Cheap Wireless modem, Zigbee or what?

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Hey all,

I was asked this week if I could come up with something simple to
trigger an alarm light in a noisy industrial environment. The direct
distance is only around 80 metres, but it's around a corner (approx 60
+ 60 metres walking) and there are quite a lot of large 3-phase motors
and inverters etc. in between.

Just FYI, I have a pair of Jaycar 433Mhz modules and a pair of Oatley
modules, also a pair of these
http://www.sparkfun.com/commerce/product_info.php?products_id15%1
These modules are for my own use (if we ever get a rainy day again :-),
and given that I don't want to spend my own (unpaid) time and
components, making circuits, writing code and testing comms, thought I
might ask you guys if there are any cheap alternatives around.

I've done some simple stuff with AVRs (using GCC as a compiler), but
again don't have the time to implement comms with error correction -
ie. manchester encoding, so instead thought something like an ATtiny13
interfaced to a radio modem may be the way to go, something like
this...
http://www.sparkfun.com/commerce/product_info.php?products_id56%0
I notice that this has - from a blurry magnification, an
ATMEGA-something chip onboard (probably for comms only).

One problem I've found - after Googling for quite a while now, is that
although they have radio modems, resellers (especially in Australia)
appear to be a bit embarrassed with the prices of these units, and thus
don't show them. I could request quotes, but really, I just want to see
prices and buy the effer!
Other good places like Dontronics and Futurlec don't seem to carry
anything appropriate.

The other way I could go is to try starting out with Zigbees so that I
have a basis for future data aquisition, but not a cheap way to go I
think.

Anyway, enough of my rambling. Any thoughts people?

Cheers, Phil.


Re: Cheap Wireless modem, Zigbee or what?


The big guys like Telstra etc  plan their RF links before implementation.

They know what is
- The minimum RF signal level required at the receiver for RELIABLE valid
data.
- What are the path losses from the transmitter to the receiver
- Add in a loss fade margin.

Based on the factors above the can compute the minimum transmitter power.

A signal fade might be if a big truck parks between the transmitter and
receiver and reduces the signal.  OR
A near by transmitter reduces the receiver sensitivity.

RF losses through walls and other structures are a significant factors....
transmitter / receiver ranges quoted in "open free space" and severely
reduce through structures.


If you just have a transmitter and receiver - what happened if another
transmitter in the 433MHz courpts the signal..
Will you send additional redundant data to make sure the transmission goes
through.

Will you use 2 transceivers  and  send data and wait for reply.


Reference: Electronic Communication Techniques (Young)


At 433MHz  the Australian legal limit is  25mW  EIRP...  hence you can
deduce what the range might be like, knowing the other factors.


Just  a few thoughts.

Joe



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Re: Cheap Wireless modem, Zigbee or what?


Hi Joe,

Thanks for your thoughs. Much appreciated.

Well, after more searching I'm still thinking that these may be the way
to go.
http://www.sparkfun.com/commerce/product_info.php?products_id56%0
and combined with a simple ad hoc network protocol with 'station ID'
checking.
Because these modules do all the low level data checks this will make
coding easier and more compact for the smaller AVR micros..

Something like...
1. Station 1 sends to Station 2 - AVAilable?
2. S2 sends ACK  to S1 - AVAilable YES/NO - OR NAK
3. S1 to S2 - CMD + ReaDY? - Switch relay 1 ON
4. S2 to S1 - VERify + AVAilable YES/NO + data_received ^
5. S1 verifies original_data against data_received
6. If data OK, S1 sends EXEcute to S2
7. S2 switches relay ON and sends OK message to S1

Simple ASCII data would probably look like...
1. S01S02AVA
2. S02S01YES
3. S01S02CMDRDYDO001ONN
4. S02S01VERAVAYESS01S02CMDRDYDO001ONN
5. Data S01S02CMDRDYDO001ONN == S01S02CMDRDYDO001ONN, so OK.
6. S01S02EXE
7. S02S01OKK
Now, these are just some initial thoughts, probably some wrong ones.

I may have example code for a simple State Machine already (somewhere
in my collection).

Cheers, Phil.

Joe G (Home) wrote:
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Re: Cheap Wireless modem, Zigbee or what?



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Simple - that would be a piece of cable wouldnt it ?

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I would not bother for this application just keep transmitting the Light
status continually


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It may be a nuisance running 80 M of cable but by the time you write the
code etc for the AVRs and worried about interference, Antennas etc I suspect
that running a length of cable may not be such a drama after all especially
if there are cable trays etc - current loop or contact closures are pretty
reliable



Re: Cheap Wireless modem, Zigbee or what?


Hi Richard,

Thanks also for your reply. Indeed the voice of reason.

Just to explain how this idea came about.
After years of coding business software my new boss decided that my
electronics hobby may be put to use at work, and damn, do I need to do
something different at work!
Yes, we could just call the 'leccos' and ask them to run cable, and
maybe I could take some photos of them doing it for posterity :-). That
would be fine (and quick), also wiping the teardrops off my keyboard
would be an easy task ;-(

Just kidding, really your thoughts make much more sense.

Thanks, and regards, Phil.


Richard Freeman wrote:

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Re: Cheap Wireless modem, Zigbee or what?


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  I would expect the call should be made a licensed cabler , most
sparkies don't have a suitable ticket , and for the small fee a cable
would expedite matters considerably .

  http://www.titab.com.au/cablerSearch.cfm
  will provide you with a suitable person in your area

Re: Cheap Wireless modem, Zigbee or what?



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Well If the cable is not connected to power distribution or
Telecommunciations network then does the sparky/whoever runs the cable
really need to be licensed ??



Re: Cheap Wireless modem, Zigbee or what?


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The law says if the cable is connected to or MAY be connected to the
telephony system the installer must be licensed
  If you have any further query perhaps you should ring the Govco Dept .?

Re: Cheap Wireless modem, Zigbee or what?




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suspect
especially

I've got to agree, in both video and comms I've often looked at and tried
wireless solutions, and the overwhelming conclusion is that in any situation
where it's possible to run a cable, you should run a cable.




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