EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: davistroy on July 03, 2010, 12:45:14 am
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Starting to design a project that would be up to 8 wireless transponders that would each reply with GPS data every 15 seconds when each is interrogated on a rotating basis - so very low bandwidth needed. The transponder would be within 2500m from the "base station" over open water, would need to be in a waterproof enclosure, and would need to run ~6 hrs per use. I'd like to go rechargeable ... weight/size are not too much of an issue (I am thinking the size of a hardback book or smaller and maybe 1kg or less). What wireless HW would the group here recommend?
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Is this for a real project, an imaginary one, or a college/uni one? All 3 have different answers.
One big problem is legislation and licensing. You are only allowed to use certain bands, at low power levels (depending on country). Depending on how long the units have to run for, the most viable way would probably be a cheap generic GPS module (Motorola Oncore, Garmin H15 or similar) which spits out RS232 data. Use a PIC/Atmel/whatever to grab this, and send the data to a cheap mobile phone as a text message.
433MHz radio modules are an option, but are restricted to very lower power (10mW) and are prone to interference. Having said that, they should just about make 2.5km over open water with a decent antenna.
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You could modify some cheap "walkie talkies" to send the data using a robust modulation scheme like FSK or DTMF. Some claim 20 or more miles of range, which is probably achievable out in the open if background noise is low enough. (Actually, get some with headset connectors so you don't have to modify them at all. Then you can simply return them if they do not work well enough.) CB radios could also be used, but they might use too much power.
The trade-off among different modulation schemes is that more sophisticated schemes have a higher bitrate and therefore reduce the transmit time, but require a higher transmit power for acceptable SNR.
As for having 8 in use at once, you could assign a separate frequency for each one. (FDMA) Or you can have each one send at a different time. (TDMA) FDMA would require 8 receivers at the base station, but is simpler. TDMA would require only one receiver, but there's a much higher chance that a fault in one node (transmitter stuck on or on at the wrong time) would bring down the entire network. A mix of the two is also possible.
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It's a real project, just messing around trying to learn something while building something (hopefully) useful for my sailing club. Hadn't thought of the FRS radios / walkie-talkies - interesting ... I'll try to see what tones my Arduino can produce through one.
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Found this ...
http://wiki.argentdata.com/index.php/Radio_Shield
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[in the U.S.] Ham radio people use APRS, so you might search for that to get an idea of designs.
As far as ANY radio frequency goes, check your local country's laws [people go to jail for not knowing what they were doing - or worse in some places].
One easy thing to do, get your amateur radio license: you will learn about radio, and get legal access to the radio spectrum to experiment with.
The ARRL has a few book titles on the subject of APRS.
http://www.aprs.org/
http://www.arrl.org/
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Had a look at this?
http://www.sparkfun.com/commerce/product_info.php?products_id=9411
Might be a little bit outside your budget, and overkill in terms of range.
Still, it's a pretty awesome little unit.
(assuming the 900MHz band is ok to use in your location)
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What country?
For that sort of range you'd probably looks at VHF, like this:
http://www.rfmodules.com.au/rm/products/vhf.htm (http://www.rfmodules.com.au/rm/products/vhf.htm)
Dave.