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Messing with 433MHz equipment

We all know and love wireless devices. For example wireless doorbells. They seem so cool yet mysterious. How do they work?

Wireless doorbells:

They come with a receiver and the transmitter that you stick near your front door. Simple! They are wireless, so they use ISM bands. Most commonly the 433 MHz range. So when you press your door bell button the bell rings. The transmitter sends data out and the doorbell responds via sound – Simple!


This is the first time when I used GNU Radio. I knew that these doorbells probably send data out with AM/OOK modulation. That was confirmed later on.

I made a quick GNURadio program to decode and record necessary signals.

I connected the rtl-sdr, pressed execute and started smashing the doorbell button. And what do you know, it works.

This was the output.


You can clearly see the repeating patterns.

Later on I created a different flow graph so could record these signals.



And then viewing them with Audacity.


I used Audacity because it works better when trying to measure timings.


Now comes the hard part. How to I send data out? These send and receive dongles for Arduino cost under 5 euros on EBay. But I wanted to build my own transmitter. No need for something powerful, it needs to work.

I found this schematic:


When collecting parts if found a wireless motion detector.


You know these things typically in buildings. Only mine was wireless. And what do you know it sends data out at 433 MHz so I popped the cover off and seemed to hit jackpot. The transmitting unit is right there no need to build my own. The transmitter has a name – Jablotron TX-3. Quick search led to results:


Whipped out my multimeter and quickly found the pinout. Nothing special data, power and ground. Sometimes Mr. Murphy sleeps or something.

I strapped it to my Arduino Nano and started creating a code that transmits right doorbell data.

Easier said than done in my case. I found like libraries dedicating to 433 transmitting but they did now work in my case because I wasn’t able to change timings or anything. So I started poking more and more. I put together some code from multiple places. So couple of hours later I was ready to test this out. This is my first Arduino project.

With Audacity I was able to measure timings. So I used all the gathered data and started writing it into a code. Uploaded it to my Arduino and the doorbell started ringing! Magic

Code source: HERE.

I also tried mimicking weather sensor data. Recorded the transmission. Analyzed it and then played it back. And Bob’s your uncle!

This applies to large amount of devices.


And here is a video:
Sorry for the shaky footage did not plan to upload this.






  1. Hi Sander
    I am also looking to decode 433 OOK Signals of my weatherstation and have been looking around for code or a GNURadio GRC file to get me started. From reading your Project it seems that you have been able to create a OOK Demodulator with GNU Radio- guess it is based around the rtl_433 you had mentioned earlier? would you maybe be willing to post/share the code you have written? Thank you, Franklyn

  2. Are you using a SDR?
    With GNU Radio I was using AM demodulation to actually see these packets. You can even record these transmissions with SDRSharp or other SDR software and then process the recordings with some audio software (for example Audacity).
    For the simplicity I used rtl_433 to decode my receptions. Also I edited these rtl_433 build files to suit my specific needs (to add date and time, etc.).
    If you are lucky the rtl_433 does the hard work for you. If it does not work out of the box you can write your own software or use rtl_433 with “-a” and work things out that way. ‎

    Also there are a lot of these kinds of topics on the internet so Google works well in this case.
    Good luck with your project and let me know how things worked out!

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