Drag Racing Christmas Tree

Recently, I was approached by my friend and co-worker Dave about a possible project.  A family member fond of drag racing had a device designed to test his drag-racing reaction time that had stopped working, and Dave wanted to know if I was interested in making a go at getting it back into action.  Here’s what it looked like before, and after, I started hacking away at it:

The Whole Package

The system consisted of a Radio Shack Model 100 Laptop hooked up to a wire-wrapped perf-board set up to control four standard 110v outlets and accept one push-button input via the bus on the laptop.  The system was electrically sound, but the software had been long lost due to the volatile nature of battery backed up RAM.

Rather than try to re-write the old software, I decided to modernize the system using a Freescale MC9S08AW60 microcontroller.  After close examination of the line voltage interface, it was determined that standard TTL level voltages were used to activate the solid-state switching setup, and pull-down resistors were already in place after the I/O interface chip tied to the laptop bus.  The laptop bus had been connected via ribbon cable to a DIP socket which was then wire-wrapped to a bus interface IC.  This provided me ready access to the perf-board and made it easy to modify.  I promptly did some surgery on the perf-board, removing the old I/O chip and re-routing 5V, GND, and the four inputs to the DIP socket.  I made a small perf-board adapter to reduce the high pin-count DIP socket to a more convenient 10-pin header.  A couple connectors mated to some spare CAT5 later, I had ready-made access to the external lighting interface which could now be re-used.  Luckily, I even had enough room inside the external light controller power-strip to install a 110v AC to 5V DC converter, which I wired directly to the built-in power switch.  Now, instead of having to plug in both the power-strip and a wall-wart for the laptop, one plug will suffice!

Inside the Display Driver
Modified Display Driver

Next, I built a prototype controller using my AW60 development board, a serial LCD, and a couple push-button switches that I would use for controlling the game-play.


As soon as the circuit was working, I designed a PCB in Eagle and shipped it out for manufacturing so I could get this finished soon enough to serve as good practice before race season.  While tweaking the software while waiting for the board, though, I discovered I needed some more buttons to make a flexible and easy-to-use UI.  Fortunately, there were some extra I/O ports I exposed on the board that weren’t being used, so when I got the boards back, I was able to attach some panel-mount switches to the board with ribbon-cable.  If I had another go at this board, I would have preferred board-mounted switches to reduce wiring, but it ended up working out just fine.

Flash forward a week and a half, and I had boards in hand.  Unfortunately, testing revealed two swapped nets on the board.  Oops!  A little PCB-surgery later, though, and it was all working fine.  In a very short time, I had a fully working prototype.  Now, all that was left was the case.  This actually proved to be the most complicated part of the entire project.

Closeup of the solder side of my PCB.

A little background is probably in order.  I’ve fiddled with electronics and programming constantly since I could get my grubby hands on any.  I’ve made countless gadgets and I make a living programming.  Nowhere in my background, however, is graphical artistry or case design.  But, this project was different.  Someone other than me would be playing with this thing and it would probably be a nice design feature if you didn’t need to hold it together with duct-tape and wire-ties for it to stay together.  Usability and case-integrity mattered.

My first instinct was to mount it in-between two pieces of acrylic separated by stand-offs.  I’ve used this approach before, and it works, but it’s not really attractive when dust and grime come in through the sides.  It’s also easy to accidentally rip out wires and connectors if there’s too easy access to the project innards.  So, after some brainstorming sessions with my wife Kelly and a few trips to Lowe’s, I decided to attach sides with some small aluminum extrusions and 4-40 nuts and bolts.

Many hours of work later, I came to the conclusion that my aluminum plan sucked.  Sure, it would look great if it was CNC machined and precision cut.  My attempts with a plexiglass cutter and a hacksaw, however, produced, to be polite, substandard results.  Though I used a miter box and did my best to make precise cuts, the aluminum lined up horribly and would have taken me the next two months of weekends to even look only mildly embarrassing.  Time to try again.

Our next attempt was much more successful.  Kelly and I ran all around town, drew out countless ideas, and finally came up with a winner.  The final plan that worked was to route 3/16″ diameter channels 3/32″ deep around the perimeter of 3/16″ acrylic which formed the top and bottom of the case.  The channels served as guides for side pieces which were cut from the same acrylic.  I didn’t have a router table or a table saw at the beginning of the project and I couldn’t afford to buy both, so we bought a small table for my router and used that for the channels and for more accurately cutting the acrylic.  After I had all the pieces cut out, the channels routed, and the edges that were to be exposed rounded over, I drilled all the mounting holes and drilled and tapped holes into the ends of the side pieces of acrylic.  I then mounted and connected everything and bolted everything together.  Success!  Finally, a working project that looked nice to boot!

Sportsman Tree
Rear angled view
Front angled view

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  1. Yamaha R1 Freak says:

    Hi, I don’t think your case design is substandard, it actually looks pretty sharp,, surely with that led display. I was wondering how it worked out. Did you also include pre-stage and stage sensors in the design? Would you mind sharing your board design and tell us more how it worked out in practice? Thanks! I am asking cause I’ve been looking to make a christmas tree myself.

  2. chembal says:

    Thanks for the feedback! I’m gradually getting better with my project enclosures! I picked up a table saw yesterday, so I’m hoping that will help make more accurately sized cuts, and I’m looking forward to playing with alternative case designs in search for the perfect case. :) So far, so good with the device. My friend reports that it’s serving it’s duty well to help practice timing and figuring out the perfect settings on his car computer before taking to the track.

    Here’s how I have it programmed:

    Plugging in a button to the 1/8″ jack on the bottom enables the user for that side of the track. Single and two user modes are selected automatically based on how many buttons are plugged in. Clicking the middle red button on the box changes mode. I made modes for Sportsman Tree, Pro Tree, and a programmable offset sportsman tree mode for dialing in your reaction time on your car’s computer.

    As far as staging goes, this is how I’ve got it working: When the button (or both buttons in the case of two user mode) is pressed, the pre-stage LEDs go on. A random time later, the stage LEDs go on. A random time later, the tree starts counting down, both on the LED tree on the board and on the remote (full size) tree. Jumping the light results in a red light on the board on the side of the plugged in user that jumped the light. Afterwords, the times are displayed. It’s working pretty slick! As soon as I get some more time, I’ll put another one together (minus the full size tree interface) and post some video. I’m pretty happy with how the interface turned out.

    I’ll think about posting the board schematics and Eagle and gerber files. I’m not sure yet, as I’ve been wondering if there might be enough interest on eBay or elsewhere to sell a few to try to justify the price of all my tool purchases to my wife. :) I’ll probably put a couple up for sale first to judge demand, and go from there. I’m tossing around ideas of kits or customizations as well, but we’ll see.

  3. Reeotch says:

    Hey, Brad, could you build me a woman… or at least an attractive man? Thanks…

  4. MdB says:

    That Rich… what a card! Nice work, Brad. Now you just need to design some babies… hehehe…

  5. K-Dawg says:

    Wow! I was about three quarters of the way through that article before I realized that this was a device to start races and not a device to attach to the base of a Christmas tree so that it can be raced down the street in some weird Iowa holiday festival. Boy am I an idiot! Anyway, I’d like to order one of those mechanical women as well. Thanks Brad! You freaking rock!

  6. MdB says:

    I have to admit the first post was I. I am the one that really wants a mechanical man to do with what I choose to. Does the finished product come with lube?

  7. Mike says:

    Chembal. Please contact me asap. I have an idea for a timing system and I am hoping it is something that you may be able to put together. jmhylton30@msn.com
    Mike Hylton

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  9. Marvin Spears says:

    A friend wants to have a Start Light Tree for his pinewood derby racetrack he sets up for scouts and other groups. He found plans for it on a website (grandprix-race-central.com). The circuit works with GrandPrix Race Manager software (version 3.0 to 6.0 – which my friend has)to provide a kick-off signal. The only thing is that it uses ultra bright colored LEDs with 12 volts via a parallel port. My friend thinks it will be too small for a large crowd to see and wants to enlarge it, but the biggest LEDs I have found are 10mm and only in red. I see you are using incandescent light bulbs and 110 VAC. What do you think? Can I convert it to drive incandescents very easily? I know electronics and some programming but I am not up on your microcontroller and its software. Can you help?

  10. chembal says:

    Hey Marvin, if you’re just interested in turning on AC loads from your parallel port, and you are familiar with electronics and electrical safety, you could just wire up a solid state relay circuit like those at http://www.discovercircuits.com/S/solidstate.htm in place of the FETs on the website you mentioned. Since your computer is doing all the coordination, no microcontrollers would be necessary. Just be careful when working with line voltages! Careful wiring and attention to current handling capacity is a must to ensure physical and fire safety! If in doubt, stick with the lower voltage LEDs described. LEDs can be amazingly bright nowadays.

    Good luck!

  11. Bob Peeples, PE says:


    I was wondering if you would look at a charity project for me along these lines. I can give you design and some dialog so far if you are interested. I originally designed it to use some sort of standard ICs, like 555 chips, but old computers with BASIC are pretty cheap. The idea was to build a pro bono prototype and then make some of the original investment back through small-scale production, and that’s where the idea of specifying a junk PC breaks down.

    I’m not looking to get rich; just not looking to get too much poorer in the process.

    You have my email,
    Bob (Bo Peep) Peeples, PE

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