Reverse engineering an ancient wireless security keypad using RTL-SDR

Instead of a Pi you can use an Arduino with a RF module. The RF transmitter modulates at a standard frequency (e.g 433.92MHz), so the Arduino only has to send the code (a few bytes) at an appropriate frequency.

Here is an example I wrote, where I use this setup to turn on and off a remote controlled socket: https://github.com/andmarios/arduino/blob/master/lidlRemoteS...

Total cost could stay below $5 if you buy cheap components from eBay.

AFAIK some/most garage doors now use rolling-key encryption, like car keys.

Nice. I have a 30 year old garage opener but I don't have a remote. I know the combination because I set the DIP switches, and I think I even dug up the trasmission frequency when I looked into it a few months ago. I don't know what sort of transmission the opener wants to receive, though. Any resources you can point me to on that topic?

Unless I'm reading this wrong, GPIO4 is being used as a radio transmitter, but it can only do between 100.025Mhz and 99.975Mhz frequencies. Garage openers work mostly on 315 and, older ones, on 390 MHz.

I don't think the pi ships with a proper software defined radio.

Probably it's just the general stability of the oscillator circuit. It seems to drift around 300 kHz around its center frequency of 340 MHz, so about +/-1‰. For such a very simple transmitter that's likely demodulated with an equally simple LC-filter and an envelope detector, it could just be be right "up to spec": http://imgur.com/l47Nm4B

Edit: % -> ‰

Technically you're right, but this is a problem that comes up a lot in wireless home automation. Thing is, what is the attack vector you want to protect against? How many burglars are going to be sitting outside your house (I'm imagining: in a black van with 'Bob's plumbing' written on the outside) analyzing RF patterns when someone comes in, reverse engineer it, then burglarize you home? My estimate is 0, even when not weighed against the alternative: if someone really thinks your house is so interesting so as to warrant several hours of waiting/observation, they'll just put a knife to your wife's throat and say 'turn off the alarm and show us the valuables'.

The vast majority of garage door openers out there use unencoded RF tech, and it's very seldom a problem. And the other side: there was a recent string of thefts of BMW and VW cars in my area. Reason? They had an inside man at the main office who slipped them copies of the remotes.

The tech is very, very rarely the weak link.

I recommend scanning your 300-440 MHz range - there's an amazing number of devices working in it. Garage remotes, car remotes, smart power meters, ACs, weather stations, tire pressure monitors, I can see 3 periodic signals at home and I don't even know what 2 of them are. (maybe picking up something from neighbours)