Another Arduino Based Track Timer

ApacheXMD Monday, 1/19/2015

I made this track timer for my son's 3rd birthday party. It's based on David's PDT (, though with a few modifications.


  • Single button operation
  • Automated start gate, self resetting
  • 1.2" tall green LED 7 segment displays
  • Displays finish places, and winning time
  • 4 lanes of Blutrack
  • Red Laser Diodes
  • Photo transistors mounted in black tubes pointed down, meaning full sun operation.
  • Rechargeable Lithium Polymer battery pack for all day play with out plugging into the wall

The housing is made from garbage from the jobsite (I'm an electrician by trade). I 3D printed parts to fit PVC pipe to make the feet. The button is a 3D printed housing of a random button I had laying around, just to make it BIG and fun. The start gate is powered by a servo from an RC car. It's linked to the timer housing with a shield CAT5 patch cable, which supplies 5V, as well as the signal wire for the servo, and the input wire for the button. The single button makes it easy for the kids to have fun with the track with minimal input from an adult. The kids load up the lanes with cars, and hit the button. The timer displays a countdown, and the gate drops. Cars hit the laser beams, and the timer displays the places. Then it shows the winning time, and the start gate resets. The kids can run it all day all by themselves.

My son and his friends, as well as a few of the adults, had a blast with this at the birthday party!

See the next post for more build pictures.


View member profile
ApacheXMD 1/19/15

Build pictures

This is a piece of crash rail from a jobsite I was working on. It's used to protect walls from carts and gurneys. This is going to be the enclosure. I needed endcaps.
I scanned the end on my scanner, converted the image to vector format, scaled it to true size...
then 3D printed a test end cap.
It fits!
Then I cut out the holes to fit the 7 segment displays from Adafruit,
I laid out the components that need to fit inside. Left to right: Ardunio Uno w/ prototype shield (ended up not using shield), switching voltage regulator, 2S Lipo battery. Beneath those, you can see the backside of the LED displays.
3D printed some brackets and mounts and things
Mounts and brackets and things acetone glued to plastic enclosure
Yay it fits!
Here you can see the USB and power ports that were part of the side end cap, no drilling required with 3D printing. Fit is perfect.
Other side is solid. Actually I forgot to model in the power switch hole, so I did end up drilling this side. :(
I was designing this on the fly, so I thought I needed some way of raising this whole thing off the track. So I printed these tube things.
This is a RJ45 coupling I was using to connect to timer to the start gate.
3D printed a foot with built in hole for RJ45 port
I decided I wanted lightening holes in the other foot (to speed up printing)
3/4" PVC pipe fits perfect
These are little tubes that will serve as mounts for the phototransistors. By pointing them down, I was hoping to avoid sunlight interference. By putting them in black tubes, I was hoping to help mitigate intereference even more. And finally, I made the tubes tiny, so only a beam from a laser beam will stand a chance at activating the phototransistor.
This is a close up of the lasers mounted in a piece of curtain rail that I found in the trash.
Everything is in place, and wired up!
I'm aligning the lasers to the phototransistors here, and running a test sequence on the displays.
Done! ... with the timer, I haven't built the start gate yet.
Here you can see some 3d printed spacers and some small hinges sandwiched between some pieces of garbage I found in the trash.
Yes, more 3d printed pieces here too. And a servo motor and a simple linkage using Traxxas ball ends.
I had a bunch of these buttons laying around so I decided to spruce them up a bit with 3D printed pieces.

After this I was running low on time, so I skipped the pictures for a while...
David's PDT project included the arduino code, but my setup was different so I had to modify the code a bit to support only two displays for four lanes, as well as the single button operation
I finished! You can see how I had to mount the track to a board to account for the height of the laser rail.

Stay tuned for a video of the track in action!

View member profile
model40fan 1/19/15

that is great stuff xmd... tracks from trash is the best ... maybe you and the rascal might want to modd race with us... come on down !

View member profile
72_Chevy_C10 1/19/15

Cool track! I really need to start messing with the arduino stuff. 

Any advice would be helpful!

  • its the 3d printer that i want... thing is MAGIC ! — model40fan
View member profile
redlinederby 1/19/15
Site manager

Holy moly, nice build. Very pro. Thanks for sharing all those photos. I think we can all get some ideas even if we don't have all the resources. I love the battery pack option, that has to make a big difference. 

So what are risks again in not using the laser? You're the first I've seen to use lasers instead of IR or just photo sensors. Lasers sound more reliable. 

One thing is for sure though...I need a 3D printer pronto. 

View member profile
redlinederby 1/19/15
Site manager

I just read through it th sensors are above the track...? And the cars roll over the laser. Interesting, good idea. I never woulda thought of that. Hmmmmm

View member profile
ApacheXMD 1/19/15

Yep lasers on the bottom. Idea is that in sunlight, you don't want the photo transistor to be in view of the Sun, so why not have it facing down? They're in a black tubular housing so that only a direct shot from the laser will activate the detector.

Infrared leds or maybe even bright red leds Probably would have worked here, but I thought lasers would be easier to align and troubleshoot if there was ever an issue.

There is a risk of kids sticking their face in the finish line and get an eyeful of laser beam, but I think it's a minimal risk and no kids tried at my son's party.

View member profile
3DBotMaker 1/19/15

Wow. Really nice set up. I love the end of race stats that show up on the display.

View member profile
Rusty 1/21/15

Great  set up and build

Very nice!!

View member profile
redlinederby 2/11/20
Site manager

Resurrection bump. With all the finish line talk happening recently, thought this tutorial from 2015 would be relevant (again). A complete tutorial and build photos. It's pretty complex if you ask me, but certainly a nice DIY project that could give new ideas to anyone looking at building their own finish line.

And if you're researching finish lines, you can always browse the Finsh Line tag collection and read articles from the archives.

  • Sooo now that you got a 3D Printer, when are we gonna see this bad boy on your fancy new store?!? :D — WorpeX
to join the conversation or sign-up now