nRF24L01 radio transceiver provides very efficient and inexpensive way to establish communication.
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TUTORIAL: How to set up the NRF24L01 wireless transceiver Module - Arduino (Part 2 - Wiring & Prog.)
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NRF24L01 RF Module Pinout, Arduino Examples, Applications, Features
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circuits4you.com: Wireless Serial using nRF24L01+