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I have a typical European failsafe bell transformer installed in my electrical box, wired to a classic ding-dong chime (works at 230v and 8v). I wonder if there is any component or a simple DIY solution that allows me to connect this legacy system to the Zigbee or MQTT network so that I can set up notifications on the mobile devices (and other systems), without migrating to an unneededly expensive (and unattractive) modern video doorbell. Note that many European-compatible video doorbells do provide this possibility (Ring, Netatmo), but I haven't found any standalone solution just for the bell notification without bells and whistles (pun intended).
In other words, is there any easy-to-use accessory on the market that monitors a wire for a current and acts as a remote switch upon detecting it? Without wanting to dive into product recommendation, can you mention some?
You can use a nodemcu with an external trigger circuit to develop your solution. Connect a relay or an electronic trigger circuit to generate trigger signal for the NODEMCU. So when the door bell rings, it sets a high trigger to the NodeMCU and this trigger can be used to send a signal to the MQTT topic.
A mobile app can subscribe to this MQTT topic to show the notification. A highlevel architecture based on my understanding is given below.
Yes, you could find something that can trigger on the voltage. But you'll have to power that too.
Consider this alternate solution (maybe easier).
Get a vibration sensor, such as the Samsung SmartThings multipurpose sensor. Set it up to monitor for vibration. Just attach it physically to the doorbell enclosure.
One thing I'm not sure of is whether there's enough vibration for the sensor to trigger.
You may endup having to also have an LED light up on the bell switch press and use that as a photoelectric trigger. I hope the vibration is enough !