Stian Eikeland bio photo

Stian Eikeland

Developer. Does consultancy work from own company. Lives in Bergen, Norway.

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Bluetooth LE garage opener

The yak I'm trying to shave here is the garage opener (keychain-)remote. You know those big clunky remotes people carry around on their keychains? I try to maintain a minimum (viable?) keychain and I absolutely hate keychain remotes and big keys with a passion.

Remotes are fine in the car, but I don't want to carry one. The garage in question here only have access through the main port - no door. And since I live on a hill, with a garage down by the road, connected by long flight of stairs, it's annoying to have to go all the way to fetch the remote when you just want to grab something real quick from the garage..

My Bluetooth LE garage opener.

I usually carry my iPhone, so my thinking was that it could work as a decent remote.

Requirements:

  • Connect to iPhone wirelessly somehow..
  • No wifi from house available (garage is in a wifi-shadow)
  • Connect to existing garage door opener expansion port.
  • Powered directly from garage door opener bus (24v max 50ma)

Connectivity

Wifi is out of the question since it's hard to reach without running cables and putting up antennas on both locations (and no, I don't want to connect to a garage-AP from my phone).

A cellular connected microcontroller would probably work, but I wanted to avoid that. And then, there's Bluetooth. And more importantly - Bluetooth Low Energy (BLE).

Microcontroller

I looked around for a microcontroller with integrated bluetooth (and preferably an available cheap dev board with an external antenna). The nRF51822 sparked my interest. It's cheap, packs a 32 bit ARM Cortex M0 core and supports BLE.

I found a small PCB with all the required supporting electronics from CJMCU (probably designed for use in a cheap drone? as it includes a LIS3DH accelerometer). By adding a 24v -> 3.3v step down voltage regulator, a switch, a led, an optocoupler (for triggering the garage door), and a few inputs for magnetic reed switches (for detecting door open or closed) I pretty much had a working prototype.

Microcontroller board and the iOS app.

App

I've never really done any serious iOS development. But I launched Xcode and managed to hobble together some lines of swift code that connects to a bonded BLE device, exchanges some secrets, and makes a "Sesame!"-button go solid blue when the garage is within range. It's not pretty, but it works.

Now - the bad parts. Unless you pay Apple, then your app will stop working after 7 days (free signing cert only lasts for a week). Paying for proper membership costs $99. Refreshing it every 7 days is too much of a chore, so I ended shelling out for a proper developer account. (This was the most expensive part of the whole project)

I also looked into CarPlay, would be pretty cool if you could open the garage from a car's interface, but sadly it seems that Apple have locked this down and requires you to register for some kind of special entitlement license. :(

Firmware

I might release the source code for the firmware, but there's a few things I want to iron out first. Overall this was probably the biggest learning experience, I've done some bluetooth stuff before, but BLE is a completely different beast (with it's own nuances, own lingo, etc..)

Making it secure was also harder than it should be, IMHO, there was way too much fidling around to make a decently secure BLE device.

I implemented security the following way:

  1. I've hooked up a physical button on the device to put it in pairing mode. You have to manually implement "pairing"-mode and "secure"-mode by selectively broadcast your services, and once in secure mode maintain a whitelist of authorized devices.
  2. After pairing you can exchange long term secure keys, this is called bonding. This makes future communication encrypted.
  3. And, in an effort of cooking bacon in butter, I exchange an application layer secret while opening the door (just as an extra layer of security in case i fucked something up in the BLE communication layer).

I'm not going to go much into details about how to do this, but I recommend looking at this BBC micro:bit firmware (also based on nRF51822) for how to set up a whitelist, pairing/bonding, broadcast services, etc.

And to be honest, what's easier if you want to break into a garage? A 2 minute job with a crowbar or hours of sig int and analysis?

Conclusion

The opener has been running for a few months now, and it's been working great for the most part. Sometimes the app is a bit slow to connect, or even require me to get quite close before it manages to detect the device. I've a few ideas how to iron this out in software (or the nuclear option of hooking it up to an outside 2.4 ghz panel antenna) - but it hasn't annoyed me enough yet to make me put in the effort :)

TLDR; Just watch the video...