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RF Lap Timer ?

Discussion in 'Electronic Design' started by zipzit, Feb 15, 2004.

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  1. zipzit

    zipzit Guest

    Looking for some design ideas...

    I'm would like to build a lap timer for athletes who travel around on
    a short track. Track size is 100M... (entire track floor size is 100 x
    200 feet).

    --I'm assuming a small device each athlete carries on tight waistband
    (like a CD/MP3 player butt pack...)Obviously smaller is better. This
    device would then transmit info on laptimes to a base station..

    --Finish and start times absolute do NOT matter. The only thing the
    coach really watches is lap time splits. These guys change lead so
    often in practice who is in first doesn't really matter, its easy to
    see who is where. Today's constant stopwatch work the coach has to do
    is pretty disruptive. Rather than focussing on the athletes, he is
    always watching the watch.

    --Track is generally indoors (i.e. likely no GPS... and I'm not into
    Pseudolites... although it is an option, I doubt it is cost

    --Needs to be reasonably inexpensive.

    --Must be non-evasive to the track. I can't place cable overhead, or
    under the floor. No way, no how. I think that kills radio frequency
    Identification (RFID) ideas.

    --Lap accuracy should be to nearest tenth of a second. I can wait for
    up to two seconds after lap measurement zone for a display. I would
    hope to get up to six athletes measuring at the same time. Lap times
    should be in range of 9 seconds up to 15 seconds/lap depending on

    The only thing I can come up with is a battery operated weak RF
    transmitter, located somewhere along the side of the track. Each
    individual unit includes a RF receiver, which looks for a signal
    minimum threshold (T). The individual recievers measure lap start
    time as (threshold boundary in (i) + threshold boundary out (i))/2 .
    Lap finish time (threshold boundary in (i+1) + threshold boundary out
    (i+1))/2. I would then send lap time to base station (laptop or PDA
    with WI-Fi or Zigbee card...)

    In ascii:

    Athlete * *
    -------> --------T(in)-------------T(out)----------->
    Path * *

    * *

    * R *
    * *

    * *

    * *

    * *
    RF Source is off track. T = RF Power Threshold * = RF power.

    This is an idea I've been toying with in my head. Anybody else have

    Many thanks, LB
    Temporily in Chicago, IL
  2. I heard that the NY Marathon gave RFID chips to the participants, and kept
    track of where they were along with way with sensor checkpoints. It kept
    track of time, and all got mapped to a website where you could look up the

    Thus, its probably a product you can go out and buy.

    Bob Monsen
  3. I attended a 4x5km relay race last year and the stick had an RFID on it. So
    it is widely available at low prices. Don't know about the accuracy though


  4. Have a look at our website There are some new
    low cost products to be introduced within 6 months. They are not on
    the website at this moment but use similar kind of equipment and mats.
    Our product are used all over the world for timing athletes, including
    NY marathon.

    Reinier Gerritsen
    The Netherlands
  5. zipzit

    zipzit Guest

    Ugh. Sorry I wasn't absolutely clear. I CAN'T TOUCH FLOOR OR CEILING
    ON THE TRACK, NO WAY, NO HOW. YES, I AM YELLING! As I understand it,
    because of this, currently packaged RFID solutions are absolutely no
    good. I know they make mats, but that won't work in my case. I can't
    touch floor (or ice) as the case may be. Does anybody really
    understand the mechanics of the RFID. I know there is a transmitter
    antenna which sends out a magnetic field, which excites the
    "transponder" chip, and causes the chip to send a signal to a receiver
    antenna. Because I can have the athletes carry a small battery pack
    unit, I don't need the B Field exciter force... What I don't
    understand is the mechanism on the signal going back to the base
    station antenna. What does this signal look like? Packet size,
    digital code, etc... Anybody been here before?

    I really want to do my processing on athlete, as opposed to processing
    at base station.

    Many thanks, and sorry for the yelling.
  6. Communication with low frequency passive transponders has two main
    "flavours": half and full duplex. Texas Instruments uses half duplex:
    50ms energizing magnetic field charges the transponders battery (a
    capacitor) to about 7V, then field stops and transponders sends ID
    code 20ms (128 bits code, FM modulation 123 and 134 KHz, 16 cycles per
    Rest of the world uses full duplex: load modulation of the energizing
    carrier (about 125KHz). The transponder modulates the resistance
    across its LC-tank circuit. There are different ways to encode the
    transponder ID, usualy some kind of Manchester coding. The code can be
    found as a tiny AM modulation on the reader antenna.

    Read range and S/N ratio is better for TI transponders, but
    communication speed and price is better for full duplex types.

    The antenna you want to get rid of is not only used to transfer energy
    to the transponder, it also locates the transponder within about 1
    meter. If you have a read range of 10 meters, where is your precision
    comming from? You'll need a UHF communication system to get long
    range. More than a few meters is not possible using lof frequency.
  7. Hi LB,
    Two possibilities. RF - you have a small base station with a highly
    directional antenna to define your 'finish' line. If you mount it on a
    pole or a wall, and aim it down, you can get pretty well defined pattern
    for limiting your trigger point. Biggest problem will be bounce to the
    far side of the track, and getting double triggers. Your individual
    runners then have a small package with received, microcontroller,
    perhaps a small LCD display for times, and even a small speaker so it
    will beep as they pass the finish line.

    Second possibility - IR LEDS. Give you a nice, well defined field, esp.
    if you raise it up and have it look down so the bounce will be up in the
    air on the other side of the track. Give it modulation, perhaps a
    time code, so it is easier to pick out the signal from any noise. Then
    you just have a photo-transistor as your reciever, as well as the same
    LCD, micro, speaker, etc.

    Edmondson Engineering
    Unique Solutions to Unusual Problems
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