I programmed my Linux laptop to make an alarm sound and blink the screen when it detects the 433 MHz signal of this cheap garage door remote controller with the RTL-SDR radio receiver dongle. The Gqrx radio receiver application shows how the signals from the two upper buttons are actually more close to 434 MHz. This is a bit like the idea on this web page, where a low frequency radio remote controller is used for sending Morse code to someone:
https://www.instructables.com/do-morse-code-with-an-rc-car-remote-and-here-it-on/
I still need to get another remote controller at another frequency, 868 MHz, and preferably with a longer range signal, to be able to send a larger variety of "messages" with these. I'm also getting an active, amlifying Mini-whip antenna that can probably detect these signals from a longer distance.
Edit: for some reason the sound in the shared Imgur video occasionally plays delayed compared to the image when I watch it.