This is a one lane slot car track lap timer I created using an Arduino Uno. The way it works is pretty simple. It uses a photo resistor which has an LED pointing at it. When the car passes in front of the photo resistor it blocks the LED light and the code in the Arduino looks for this difference and trips the timer. When the timer is tripped it outputs the last lap time and best lap time on a standard LCD display. Also, when a new best lap time is recored the piezo buzzer I have attached outputs a slightly different tone than it does for normal laps. As you can see this project was mocked up pretty quickly with the sensor and LED just taped in place. If this were a permanent project I would have opted to use two hall effect sensors placed under each lane. Not only would that make things a lot neater but it would also allow side by side racing with both lanes being timed at the same time.