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Short range mobile transmitter/receiver

  1. Feb 15, 2010 #1

    I am looking for information on a concept I am writing software for. Since I don't know much about EE, I though I'd get suggestions.

    I wanted to know that best setup I could use for a short range point to multipoint broadcast. The example I have is go-cart racing.
    Suppose you are racing around a track with 2 other drivers. You wanted to be warned when someone was in proximity to you. If you were leading in the 'red' car and the 'green' car came withing 1000 feet (arbitrary) of you, a dash light (green in this case) would light up. When the 'green' driver moves out of range, the light goes out. If the 'blue' driver moves in range then the blue dash light comes on. If they are both close, both lights are on.

    I assumed at first the simple fix would be to use 3 different waves (bands), one for each driver (assuming you the red car were behind the others), but that could end up requiring an unreasonable amount of hardware. I instead will use the device as a carrier wave for code that will identify the car behind. (much like a garage door opener is coded)

    For starters, I wanted to know:

    What band range do I need to work in assuming the distance is less than half a mile or so? What do I need to keep the signal from being omnidirectional? (I wanted it to be a relatively focused forward beam.)

    That's all for starters. I have more questions, but they will be based on the solutions given.

    I have attached a drawing (forgive me, I am not trying to insult your intelligence, I just thought it would be clearer)

    Thank you so very much.

    Attached Files:

  2. jcsd
  3. Feb 15, 2010 #2


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    Staff: Mentor

    Welcome to the PF. There are several issues. First, you would likely need to use a licensed RF band -- there are some unlicensed bands (like WiFi microwave oven ISM band), but they are pretty cluttered already.

    Second, getting range data from an RF link is problematic, due to multi-path reflections causing degredation/attenuation of the signals. Especially on a closed road course, the "distance" between cars should be measured along the course, not through the air as "the crow flies".

    So one solution would be for a central control station to know where the cars are on the course, and to send the information to the cars to light up the appropriate lights. You can use simple existing RFID technology to read cards on the cars as they pass points on the track, and then send a simple transmissionn to the cars telling them what lights to turn on. I'm not sure what the best frequency would be for this transmission -- again, it would probably need to be a licensed band. It's quite common for racetracks to have licensed band access for walkie talkies and other race-related radio traffic.
  4. Feb 15, 2010 #3
    Thank you berkman for the rapid response:

    Ok, I made the mistake of letting my example limit the scope. I need the technology to do this on the street. The go-cart analogy I thought would make it clear. I suppose I actually need it to work in a street environment in order to get the exact effect. If an FCC license is needed, although difficult, shouldn't present an insurmountable problem. I need the best case scenario here.

    Thank you
  5. Feb 15, 2010 #4
    Perhaps a better approach would be to bounce radar off the cars behind you. That wouldn't require radios in all the cars. You also have to consider the effect if everyone on the road were transmitting radar signals all the time.
  6. Feb 15, 2010 #5


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    Staff: Mentor

    Sounds like GPS in the cars may be the best bet. This is already used by HAM radio operators at public events (like long bicycle races) to keep track ow where all the operators are on the course. Would GPS accuracy be good enough for your purposes?
  7. Feb 15, 2010 #6


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    Staff: Mentor

  8. Feb 15, 2010 #7
    GPS hobby kits are widely available too if you actually want to build it. Check out http://www.sparkfun.com/commerce/categories.php?c=4". I suggest you get the evaluation board because it will have the GPS chip and all the associated bits needed to run it.

    BTW, would a laser range finder do the job? That would work even when the GPS signal is blocked in tunnels and things like that. You also don't have to get licensing from the FCC.
    Last edited by a moderator: Apr 24, 2017
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