The goal of Ilias’ project was to be able to use the RTL-SDR and MATLAB to uncover the details of a 433 MHz transmitter he bought on Ebay. He wanted to see if he could determine the protocol and recover the data before even looking at the transmitter’s library code.
To do this he first used SDR# to record the data sent at 433 MHz. Then by looking at the waveform in the Audacity audio editor he was able to determine that the signal was on-off-key (OOK) modulated and from this knowledge he was able to manually recover the binary string. Next he used MATLAB to create a program that can automatically decode the received OOK signal. His post goes into further detail about the signal processing steps he took in MATLAB.
First Tomasz used his RTL-SDR with SDR# to capture a few sound files of the gate remote which transmits at 433 MHz. Then he viewed the sound waveform’s in Audacity, a free audio editing program. Just by looking at the waveform he was able to determine that the signal was On-Off Key (OOK) modulated and that each frame of the transmission was the same, meaning that no security scheme was used.
Next he wrote down the transmission parameters that he learned from his analysis and built a simple 433 MHz transmitter which he connected to a microcontroller. After programming his microcontroller to send a copied signal he was able to open the gate.
At Tel-Aviv University in Israel, two students undertook a class project where they were able to use an RTL-SDR to record a garage door opener signal and then use a Texas Instruments (TI) Chronos watch to retransmit a copy of the signal. Their report can be found here (pdf). The TI Chronos is a wrist watch with a built in programmable ISM band RF transmitter.
The students report contains an analysis of the signal which may be of use to anyone interested in decoding their own ISM band signals and they also describe a method used to automatically obtain the required parameters for programming the TI Chronos with the signal to be copied. The abstract of their report is as follows
We present a simple and affordable way of copying remote controls widely used for parking lot gates, garage doors and other simple systems. These simple remote controls usually use a fixed code (as opposed to the more secured rolling code used for car keys remote controls) and a simple On-Off Keying (OOK) modulation, over 433.92MHz in the ISM band. We suggest the use of the TI-Chronos wrist-watch platform for the emulation of the remote control, as this platform transmits in the same band, and can be programmed to emulate different modulations and to send user pre-defined signals.
In this report we show the complete process for copying a remote control into the Chronos platform. This process utilizes only a standard PC and low-cost hardware (less than $75 all together), alongside free software, and additional software developed by us. The process starts with recording the original remote control RF signal. It continues with automatic analysis of the recording, extracting the needed parameters of the signal. Finishing the process, we set the Chronos with those parameters. We demonstrate the copy process using a 4-channel remote control and its receiver board.