ASK RF Link 

Created 12/1/11 by Glyn Hudson 

Xbees are great, but they are expensive. If all you require is a one-way link between two Arduinos, low cost RF transmission is the way to go. 

Hardware

There are several types of RF transmitters and receivers

I have been using  the MO-SAWR-A transmitter module and the MO-RX3400-A receiver module. These are available in 434Mhz and 315Mhz from coolcomponents at a very reasonable £8.80 for a pair (12/1/11). They are also available from Sparkfun.

The modules use ASK (amplitude shift keying), they have a maximum bit rate of 4800 bps and a very impressive range of about 150 metres in open air. 

The modules are wired as follows: 

The antenna should be 30cm long. Don't be tempted to coil the antenna, as it reduces performance.

Software 

The VirtualWire Arduino library is used to package data to be transmitted into a data frame. A sync preamble is prepended, and a checksum appended, to the data. The library was written by Mike McCauley. 

http://www.pjrc.com/teensy/td_libs_VirtualWire.html

Documentation for VirtualWire library

The library contains a simple string transmitting example.

Additional example code:

Range Tester - by Suneil Tagore

The range test transmitter code transmits the integers 1-9 in order repeatedly. The receiver code checks to see if the integers were received in order. If a transmission is dropped it is flagged. The number of dropped transmissions can be counted over a period of time to determine the transmission reliability rate. 

Multiple value transmission (four values)  - by Trystan Lea 

It is often useful to transmit multiple values such as power, temperature etc. I have used this code to make a wireless home energy monitor display. The code works by adding a letter such as 'A', 'B', 'C' or 'D' before each value. The transmission can then be separated into the individual values at the receiver.  

 Future development 

A more advanced library is being developed by Miles Burton here