“A 555 Timer to Set the Light Speed The circuit that sets the reaction game’s speed will be built around a 555 timer, and it’s similar to the circuits you built in Chapter 8. The components in this circuit diagram will set the game to a “medium” speed: it’s not super fast, and it’s not super slow. Every time the output from the 555 timer goes from low to high, the light moves one step to the side. The number of times the output from the 555 timer goes high per second is the frequency of the output. As I showed in Chapter 8, the formula for calculating the frequency of the output of the 555 timer is The following values from the 555 timer circuit diagram correspond to that formula: R1 = 100 kΩ R2 = 10 kΩ C1 = 1 µF Plug these into the formula, keeping in mind that 1 µF = 0.000001 F and 120 kΩ = 120,000 Ω, and you get this: This means the output will go high 12 times per second and the light will change places 12 times per second. You can experiment with the component values for R1, R2, and C1 later to speed up or slow down the game.”