First, go to the Micro:bit website. Click on 'New Project' and name your project as 'Reaction Timer'.

Let's start by creating a welcome message that will be displayed when you power on your Microbit.

Add the following code to your project:

basic.showString("Get Ready")

After the welcome message, we are going to create a countdown from 3 to 1 and then show a line on the screen.

Add the following new code to your project:

basic.showString("Get Ready")
basic.showNumber(3)
basic.pause(1000)
basic.showNumber(2)
basic.pause(1000)
basic.showNumber(1)
basic.pause(1000)
basic.showLeds(`
    . . . . .
    . . . . .
    # # # # #
    . . . . .
    . . . . .
    `)

To make the game unpredictable, let's add a random delay after the countdown. 

Add the following new code to create a random delay between 1 to 5 seconds (1,000 to 5,000 milliseconds) and then show the signal (a fully lit up LED screen) to the player.

basic.showString("Get Ready")
basic.showNumber(3)
basic.pause(1000)
basic.showNumber(2)
basic.pause(1000)
basic.showNumber(1)
basic.pause(1000)
basic.showLeds(`
    . . . . .
    . . . . .
    # # # # #
    . . . . .
    . . . . .
    `)
basic.pause(Math.randomRange(1000, 5000))
basic.showLeds(`
    # # # # #
    # # # # #
    # # # # #
    # # # # #
    # # # # #
    `)

We will need 2 variables to store some time stamps:

1. startTime - to store when the signal is shown to the player.
2. reactionTime - to store when the player presses the A button.

Create these variables now and then add the following new code to set the startTime variable.

let startTime = 0
basic.showString("Get Ready")
basic.showNumber(3)
basic.pause(1000)
basic.showNumber(2)
basic.pause(1000)
basic.showNumber(1)
basic.pause(1000)
basic.showLeds(`
    . . . . .
    . . . . .
    # # # # #
    . . . . .
    . . . . .
    `)
basic.pause(randint(1000, 5000))
basic.showLeds(`
    # # # # #
    # # # # #
    # # # # #
    # # # # #
    # # # # #
    `)
startTime = input.runningTime()

Microbit: How to create a variable

---

In the Variables toolbox, create a new variable by clicking the 'Make a Variable' button.

Once you click this button a box will appear asking what you want to call your variable. Give it a name that reminds you what you will be using it for. For example, if you wanted to keep track of your score in a game, you would create a variable called 'score'.