Can Someone Buzz Me In?

For those of us who live in older multi-unit condos and apartment buildings, we’re familiar with the old doorbell intercoms and buzzers. Typically someone comes to the front door, presses a door number, you pick up an phone-like handset, say, “who is it?” and then buzz them in by holding down a button which briefly unlocks the front door.

With AirBnB and daily Amazon deliveries, this system sucks because you have to be home to let someone in. The problems is made worse in my building because we only get a few non-duplicatable keys and the threat that, if you lose one, you’ll have to pay $15,000 to have the building re-keyed and new keys distributed to everyone in the building.

I wanted something better so I created doorman, a mobile web app that unlocks the front door from anywhere.

Here’s what I used:

As a diagram, it looks something like this:

The password list kept on the Pi is easy to update so you can add and revoke access with a few key stokes.

It’s probably worth noting that anything you can safely control with a relay and a python script could use this same set up. It’s convenient that the intercom handset is 12V DC so it’s easy to play with. Before you go through the bother of setting all this up, it’s worth shorting ground and door release terminals on your intercom handset and making sure it works like mine. I was able to find all the wiring diagrams for my intercom online. Make sure you know what you’re doing. Don’t electrocute yourself or fry your intercom.

To set up it up for yourself:

(2) Get a Raspberry Pi and Relay card,

At this point you’ll want to test your work, go to the Pusher.com Debug Console, press Show Event Creator and modify the data attribute of the JSON block to be one of the passwords. Click Send Event. If it’s all working well, you Pi should execute the command in the doorbell.properties file.

Here’s how things wired up

The Java app on the Pi just executes a shell command so it’s best to test without unlocking the door, then run the python script to make sure the door unlock works. Once you know the two pieces work correctly independently, modify the doorbell.properties file and restart the Java app to test it out.

I could do some future work by preventing a brute force attack by requiring a long key that’s shared by all users that’s verified by web app and add some per-IP rate limiting. From a computer security standpoint, this isn’t strong authentication with two factors, but it’s better than the security on an electric garage door.

If you decide to set this up yourself. Make sure to configure the /etc/rc.local to re-launch the Java app on start up of the Raspberry Pi. You don’t want power outages to lock you out of the house.

The system does rely on power and networking so it’s not a great alternative to not having any keys, but a nice way to let the Amazon delivery guy in while you’re at the office or if you want to go on a run without your keys.

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