Tutorial: How to Make a Cheap Personal Lap Timer

[Spoon]

This post is really just an extension of an idea already documented here. I have updated the design with pictures of the process and some personalizations/improvements of my own.

Tools:

Screwdriver
Soldering Iron
Dremel Tool or small file
Drill and Bit Set

Supplies:

Hobbico MicroSizers Lap Timer ($5.28)
Laser Pointer ($6.50)
K&S Aluminum Tube 9/16" ($2.00)

Radio Shack Photo transistor/ Detector ($3.29)
Straws (free at Wendy's)
Simple on/off switch ( salvaged from other RC stuff )
Velcro ( ~$2.00 )

As of the writing of this description, the lap timer was available from E-bay for $5.28 shipped. I also got the laser pointer + 10 batteries for $6.50 shipped.

Modification of the Lap Timer

All you will need from the lap timer is the main unit itself. You can discard the reflective piece used for the Microsizers. I installed the batteries just to make sure the timer was working before I began the modifications.

Hobbico Timer Box
Hobbico Timer Contents

First, unscrew the 5 screws in the back to disassemble the timer into it's two main pieces. You can leave the screws and the front piece alone for a while. We are mainly concerned with the section that hold all of the electronics and the display.

Remove Screws
This is all you need for now

Once you have it apart, remove the screw that holds the transmitter, receiver assembly. It's highlighted by the red arrow below.

Remove this Screw
Detector and Emitter

You can clip the wires going to the Emitter and throw it away. The laser pointer will serve as the emitter for our timer. You should also de-solder the leads to the collector but pay attention to which wires go where. For my setup, the green wire went to the longer lead on the detector.

Wire Connections 1

As recommended in the original reference, I used telephone wire to make the connection to the detector circuitry. I would have preferred to solder the Tele. wire directly to the board, but I couldn't remove the display without destroying something. So I just soldered to the short wires going to the board. Also, be sure to put a know in the tele wire, so you do put strain on the solder joint if the wires get pulled. One thing that I noticed here is that since you are working in such a small area, it helps to cut the green wire just a little shorter to make up for the difference in the leads of the detector.

Here's a couple of shots of the wires being soldered. First, I twisted them together, then applied some solder. Try to work quickly or you will start to melt the housing on the wires.

Wire Connections 2
Wire Connections 3

Attach the other end of the detector circuit.

I did this a little differently than the original reference. I tried to use a pen tube from a BIC like the author suggested but I couldn't get consistent accurate readings. I figured I had several problems.

1. The detector is fairly small and it was difficult to get it to sit straight in the BIC tube. I think the sensitivity of the detector is greatly reduced if the light is hitting off-angle.

2. The BIC tube wasn't really blocking light from the room so laps would get missed when the laser was blocked.

3. I was having trouble aiming the laser at the small detector.

I decided to drop by the local Radio Shack and pick up their Emitter/Detector Kit for $3.29. The detector is a little larger than the one in the Hobbico timer so I thought it would be an easier target to hit. I also went right down the street to a hobby shop to pick up a short tube of Aluminum. I think the 9/16 size fit the detector quite well. I would recommend getting the detector first and then checking the tube size before you buy.

Hobbico Detector
Radio Shack Detector
Aluminum Tube

With a good tight fit and a larger reflective tube, it is easier to get the laser directly on the detector. Note: brass is cheaper, but it is also more likely to bend.

I also cooked up a scheme for mounting the detector and laser that would help fix any alignment problems. But that's later.

Once you have the Radio Shack or Hobbico detector wired, you can close up the timer.

Setup Laser and Detector

You can really do whatever you want here but I had some scrap wood that really worked well for my cause.

First, mount the detector in the tube of your choice. (In my case the K&S aluminum) and tape the end.

Taped End

Then, use a drill bit that is about the same size as the outer diameter of your tube and drill a hole as straight as you can through a short piece of 4x4. A drill press would be great here. If it's not perfect, that's okay because we can adjust with the laser.

Test fit your detector assembly into the hole in the 4x4. It doesn't have to be tight, just snug and stable. Mine slides in with minimal effort, but stops where the first layer of tape is applied. See the pictures below.

Test Fit into Mount


The next thing that you want to do is mount the laser so it's adjustable. I used Velcro. One strip on the laser and 2 strips on another block of wood let me position the laser to shoot directly through the hole. This is where it's important that you can slide the detector assembly out of the mount.

Laser Mount

To line things up, I turn on the laser, remove the detector and make sure the laser shines through the 4x4 to the wall or track behind it. As long as it goes straight through you are going to have a good clean shot at the detector.

Laser Alignment

Don't look through the hole to check the laser! That would be dumb. Put your hand behind the hole or just look at the wall while you adjust it.


Here is the entire setup. --Whole Setup

Optional Laser Mod - Highly Recommended

Here is where you need the Dremel. I decided that it was a huge pain to have to hold the laser button down with tape so I dug up an old switch that I had from another RC something or other and wired that to the laser. There was some necessary and unnecessary destruction involved. First, I thought maybe I could unscrew the laser assembly from the housing. WRONG! I think it's all plastic and it's all glued in and it's very brittle. See the picture below.

Mangled Laser Pointer. Don't do this!

My next plan was to drill in the backside of the laser and see if I could remove/replace the push button switch. I couldn't remove the switch but I was able to bypass it with my toggle switch.

Small Start Hole/Final Hole

You have to drill a hole into the back big enough to get your soldering iron into and solder a wire to each of the metal tabs on the circuit board. If you are handy with a Multimeter, you can check the connectivity between the two tabs when the push switch is open and closed. If the status changes, those are the two tabs that you want.

Wires Attached
Wired Switch

Once you get the switch soldered to the tabs you can tape everything back up and test out your setup. You should be able to activate the laser with either of the switches.

Final Setup


Obvious Stuff:

To count the laps you are going to need to place tape or something on the antenna of the car. I plan on getting some straws to put over the antenna so I don't have to untape the antenna to change the batteries. If you want more room, you could set the laser/detector combo pretty high on the antenna and you would still have room to change batteries and make simple adjustments.

Again, don't ever look directly into the laser.

Also, try not to burn yourself with the soldering iron.