Parts:
2x 15" Tomy straights
X" of 5 core shielded computer cable
4 miniature glass reeds (12mm long)
15pin HD solder tail connector
Backshell and thumbscrews to suit
2x 100kohm resistors
Superglue and Hot glue gun
1/16" sheet styrene
Solder

Heres a few pics of how it went together, I can't actually show you the finished counter as I sealed it with a piece of styrene for protection once it tested OK. I learnt the hard way how fragile the reeds are. The pics are pretty much self explanatory although in the reeds location pic the actual reeds are fairly difficult to see. To construct the section I superglued (Loctite 401) two 15" Tomy AFX straights together (Using the molding lines/ribs underneath as a guide to keep the track lined up evenly).

With a Dremel, or similar tool, make a hole approximately 3/4" from one of the center ribs to suit your 5 core flexible computer cable (shielded). Strip nine inches of the cable and remove the shielding. Lay the wire across the track bending one wire off at each switch position and allow one wire (the common) to go straight across all four lanes. Using your Dremel make grooves in the bottom of the slots (Don't cut right through) and the centre glued section which allows the wires to be recessed. Bare the common wire in the first three Switch positions and cut to length and bare the end. Solder a reed to each bare spot on the common wire. Don't cut the reed wires shorter and do not bend them near the glass Bubble or it will break. Make sure the Reeds will sit on the bottom of the track with the gap up (Visible when looking down on it, like this ==== ). Cut the other wires to length and solder to the other ends of their respective reeds.

IMPORTANT: Take note of which wire color goes to which lane NUMBER.

Super glue your cable into the entry hole you made. Now using a SMALL dob of hot glue tack each end of the reeds to the underside of the Track. On the other end of your cable (length to suit) Strip Approx. 1 1/2" of cable and Solder the common wire to pin 4 of a HD-15 Solder tail connector, and lane 1 to pin 2, lane 2 to pin 7, lane 3 to pin10 and lane 4 to pin 14. On pin 1 solder two resistors of around 100kohm each, (I used 120k and they worked fine). Solder the other ends of each resistor to pins 3 and 6, remembering that they have to fit inside your plug casing. Assemble plug shell and plug in to the game port.

The following applies only to Gregory Braun's Lap Timer 2000 available from www.gregorybraun.com

Following the instructions in Lap Timer2000 set up the joystick port and the software to suit your track. (For your info I have used a timer resolution of 1ms and a debounce delay of 100 milliseconds with no problems) Test every lane ensuring that you get no missed counts or crossed counts( laps counting on two lanes while running on one) Both these faults can be caused by incorrect positioning of the reeds, the reason why they are only tacked in place. Once you are happy with your results (Stop driving NOW, we're not finished yet :) ) Hot glue the reeds into their final positions on the wiring only. This allows for easier replacement should the need ever arise.

Now cut a piece of 1/16" sheet styrene 90mm x 150mm (sorry don't know the conversion here) and superglue to the bottom of the track , it fits over the reeds on the middle two ribs. (the section where the AURORA brand mark is) Now run and have Fun, Total cost for this lap counter was about AUS$20. I masked mine and sprayed the checks on with an enamel Spray can, then ran some laps with dirty tyres (we all have those don't we) while the paint was just tacky for that well used appearance.

Note: These instructions can also work for non-computer timer/counter systems. Simply run the wires straight from the reed switches to the units, instead of setting up the computer interface.