Many apartment buildings in urban areas have intercom systems that give residents the ability to unlock the building’s front door when a visitor calls. The visitor selects the resident from a list and the system connects them to that resident’s intercom. The idea is that the resident will recognize their visitor and push a button on the intercom to allow them entry. herzogshandicraft’s handy device utilizes the intercom system to automatically unlock the door when it detects a specific audio pattern.

This device serves two purposes: it lets the resident gain entry to the building without needing a key and it lets the resident give access to visitors when they aren’t home. It listens for a pattern of noise created by the visitor pressing and releasing the “talk” button on the intercom. When they press that button, audible noise comes through—even when nobody is speaking. In this case, it listens for noise above a threshold (the button pressed) for one second, no noise (button released) for one second, and then noise again for another second. If it hears that pattern, it uses a servo to press the door unlock button on the intercom.

An Arduino Nano RP2040 Connect controls the device and was chosen because it has a built-in microphone. It would also be possible to utilize the Arduino’s onboard wireless adapter to log events if the user wants to track them. The only other components are the small hobby servo motor, a mini breadboard, and a USB power supply. Those fit on a custom 3D-printed frame that attaches to the intercom panel. It’s an affordable and simple way to add some intelligence to an apartment intercom and users can always reconfigure the unlock pattern to suit their own security requirements.