Add 'line following' tutorial (#435)

docs/tutorials.md

@@ -54,5 +54,11 @@ Step by step guides to coding your @boardname@.
   "cardType": "tutorial",
   "url":"/tutorials/what-color",
   "imageUrl":"/static/tutorials/what-color.png"
+}, {
+  "name": "Line Following",
+  "description": "Use the Color sensor to make a robot follow a line.",
+  "cardType": "tutorial",
+  "url":"/tutorials/line-following",
+  "imageUrl":"/static/tutorials/line-following.png"
 }]
 ```

docs/tutorials/line-following.md

@@ -0,0 +1,111 @@
+# 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!