Zigbee Fire Alarm Detector

My home is already equipped with 4 EI 146 smoke detectors, which are interconnected by a separate signal wire (IC labelled). So when one of the smoke detectors detects smoke, all connected smoke detectors will also start making noise. This instructable is inspired by this project: https://www.instructables.com/Tiny-Fire-Alarm-Dete...

- At least one EI 146 smoke detector
- Xiaomi Aqara (or similar) door/window sensor
- PC817 optocoupler
- 4.7kΩ resistor
- Some wire, different colors
- Isolation tape or shrink tubing

First, open up the sensor. Remove the battery, and desolder the glass Reed sensor. Solder two wires to the place where first the reed sensor was connected. Those wires will be connected to the switching side of the optocoupler (green wires). Next, solder the resistor to the collector of the optocoupler. The yellow wire will be connector to the other side of the resistor, this is my IC line. Also connect a wire to the emittor of the optocoupler. This (blue wire) will be connected to the Neutral wire. Make sure everything is isolated very well, especially the yellow and blue lines!!
To make everything look professional, drill or grind a hole in the housing of the sensor.

- Remove the sensor from the base (check the manual how to do this!)
- Remove the buttom inlet from the sensor and drill a hole in it
- Remove the connector cap and connect the wires thoroughly. Official the connector is not build for holding two wires, so make sure to do this in a safe way!

That's all. The sensor should now switch on when the alarm is triggered!
It can be wise, also to notify when the battery is empty, but it should last 1-2 years in my experience!