Adding Smart Notifications to an Old Apartment Intercom System
Hearing when someone is at the door isn't always possible, which is why GreatScott created a device that sends push notifications instead.
The idea
While going about soldering, assembling a project, or designing something completely new, YouTuber and maker GreatScott! loves to listen to music with earbuds, making him oblivious to when a person is trying to get his attention at the front door, such as a component delivery. Although a video doorbell would be ideal, his apartment's existing intercom system would not allow for any substantial modifications, thus making this solution out-of-reach. Rather, he would create a small device that could send him a mobile push notification if activity is detected at the intercom.
Design overview
Because the doorbell being pressed causes the intercom to make a noise, finding out which part of the circuit changes in voltage and sending the resulting signal to a microcontroller was easy. Once this aspect is known, a connected ESP8266, powered via 5V from the intercom, can send data over WiFi to some kind of API service and trigger a WhatsApp message to be sent.
Investigating the doorbell
Just behind the intercom handset's facade is a small control PCB with a set of four wires running into it. The first two belong to the TwinBus communication system that connects it to the rest of the intercom handsets and the central control system in the basement. The other two are for the floor's bell ring button. Due to the proprietary nature of the TwinBus protocol, GreatScott! relied on some previous reverse engineering done by others to discover that one pin gets set to 5V whenever the ringer is active.
Generating notifications
The second and last step of this project was figuring out a suitable way to get notifications sent from the ESP8266 to a mobile phone. Home Assistant combined with its simple API turned out to be too slow and unreliable, which meant that GreatScott! had to choose an external service to sent WhatsApp messages. But there were some challenges, as both Twilio and CallMeBot either required money or took several seconds between the switch being active and the message being received. Instead, he opted to use Telegram's Universal Telegram Bot Library in combination with its API so that messages could be sent over HTTP.
Final assembly and testing
After adding a voltage divider across the signal line to reduce its voltage from 5V to 3.3V and attaching a 5V power supply just below it, GreatScott! soldered together everything and placed the components back into their respective enclosures. To see more about how this project was designed and built, you can watch GreatScott's video here on YouTube.