Modules which allows you to communicate at 433MHz band are extremely cheap. Transceiver set you can get for under a 1$. Adding them up with Arduino Mini Pro (~4$) you can create wireless sensors for humidity, temperature and present them in any form you like.
While working with those transceivers I wanted to check precisely in what part of 433MHz band it works. I used a DVB-T USB Dongle (chipset rtl2832u and r820t) as a simple spectrum analyzer. CubicSDR software (CubicSDR – home page) allows you to follow waterfall live. I generated the signal using my car key and garage gateway remotes to compare TX power of the transmitters. With 6,5cm of a copper wire I have built a simple half-wave dipol antenna and I generated square wave on transmitter module of Arduino. It seemed that this setup completely jams 433MHz remotes (up to ~200m radius!).
I can only think of one rational usage – as a jammer for ‘cloner’ stuff which car thieves likes to use. You can build such module, add bluetooth communication and enable/disable it remotely with smartphone. Leaving it enabled in car (without antenna!) will make it impossible to open car with any key working at 433MHz. After disabling it over bluetooth you can open the car normally.
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int powerPin = 13;
tone(8, 6700); // generate square wave
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This was built quickly just as a PoC. Working radius – ~10m. It was connected to smartphone over OTG cable.
This one is enclosed within a PVC case with antenna cable taken outside. It has it’s own power supply and on/off switch. This version jams 433MHz band up to ~200m radius. You can also see a screenshot from CubicSDR software while generating square wave.
Link for YouTube channel – working device:
YouTube – Arduino 433MHz Jammer