
Optocoupler x1
Some resistors

Instead of NodeMCU, you can buy almost any ESP with enough count of GPIO (for example, Wemos D1 Mini). Button and LED are optional.

Example Parts

Scheme

Thanks to Oleg Yu

Configuration and build

Fill in WiFi and MQTT credentials in software.h
Fill in upload_port in platformio.ini
Use PlatformIO to build and upload firmware

Status MQTT messages (domofon/status)

'R' - ready; sent after successfull boot-up or after receiving of 'P' message
'L' - last will message; send when device goes offline

Incoming MQTT messages (domofon/in)

'O' - door open command
'N' - call reject command (door will not open)
'P' - ping command (answers with 'R')

Outgoing MQTT messages (domofon/out)

'C' - call; sent after detecting of incoming intercom call
'H' - hangup; sent after detected incoming call finished
'B' - button; sent when "door open" has been performed by green hw button press
'J' - reJected; sent when incoming call has been rejected by red hw button press
'S' - success; sent in response to 'O' or 'N' command
'F' - fail; sent in response to 'O' or 'N' command (this means that 'O' or 'N' command has been received but no incoming call detected)

Home Assistant integration

Full configuration with sensor, switches and automations

Put this file into /config/packages/domofon.yaml and correct notification service in automations.

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Smart intercom based on ESP8266