Implementing a simple radio data link- Where to start?

[McPete]

Hi All,

I've been asked to put together a system to send a temperature reading from a remote building, back to a building management system, which wants a 0-10V or 4-20mA input. The distance involved is less than 100 metres. The resolution doesn't need to be greater than 0.1 of a degree, and the accuracy doesn't need to be fantastic either. The range is -10C to around 50C.

Unfortunately, having been in service/repair jobs in career so far, I haven't had to do much of this kind of stuff.

The part I'm not so sure about is the development of embedded software to extract the data from a sensor, funnel that into the radio module, then on the other side, decode that, put it into an ADC, and get an actual voltage out.

What I need is a poke in the right direction of how to start this whole project, and and a bit of guidance as I go. The micro I've done some classes on is the TI MSP430, so I'd be inclined to start with it, but if you have a better idea, I'm open to it.

My initial inclinations are these;
*Poll an SPI interfaced temperature sensor, read the data from that into a register in my transmit-side uC. I like the look of the LM70 for this.
*Stream that data (Plus some sort of address/checksum?- As I understand it, the 433/960Mhz bands are pretty flooded with signals) into the RF module
*Receive and validate that data at the RX.
*Pass that onto an DAC, which may need to be fed into an op amp to get it to the required levels. I could probably use a cheap parallel DAC for this, as I expect I'll have a bunch of IO pins and not much else to connect up.

Am I on the right track with those inclinations? Where should I start?

Thanks,
Peter

[homebrew]

Hi Peter,

Your ideas look good to me in principle. I would suggest to also use an I2C or SPI capable DAC as you really don't need much speed here. Take care of the resolution though. You will need at least 10 bits if you want anything near 0.1 degrees resolution.
If it is some sort of single device prototype hack, I would probably go with an Arduino. They are quite cheap and reliable. And all sorts of I/O shields are available. And code example, documentation, community etc. etc ...

What worries me more is the distance. Around 100m probably also through concrete walls seems no easy task for some cheap ass RF module. (I can't even have wireless in front of my house because of concrete walls and there we're talking about 100mW RF). I think you probably need direct line of sight.

Cheers,
Peter

[Whuffo]

You should start by checking your national radio licensing requirements. License free operation limits output power / radiated signal to the point where 100 meters is pretty much out of reach unless you use highly directional antennas and have line of sight access.

[GK]

We use both licensed and non-licensed data radios from STI engineering (formerly RF innovations) for this kind of thing.

http://www.stiengineering.com.au/products/default.aspx