Close up of the display showing the counts for four tracks. In this mode it counts up to 99, but when the lap counts hit 100 the display changes to an abbreviated version showing Ln1, Ln2, etc. for the top row (no "Lane") and just the lap counts for the bottom row (no "Laps").
The middle board is the one connecting the Arduino analog inputs to the reed switches underneath the track (the bottom board is the Arduino Uno). It is the protoshield board from Adafruit and comes with the pin headers allowing it to be stacked/sandwiched like this.
This is the full setup. I just use the USB power from an old phone USB charger and I made the cable connecting the sensors pad to the sensor board using some CAT5 cable I had lying around. I only needed five wires and it had more than that so I just clipped off the wires I didn't need. I put a connector in between allowing me to disconnect the track easily when I wanted to reprogram the Arduino. I use the Reset switch on the LCD shield to set the lap counters to zero.
This sensor pad is just a sheet of computer paper cut to the size of the track and markings for the reed switches to go. The reed switches are taped (scotch tape) to the paper in the right spots - underneath one of the track rails, since the car magnets are intended to "stick" to them. Any wiring/solder connections are covered with tape as well.
This is the shield I used but nothing is critical about it besides the price being under $10. It just uses the standard Arduino LCD library so no special drivers are needed.
This is the prototyping shield I like to use. It is from Adafruit and is under $10 so it is cheap. It comes unassembled so I just put the pieces on I needed (connectors) before soldering my pull-up resistors and wires for the sensor pad.
The reed switch is probably the most critical piece of this project. I actually had a hard time finding these little jewels. The ones I found were at frys.com and were called Philmore Open Reed Switch. I think the actual part number I used was Philmore 30-17154. There is virtually no current that flows through them (0.5 microamps) so there is nothing to worry about regarding current rating.
This switch has connections for normally open (NO) or normally closed (NC). Only the NO lead is needed so I cut off the NC lead and just used the NO lead. I used an ohmmeter to figure out which lead needed to be cut off (it was the longer of the two leads on the one side), the lead that needs to be used should show as an "open" when no magnet is used and "closed" when a magnet is used.
There is really no reason to get this same switch, a standard normally open reed switch (with only two leads instead of three) will work just fine. I found a Philmore 30-17152 that is much cheaper and only has the connections you need.
Here are some links on the switches I found.
The circuit is very simple. There are four pull-up resistors (10k in this case) that hold the Arduino analog pins A1-A4 at 5 volts normally. When the switches close (like when a car/magnet passes over the reed switch) the analog pin is taken down to ground level.