“A Counter to Turn the LEDs On To control the LEDs, you’ll use a decade counter, which is an IC that counts input pulses. Every time the clock input on pin 14 goes from low to high, the counter increments by one. It counts from 0 to 9, and it has 10 outputs marked 0 to 9. For example, when the counter has counted three input pulses, output 3 (that is, pin 7) is high, and the other pins are low. If you connect an LED to output 3, then when the counter is at three, the LED will turn on. If you connect LEDs to several output pins, then as the counter increases, the LEDs turn on in order, according to their output pins. When the counter is at 9 and receives a 10th input pulse, it goes back to 0 and turns the output pins on in order again. But the counter counts pulses only if pin 13 is low. This means you can use pin 13 to tell the game when to start moving the light across the LEDs and when to stop the light. Each output has a resistor to reduce the current through the LED and make sure the LED doesn’t get destroyed. Because two output pins connect to each LED, the resistors keep the voltage to each LED high, even though one output will be low and one will be high. The resistors also ensure that two outputs aren’t connected directly together, which could damage the IC when one output is high and the other low.”