There are many ways to communicate with remote Arduinos. I have found that many of them hit a slightly higher price point than what I would like. Also, ethernet shields (and wifi dito?) consume many valuable hardware pins. I will describe how to communicate host to Arduino using commercially available wireless serial ports. They are cheap(ish) and only use the rx/tx pins. The drawback is that the air-protocol isn't encrypted so I wouldn't use it for sensitive applications. I use it to control lighting. I would be more impressed than offended if a neighbour hacked my indoor lighting. Serial ports (even wireless) are designed for point to point communication. It is possible to use in one-to many or other constellations but certaing design consideration have to be taken. My Arduino library referenced in step 5 supports one-to many.
The described project is for one-way or two-way communication using (i. e. transciever). If you know you will only send commands "blindly" you can attach a transmitter to the PC end and a receiver to the Arduino. Those modules are really cheap.
Step 1: What you need
- USB serial adapter
- Two or more Wireless RS232 modules (e. g. EWRF 3022)
- Arduino. It's a Nano in the picture but any model should due.
- A gadget box
- Soldering equipment
- Basic Arduino skills