I have an old doorbell with the transformer in the basement and the little pistons with the chime. I’ve been coveting the smart doorbells, but then I had an idea. Knowing that the smart door sensors use a magnet to trip the reed switch, I wondered if the little electromagnet that dings the chime would be enough to trigger the door senssor. IT DOES! Now it notifies our phones and I can trigger other automations.
You can see the copper coil in the first picture and where I set the sensor in the second. So simple, it’s stupid. YMMV.