Make Your Ancient Intercom/Door-Control Usable Again

If you live in an apartment building with a locking lobby door, you most likely have some sort of intercom system that can also be used to unlock that door to let guests in. If your apartment building was built sometime in the 20th century, and a new system hasn’t been retrofitted, it’s likely that the intercom you have is virtually unusable. Those old systems required you to actually be at the intercom to let anyone in—a major annoyance in our modern wireless world.

Luke Hoersten had that problem, and decided to take the solution into his own hands. His intercom has four functions: a visitor can buzz him, he can unlock the door, he can listen to the visitor, and he can talk to them. All of those functions are handled by five wires, which carry an unusual 52V.

Hoersten knew he wanted to use a Raspberry Pi to make the doorbell and unlocking features accessible from his phone, but couldn’t control those functions directly because of the high-voltage of the circuit. Switching high-voltage circuits with low-voltage controls is exactly what relays are for, so Hoersten ordered a Pimoroni Automation pHAT. To read the doorbell, an analog-to-digital converter (ADC) makes the signal readable by the Pimoroni.

Those functions are handled by a Python script on the Raspberry Pi, which uses Pimoroni’s own functions and API. Finally, that’s all integrated with Apple HomeKit to make it easy to access. The system works well how it is, but Hoersten plans to get the listening/talking features working soon, too.