Make this optoelectronic critic and end ever those squabbles over who actually won the competition. Install Q1 and Q3 phototransistors at the end position, exactly in separate roads of your slot-car track so that each light-sensitive front of each device is facing upwards.
The most reliable technique would be to cut a tiny hole into the track for several phototransistor, and attach each unit flush with the track’s outside. Arrange for light to come on both Q1 and Q3; a little desk lamp will run great, exactly ambient room light will regularly satisfy. Press S1 and both diode LEDs directions go off. The first car to cross the end line prevents the light beam and makes the appropriate LED to light up.
Slot car race referee parts list:
R1, R2 = 18KΩ
R3 = 3K9
R4, R5 = 330Ω
C1 = 0.1uF/35V
Q1, Q3 = FPT 100
Q2, Q4 = 2N3904
IC1 = SN7474 dual D flip-flop
LED1, LED2 = LED diode
S1 = normally open SPST pushbutton switch