This is a DIY slot car lap timing system using items you might have laying around. If not everything is very inexpensive to purchase.
It is also very easy to build if you have basic soldering skills.

Mine is for a routed HO copper tape track, but can be adapted for braided track, in any scale. Might even be able to do with rails with a little more modification to the track.

Supplies

-USB game pad. Almost any cheap controller will work. I got mine used a couple years ago for $5.
-Roll of copper 1/4" (6mm) striping tape. You might already have some if you built your own routed copper tape track.
-1-4 pieces of double wire for going from the gamepad to track. I used an old power supply i had laying around.
-Soldering iron and solder.
(Optional) flux paste
-Drill with 1/4" - 5/16" drill bit. Can go a little bigger if need be but not too much. We want the hole as close to the size of the copper tape (mostly for aesthetics)
- Lap Timer 2000- Timing software
-JoyToKey - To turn gamepad button presses into keyboard keystrokes
-PC to run it. I'm using an old laptop.

Step 1: Prepare the Software

Download the software, Lap Timer 2000.
Once its installed, go to File, then Options..
You can mess with most settings when you're finished with the timer, and ready to start racing.
For now we'll focus on Port, and Track.
In the Port settings, change to keyboard and you can leave as 1, 2, 3, and 4 (recommended) or set to anything you'd like. Id leave then as-is.
Next hit the Track tab and pick how many lanes your track has (1-4)
Hit ok and you're done.
Leave open for now.
Plug your controller in and let Windows install the drivers. Make sure it works under the device settings.

Download JoyToKey software. Its shareware, but will work as needed (donate to the creator if you can. Its a great piece of software)
Once its installed, open up and hit the options tab. Change Number of Joysticks to 1.
Hit configure, then Advanced settings for each device. Next under Please select a device make sure its the controller you just installed. If it is, hit ok. Back on the main screen click Joystick 1. Press some buttons and see which ones light up numbers 1-4.
Double click one that lights up Button 1 and then press keyboard key 1 in the top most left box. You'll see its yellow in my picture.
Do this for every lane you have, 1-4.

Start a race in Lap Timer 2000 and press each button to make sure it worked. If so proceed to step 2 of this ible. If not go back and check every step.

Step 2: Prepare Gamepad

Unplug gamepad and remove casing. Typically 6-8 Phillips screws in back.
Remove pcb from casing being careful to not rip off the 4 usb wires. Tape the wires down to back of pcb to prevent this.
You'll notice under each button there is pads that you can solder on. 2 per button. When a button us pressed it bridges the two and opens the circuit.
Get a piece of the double wire and strip the ends off both sides.
Get your soldering iron ready.
Tin all 4 ends of one length of cable and tin the two pads for button 1 corresponding to your button 1. Might be in a different location than mine pictured.
Your controller might also look different.
Once you have the wires soldered on to button 1, touch the other two ends together and see if it lights up in JoyToKey. It is bridging the circuit. Button 1 should highlight yellow in JoyToKey.
If not, check your solder points making sure you have a good solder joint.
Do this for the rest of the buttons for as many lanes as you have.

You should now be ready for step 3 if every button worked

Step 3: Dead Strip on Track

Find a section of track where you want to start and finish races.

Cut off about 1 1/2"-2" of the copper tape.
Drill a hole close to one end of tape that's left on the track. Reference the seconds picture for this.
Cut off a decent length of copper tape. Enough to fill in the dead strip on top and to hang below underneath the track.
I like to push the tape from underneath with the backing still on. Position tape where it should be then remove enough backing to stick it to the track. Now go underneath the track and peel off rest of backing and stick to the underside making sure they don't touch.

Make sure it doesn't touch either end of the powered copper tape on top either. This will send power to the controller and simulate multiple button presses, or even worse, damage the controller. The pictures will show you exactly how it should look on top and bottom.

Now solder either wire for button 1 to each tape piece. Polarity doesn't matter here.
Once done, plug controller in, fire up Lap Timer 2000, start a race and see if it counts the lap.
If you did everything correctly it should work perfectly. If not, go back step by step to see what your issue might be.

Repeat for other lanes and you are done!