I have a Friedland Type 6 AC door bell chime. 16V AC. Press door bell and solenoid moves and hits metal plate making a sound. Both fire if front door and one fires on rear door.
To connect to HA I first tried reed switches over the solenoids to see if they would open/close with the magnetic field but no luck. Tried vibration switches but too many false positives and false negatives. Contact sensor between solenoid and metal bar didn’t work.
Finally settled on optocouplers connected to a 433mhz door sensor.
Found a good Video on the optocoupler. Big Clive Optocoupler
Problem was getting a signal through to the DC side. Had to reduce the resistor on optocoupler down to about 900ohms on rear door and 3K on front door to get enough current. Replaced the 150K original resistor. Worked my way down from 10k resistor. AC only on for less than 1 second so risk of burning it out is low as it never heats up.
Removed reed switch and tamper switch to give me 2 signals for front and back door.
When door causes 433 sensor to fire it gets picked up by either my sonoffRF bridge or OpenMQTTGateway-RTL sensor and sends it to HA by MQTT.
This is it all hanging out.
Just enough room to fit the 2 optocouplers in original case but had to make 4 small holes in top of Friedland cover to fit wires though to get the 433 door sensor sit on top of the cover. Will replace the dupont jumper wires with something more flexible.