did sth similar a while back but was super lazy so i just got one of those doorbell extenders that you can attach. first i used dvbt stick to capture signal then. second solution was a simple lightsensor i glued the doorbell extender thing as it made a blue LED blink once when the door rings. both things work but frenk can even turn his one off. not sure if i would be willing to connect the relay he uses. very cool project nevertheless.
Yeah, and they're assuming you're going to power it off of a 5v line. In most instances you'd want to power it off the existing doorbell transformer, so you have to step down the voltage and convert it to DC somehow. That's $5-10 right off the bat.
Additionally, you'd want to have the esp chip and relay at the doorbell transformer, not at the actual doorbell. You'd need to have a second relay or power transformer for this, but that's not a huge deal.
The project would likely be around $10-20, assuming you're not going to 3D print an enclosure. Plus, if you already have a front door camera, chances are you'd already have a doorbell camera that can already do this.
Source: building custom smart devices is my hobby.
