Reverse engineering a wireless burglar alarm, part 1

After Adam’s recent post on reverse engineering a wireless doorbell, I thought I would take you through a similar process, but with a different system.

This is a Response SL2 wireless burglar alarm system, purchased from Amazon in late 2011. They seem fairly popular and well-reviewed.

We start our research without even touching the alarm, by using google. Their product page:

Operates on the 868MHz frequency with 20 Bit ID code and 1 million unique codes for added security

 So we already have a hint to where to look in the spectrum. Interesting how they say both “20 Bit ID code” and “1 million unique codes” – it is redundant information, but “20 Bit” sounds technical, so they probably add it for impact.

Notice that a number of keywords haven’t been used here:

  • Narrowband
  • FHSS
  • Spread spectrum
  • Rolling code
  • KeeLoq
  • Encrypted
  • Bi-directional

It’s likely that if the alarm used any of these, they would be making it known.

(As an aside, Friedland did initially tell me the SL series of alarms used rolling code – being a bit free and loose with specs seems to be common in the alarm world…)


Let’s start off looking at this as if we couldn’t access any of the components. I’ll use my RF Explorer spectrum analyser to take a look at what is going on. The RF Explorer can be used standalone, but if you are next to a PC, the Windows client is really great and much quicker to use.

RF Explorer (wide)

I start by tuning it to 868MHz with a span of 5MHz. This will be wide enough to receive the transmission if it uses spread spectrum, multiple channels, or if the frequency is out. I trigger the alarm, and as you can see I end up with a broad peak at around 868.3MHz. Let’s go in further.

SL2 RF explorer2

Now I can see that I have two peaks, spaced about 100kHz apart. From this static image, you can’t clearly see what is happening over time – is this 2-FSK modulation (where data is encoded as two frequencies), or is it AM on two frequencies (rubbish frequency hopping spread spectrum)? The RF Explorer can’t really tell me – so it’s about time I whipped out my software defined radio


Why did I start with the RF Explorer? The amplitude measurements are far more accurate and it can show me a much wider range (600MHz) of spectrum all in one go.

This is a TV tuner dongle. Some rather clever people worked out that it has a wideband RF frontend and can be tuned from about 25MHz to about 1750MHz, presenting us with a ~2.8Mbps baseband signal to work with. It is essentially the same as the FunCube used by Adam, but a lot cheaper. It does have some quirks and limitations though – it can be very deaf in certain areas and you see spurious signals due to the super-heterodyne design of the receiver.


I can now see the signal from the alarm in a piece of software called SDR# – this not only shows me a spectrum analyser view (amplitude vs frequency), but also a waterfall diagram (frequency vs time). I can see that the two peaks are nearly continuous – I am likely dealing with 2-FSK modulation.

I can record the baseband signal and look at it in an audio editor as well:

Audacity screenshot

This clearly looks like data now.

At this moment, I have several options:

  • Use gnuradio to build a software receiver for 2-FSK. I will still be none the wiser to the protocol, and it will be receive only.
  • Crack open the alarm and take a further look…

Part two will show how I opened the alarm and started to work out what the system was doing….

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