The finish line

The finish line has two components per lane: a laser beam and a photoresistor.  These components are mounted on a movable bracket underneath the derby timer to accommodate tracks with different lane spacings.  The laser is mounted on a flexible rod to allow the laser to be aimed precisely.

Laser beams are used in this design because the derby timer is portable and not permanently fixed to the track.  A portable design is convenient if you want to loan your timer out to your nephew’s cub scout pack.  The drawback to a portable design is that it must work without requiring modifications to the track, such as drilling holes in the track and installing lights and sensors.

Lasers shine onto the track from above and are reflected back up toward the sensors mounted underneath the derby timer.  Laser light is extremely bright and can be aimed precisely, so there is no need to use a high powered laser.  The derby timer uses Class-2 lasers like the ones found in handheld laser pointers.  Laser light can be very hazardous.  Take all necessary precautions to avoid looking at the laser beam, and do not use a more powerful laser, such as Class-3 laser.

Why measure to a millisecond?

The derby timer uses a microprocessor that can process millions of instructions per second.  Therefore making the derby timer measure in increments of less than a millisecond (0.001 second) is easily doable given the timer electronics.  So, why not go for 0.0001 of a second?

Consider a track that is 28 feet long, with a starting line that is 4 feet above the finish line.  A car that finishes the race in 2.8 seconds has an average speed of 10 feet/second, but is going about 16 feet/second at the end of the race.  In one millisecond, the car will cover 5 millimeters (1/4 inch).

The small laser beams used at the finish line are about 1 mm in diameter.  In order to measure to 0.0001 of a second accurately, the lasers would need to be aligned within 0.5 mm.  This could be done on the workshop bench, but would prove a little tricky (and unnecessary) in a gymnasium full of cubs scouts.

Of the 72 heat times recorded at my son’s pinewood derby, 30 head-to-head times were within 0.01 second on the same lane.  Only 2 of the 72 times were within 0.001 second on the same lane.  Measuring to 0.001 second makes the likelihood of a tie very remote, particularly in a race-off situation with only a few cars competing.

Conclusion: One millisecond is an ideal level of accuracy that is relatively easy to measure and does a good job of discriminating between close finishes.

Another thing to consider is that displaying 0.001 second requires one less character on the LCD display than 0.0001 second.  Display space is expensive.  You will want the largest display that fits your budget, so use the space wisely.

Pinewood Derby Timer

This blog is dedicated to building a digital timer for pinewood derby racing.  The pinewood derby is a cub scout activity that is held every year all across the United States.  Although I am a big fan of scouting, and my son is a cub scout, please do not interpret anything in this blog as an endorsement by the cub scouts or the boy scouts for what I have done.

I am not aware of any official scout rules that prescribe how a pinewood derby racing event shall be officiated.  Many race events are sucessfully held without any timing equipment.  I do not want to imply that having a timer is essential to running a successful derby activity.  It is not.  My main motivation for building the timer is the challenge, fun, and learning experience that it provided.  And besides, it is really cool on race day!

Here is a picture of the finished project in action:

Derby car speeds for the finish!

 

Introduction: Lessons Learned

The purpose of this blog is to share my lessons learned from a hobby project that I started last year.  The project was ultimately a success.  However, there was a whole lot of trial and error along the way.  I want to share what I have learned, so that others can have a head start if they are planning to do something similar.

I don’t really intend to create a step-by-step guide for this project.  If I were to create a step-by-step guide, the success of my blog would depend on how well a reader could replicate my project.  This project requires some skill with computers, electronics, and basic woodworking.  For a beginner, replicating this project would require a lot more details than I am willing to provide.  However, I do think that for someone who is already thinking of building a derby timer, the ideas here will help them get to their goal a lot faster.

Good luck, and much success.