M2M 433Mhz RF Link - DORJI Review

There are a ton of different ways of getting Machine-To-Machine communications, everything from WiFi to Bluetooth. However most systems employ some sort of protocol layer. These make certain that the receiving and transmitting pair are talking to each other. They can be fiddly to set up or restrictive. If only there was a way to just have a open channel.!? Well the DORJI 433Mhz Data Radio Modem is just that. So who does it work, what's it like then?

It was a tweet from Tindie that got my interest going. You may have seen that I’m working on the EarthRover project and I was looking at RF links that I could use. Ones that are easy to use and designed for low power usage. I’d looked at designing a straight RF link transmitter as other options were not what I wanted. You see devices like the XBee take time for the underlying protocol to connect the devices, same way a WiFi unit logs into a network. However I don't have secure data to protect and need basic fast communications.

The DORJI units allow for straight UART connections at a range of speeds. The RF side is all sorted for you by the onboard microcontroller. Operating at 433Mhz it has great range and can transfer data at up to 40Kbps. The datasheets also says the unit can drive its output to a maximum of 20dBm and achieve -121dBm at 1Kbps.

The really interesting thing for me was the power usage of these units. In normal mode the unit draws around 16mA for receiving which is typical for most RF units I’ve seen. However transmitting at fully 20dBm will draw 95mA. However the power output can be adjusted to one of eight steps or 3dBm allowing for great reductions in power usage. This will reduce range but means you can choose to only use the power you need.

As a range test I connect one unit via the USB adaptor to my laptop and the other was connected to a mbed. The mbed would echo any data received. With a data rate of 10Kbps set I headed off out the house and down the road. With the mbed unit inside my house (remember in uk we have brick houses and little space between them) I got a easy 200m down the road before seeing a drop in data. With the unit in the back garden I repeated the test and, well, decided that about 0.4 of a mile across a field that the range was more than enough. From a free online range calculator I worked out that I should be able to get 3 miles range. But this was not tested.

Coming back to the power draw, there is more than one mode the units can be used in. There is the ulta sleep mode where basically the unit if of and the drops the current to

For me the nice thing wa the speed of response. Once powered on and set into the required mode (taking milli seconds) you can start sending and receiving straight away. This makes the link completely open and transparent.

So keep an eye out as you will see these unit in use on the EarthRover project!

Thanks

Paul (aka @monpjc)

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