# Line Following

## Introduction @fullscreen

Make a program to follow a line using the Color sensor and reflected light.

![Brick with color sensors tracking a yellow line](/static/tutorials/line-following/line-following.gif)

## Step 1

We are going to test and see if the reflected light is white (outside the line) or black (on the line), and drive our robot accordingly. Open the ``||logic:Logic||`` Toolbox drawer. From the **Conditionals** section, drag out an ``||logic:If then else||`` block onto the Workspace, and drop it into the ``||loops:forever||`` loop.

```blocks
forever(function () { 
    if (true) { 
         
    } else { 
         
    } 
})
```

## Step 2

Open the ``||logic:Logic||`` Toolbox drawer again. From the **Comparison** section, drag out ``||logic:0 < 0||`` comparison block onto the Workspace, and drop it into the ``||logic:if then else||`` block, replacing ``true``. 

```blocks
forever(function () { 
    if (0 < 0) { 
         
    } else { 
         
    } 
})
```

## Step 3

Open the ``||sensors:Sensors||`` Toolbox drawer. From the **Color Sensor** section, drag out a ``||sensors:color sensor light||`` value block onto the Workspace, and drop it into the first slot of the ``||logic:0 < 0||`` comparison block, replacing the `0`.

```blocks
forever(function () { 
    if (sensors.color3.light(LightIntensityMode.Reflected) < 0) { 
         
    } else { 
         
    } 
})
```

## Step 4

If the value of the reflected light is greater than 40% (white or very light) then our robot is outside the line, and we should steer it to the left. In the ``||logic:0 < 0||`` comparison block, click the operator drop-down menu to change from ``<`` to ``>`` comparison, and change the compared value to `40` replacing `0`. 
 
![Logic operator dropdown for conditional block](/static/tutorials/line-following/if-then-else-dropdown.png)

```blocks
forever(function () { 
    if (sensors.color3.light(LightIntensityMode.Reflected) > 40) { 
         
    } else { 
         
    } 
}) 
```

## Step 5

Open the ``||motors:Motors||`` Toolbox drawer. Drag out **2** ``||motors:tank large motors||`` blocks onto the Workspace, and drop one of them into the ``||logic:if||`` part, and the other into the ``||logic:else||`` part of the ``||logic:if then else||`` block.

```blocks
forever(function () { 
    if (sensors.color3.light(LightIntensityMode.Reflected) > 40) { 
        motors.largeBC.tank(50, 50) 
    } else { 
        motors.largeBC.tank(50, 50) 
    } 
}) 
```

## Step 6

In the first ``||motors:tank large motors||`` block in the ``||logic:if||`` clause, change the speed values of the motors from ``50%``, ``50%`` to ``5%``, ``15%``. This slows down the robot, and steers it to the left (because the **C** motor is driving faster than the **B** motor).

```blocks
forever(function () { 
    if (sensors.color3.light(LightIntensityMode.Reflected) > 40) { 
        motors.largeBC.tank(5, 15) 
    } else { 
        motors.largeBC.tank(50, 50) 
    } 
}) 
```

# Step 7

In the second ``||motors:tank large motors||`` block in the ``||logic:else||`` clause, change the speed values of the motors from ``50%``, ``50%`` to ``15%``, ``5%``.  This slows down the robot, and steers it to the right (because the **B** motor is driving faster than the **C** motor).

```blocks
forever(function () { 
    if (sensors.color3.light(LightIntensityMode.Reflected) > 40) { 
        motors.largeBC.tank(5, 15) 
    } else { 
        motors.largeBC.tank(15, 5) 
    } 
}) 
```

## Step 8

Now, let’s download our program to the brick. Plug your @boardname@ into the computer with the USB cable, and click the blue **Download** button in the bottom left of your screen. Follow the directions to save your program to the brick. Attach a Color Sensor to Port 3 of your brick, and attach your brick to a driving base with large motors attached to Ports B and C. See the instructions for building a _Driving Base with Color Sensor Down_. Test your program by positioning your robot to the right of a dark, thick line and then let it drive!