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.