Bluetooth

If you need your project to talk to the outside world, Bluetooth modules offer inexpensive yet robust communications. Many pairs of modules may be talking in the same room, and HC05 modules can also be paired with Andriod phones and many laptops that have classic Bluetooth. 

IMPORTANT NOTE: HC05 is "classic" Bluetooth and is not compatible with most Apple products. So, you can pair two HC05 modules to get Arduino to Arduino communication, or you can use one to pair with an Android app, but you can't pair these to iPhone or Mac in most cases.

EASY ARDUINO WIRELESS
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