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Pinewood Derby Finish Line Detector

winner cr2A few years ago the finish line detector at my church’s AWANA Grand Prix track bit the dust. AWANA Grand Prix is the same thing as the Pinewood Derby to scouting folks. We did without for a couple years until I finally got all the pieces together for a new one. First I’ll explain the end product; then comment on my wish list for a future version. Special thanks to my coworkers Gerry Powell and David Fowler for their electrical engineering design help on this project.

This article was submitted by Ed Wilson as part of the “Hobby parts for articles” program. Write something of interest to electronic hobbyist and receive parts for your next project.

Here is the finished system in action. It worked flawlessly.

winner cr2

The system uses two thru-beam optical sensors to sense the cars as they break the beam at the end of the track. The sensors were tied up with two D-type flip-flops. The flip-flops were wired so that the winner would light an LED and hold the other flip-flop in reset. A simple push-button switch was used to clear the flip-flop’s for the next race.

Here is the schematic and parts list. The LED’s we used were too small and dim; it’s what we could find at the time. Hope this helps if anyone is looking for a simple finish line detector. Hopefully you’ll have more time to spin a proper board and make things a little neater. Good Luck.

  • C4093—-U1
  • C4013—-U2
  • 10K—-R5,R6
  • 1K—-R4,R7,R8
  • Switch—-S1
  • +12v power source,B1
  • LED—-D1,D2
  • Sensor—-Q1,Q2 (SunX EX-13EBD)
  • LED source (not shown)


Posted in Hacks, Projects.

9 Responses

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  1. Brian says


    User friendly? You can read that chart?

  2. marcos says


    i think this is wonderfull since there are very complicated systems out there.i’m in the process of finishing a track and i think this is the simplest way to go, but i tried to print out the circuit and it didn,t print complete, can you e-mail it to me? i would really appreciate it and i will send you photos of the finish track in return.thanks.


  3. John says

    10K pull up resistors for the sensors are way too low, and require powerful IR illumination, which is wasteful with battery operation. Go to 22K or 27K, and see the difference.

  4. Kristi says

    This is awesome. We wrote up a post for it!

    Hooray for DIY.

    – Kristi (DIY:happy).

  5. NGinuity says

    “Now all you need is a method of starting the racers at the same time ”

    The Pinewood Derby track usually has a starting gate of some sort doesn’t it?

  6. Jac says

    Looks good, try using cheap laser pointers instead of the LEDS, nice and powerful and a good focused beam. Or if you want to go all out, buy a nice IR LED and its relevant phototransitor, which will reduce the chances of ambient light affecting the results. Now all you need is a method of starting the racers at the same time 😛

  7. Ed says

    Received the MSP430 kit.


  8. NGinuity says

    I like it. This would eliminate so many arguments (from parents anyway), at Cub Scout Pinewood Derby meets. 🙂

  9. brandonu says

    Nice job! Great project, looks uncomplicated and user-friendly!