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Ideas for a lap counter for RC cars?

Discussion in 'Electronic Design' started by Joe G., May 11, 2005.

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  1. Joe G.

    Joe G. Guest


    I am trying to design a lap counter for radio controlled cars. So
    every time one of a dozen cars or so goes over the start/finish line
    another "lap" is counted for that car. I plan to use a Windows based
    PC (an old 166MHZ) to track the laps and times, etc. I think I can
    handle the computer programming end of things (as a VB developer) but I
    am not an electronics expert. The biggest puzzle I have right now is
    figuring out how to transmit the cars ID (if you will) only 3-12 inches
    to a receiver near the start/finish line. I am hoping for some help
    from you experts pointing me in the best direction.

    Here are some important points I had concerning the project so far:
    1. Can count laps seperately for up to 12 cars. So it has to know
    which car just crossed the line, etc.
    2. It has to be reliable. No missed laps etc. So I figured dragging a
    light "brush" on the bottom of the car is out. It may not make good
    contact etc.
    3. I considered RF and/or IR transmitter/receiver type of design. RF
    seems nice because bright lights and/or line of sight isn't important
    like IR. IR may have some issues if several cars are sending pulses to
    the receiver at once. Example: a 40HZ cycle from car 1 and a 20 HZ
    cycle from car 2 could look like a 60HZ cycle right?
    4. Since RF is usually trying to transmit distances greater than the
    3-12 inches required for this project, are there some other types of
    circuit that I may be missing that would transmit this very short
    distance (circuits that one may not usually think of as an RF
    transmitter...but will transmit this short distance...if you know what
    I mean)

    If you can share any thoughts, ideas, advice or suggestions that would
    help point me in the right direction I would greatly appreciate it.


    Joe G.
  2. Luhan Monat

    Luhan Monat Guest


    What about optically? Put some time of simple 'bar code' on the bottom
    of each car. Read it with a reflecting type ir emitter/detector and
    decode the pattern data in the computer.
  3. Luhan Monat

    Luhan Monat Guest

    Put some type of simple 'bar code' ....
  4. Jim Thompson

    Jim Thompson Guest

    Good idea, Luhan! Let the bar code be the "clock". Can a PIC or CPU
    handle the interrupts quickly enough to establish "pecking order" at
    each lap... if several hit simultaneously?

    ...Jim Thompson

  5. Most likely. At 1000 inches per second (a plausible maximum speed for
    a slot car these days, apparently), 0.01" is 10 microseconds. It
    should be possible to handle, say, 4 lanes with that resolution even
    using polling with a micro with a 100ns instruction cycle (eg. a 40MHz
    18F PIC with 10MHz crystal). Personally, I think I'd stuff tokens
    representing the direction and time stamps of each transition on each
    channel into a circular queue and deal with them at my leisure,
    leaving only one really fast section of code (a single hardware
    timer-triggered ISR).

    Best regards,
    Spehro Pefhany
  6. Luhan Monat

    Luhan Monat Guest

    If the 'bar code' is simple enough (maybe 6 bars) the timing is somewhat
    easier. I'd say a 20mhz PIC would have no trouble. The que idea looks
    good because there is a lot of 'free' time between readings.
  7. Guest

    A co-worker races people-controlled cars in the SCCA and has recently
    talked about some kind of transponder system they are using for lap
    times. I can't remember if it's RF or infrared. Each car has a
    relatively small and inexpensive transmitter, and I think the
    receiver(s) plug(s) into a laptop. This is off-the-shelf stuff. In
    your case, though, the weight of even a small transmitter might be

    You might look into "RFID" tags. The idea here is that there is a
    small, cheap, passive tag that gets stuck on the can of beans or
    toy car or whatever, and a reader that sends out a burst of RF that
    energizes the tag and causes it to transmit its serial number (or
    whatever) back. I think you can get these that work over relatively
    short distances, so the reader at the finish line won't count cars
    on the other side of the track.

    The barcode idea might work for slot cars, but I think you are talking
    about cars with rubber tires that don't run on a fixed track. You
    might be able to make the barcodes work IF the finish line is maybe
    only one car wide.

    Maybe you could put racing numbers on the top of each car (say a 2"
    white square with 1" tall black numbers in the middle), hang a
    camera over the finish line, and apply OCR to the digitized pictures.

    For the RFID, bar code, or OCR ideas, you could shoot an infrared
    beam across the track to decide when to turn on the RFID reader /
    turn on the scanner / take a picture.
    This can happen. The trick is to select frequencies that don't add up
    to one another or are harmonics (2x, 3x, 4x, etc) of one another. Look
    up the frequencies for Touch-Tone (tm) phone dialing for an example.

    One thought... with any system like this, you will probably be able to
    count laps fairly easily, but lap _times_ will be harder. To count a
    lap all you need to know is that the car got close to the line; to do
    timing you have to know exactly when the front edge (or whatever) of the
    car passed the line, which is trickier to detect, especially if two
    cars can pass the line at almost the same time.

    Matt Roberds
  8. I was going to build something similar to

    Looks fairly easy to do

  9. Michael

    Michael Guest

    Curcuit Cellar published a construction article on such a system (for mud bikes,
    Sorry, I don't remember issue# or date. Within the past 5 years, I would guess.
  10. Mebart

    Mebart Guest

    Hi Joe,

    We worked on an RFID system a few years ago for a local store owner
    who believed the system was not detecting some tags.

    What we found was the 6 or more tags in the same bag with one tag in
    the patrons pocket would fail to detect all the tags. The result was
    collisions and some tags are not detected.

    We reported our findings to the store owner and he followed through
    with some RFID people.

    Older RFID tags have collision problems. Newer tags have a globally
    unique digital signature and transmit twice, both transmit times are
    random so there is little chance of a collision. RFID is probably the
    answer, an RF source to power the tags is easy to build, the receiver
    is a little more difficult but easily doable. The tags are small and
    don't need external power, and your local Wal Mart and or your local
    record store has an endless supply.


    As luck would have it, I also design low power photonics. I don't know
    how big your cars are and how wide the finish line is though. If the
    cars are small, even the simplest transmitter is probably to large and
    the coverage of the transmitter and field of view of the receiver can
    be a problem too. An LED transmitter would need the same type of
    anticollision provisions built into it as the RFID systems have.


    There is a third possibility if the cars are large enough and have dc
    power available to power a transmitter. That option is to use a Hall
    magnetic detector and transmitter in the car. As the car passes over
    magnets in the race track, it sends out a unique identifier code to
    the receiver located at the computer. Lynx makes the 433 Mhz
    transmiter chips that use SAW devices to set their operating
    frequency. You would need a unique identifier generator chip (Maxim
    and others) and some anticollision measures too.


    A note on anticollision technology...... You don't need a James Bond
    secure and accurate system. If you have a capacitor charge to
    determine the first transmit time and the discharge to determine the
    second transmit time, you end up with 2 random signals. After all, you
    aren't trying to time supersonic aircraft and a few milliseconds error
    is probably acceptable.


  11. Mebart

    Mebart Guest

    But the flag weight and wind resistance is not trivial, especially on
    smaller cars. Even the lightest weight flag and flag pole will affect
    handling in tight corners. I'd hate to be the unlucky one who has to
    use the highest flag.

    Also, it's limited to a max of six users. What if number 7 comes along
    and wants to race in place of racer number 5? Each car owner needs an
    assortment of flags and needs to be able to change them easily.

    It looks fine for 2 or 3 racers on an home track, but for larger scale
    exhibitions and events, the system has problems.

    Good luck.

  12. Joe G.

    Joe G. Guest

  13. dalai lamah

    dalai lamah Guest

    Un bel giorno Joe G. digitò:
    IMHO the easiest solution for a good precision is to put a magnetic stripe
    on the finish line, detect the passages with an unexpensive sensor mounted
    on each RC car (just like go-karts), and send a short RF burst to the fixed
    station at a different frequency for each car. On the fixed station you can
    use various ways to detect each burst, for example a module that "scans"
    your range of frequencies: the modules studied for frequency hopping
    applications are very fast in switching carrier frequency (tens or hundreds
    of us), look for example
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