Added a railway crossing project (#407)

docs/projects.md

@@ -85,6 +85,10 @@ Fun games to build with your @boardname@.
   "name": "Milky Monster",
   "url":"/projects/milky-monster",
   "imageUrl":"/static/mb/projects/milky-monster.jpg"
+}, {
+  "name": "Railway crossing",
+  "url":"/projects/railway-crossing",
+  "imageUrl":"/static/mb/projects/railway-crossing.png"
 }]
 ```
 

docs/projects/railway-crossing.md

@@ -0,0 +1,178 @@
+# Railway crossing
+
+### ~
+
+https://youtu.be/xBInZN2ZWRI
+
+## Duration
+
+7 steps, about 15-30 minutes each.
+
+## Step 1: Light sensor
+
+We are going to use the light sensor to detect if a train is passing. We will do this by detecting the shadow of the train.
+
+Let's first explore how the light sensor works by downloading the following program onto our @boardname@.
+
+```block
+input.onButtonPressed(Button.A, () => {
+    basic.showNumber(input.lightLevel())
+})
+```
+
+Now press button A and the @boardname@ responds with a number. This number indicates the amount of light that falls
+on the LEDs (yes, LEDs can also be used to detect light). If the number that you receive is either 255 or 0 it means
+the brightness sensor is initializing, so you need to press the button once more.
+
+## Step 2: The scene
+
+NOTE: If you do not possess a train, you can also use a car or even your hand to create a shadow.
+
+Detecting a shadow can be a bit tricky. For this to work we need to have a bright lamp on the other side of the train,
+that is not too high. A desk lamp should work. Furthermore, we do not want too much light from other light sources,
+so you may have to close curtains or blinds and dim ceiling lamps.
+
+You should now see a shadow of the train that is big enough to place your @boardname@ in. Make sure the board is laying
+down flat on the table and write down the amount of light that is measured in the shadow and the amount of light
+that is measured when there is no shadow. The number in the light should be at least 2 times the number in the shadow.
+
+## Step 3: Detecting the train
+
+![Train detection](/static/mb/projects/railway-crossing/railway-crossing-action.png)
+
+We are going to turn on the top-left LED on our @boardname@ when a train is passing by.
+For this you need to pick a threshold. This should be a number that is roughly in between the 
+two numbers you wrote down in step 2. 
+For example, if the brightness in the shadow was 20 and in the light it was 60, you should use 40 as the threshold. 
+
+Add the following blocks to your program to make the top-left led indicate if a train is detected. 
+Replace 40 with your threshold.
+
+```block
+basic.forever(() => {
+    if (input.lightLevel() < 40) {
+        led.plot(0, 0)
+    } else {
+        led.unplot(0, 0)
+    }
+})
+```
+
+Now try it out! If the led turns on even when no train passes, you should try lowering the threshold.
+If the led does not turn on when a train passes, you should try increasing the threshold.
+
+NOTE: If you cannot get this to work reliably, you can still continue with the rest of the steps and
+close the railway crossing using a button.
+
+## Step 4: Making the lights
+
+For the lights we need the following materials:
+
+ - A piece of cardboard (There is nothing wrong with recycling!)
+ - Two red LEDs, preferably 5mm with diffused light and long connectors
+ - A resistor that roughly matches the LEDs at 3 Volt
+ - Crocodile clips
+ - glue
+ - tape
+
+https://youtu.be/No1qK51tHNQ
+
+The movie shows how to build the pole. A short summary:
+ - Build the pole using cardboard
+ - Put a resistor at the bottom of the pole and add 2 LEDs in parallel above it. 
+   These LEDs should be placed in opposite directions.
+ - Use the crocodile clips to complete the circuit.
+ - Now connect the circuit to the 3V and GND pins of the @boardname@. One of the LEDs should now light up.
+ - Swap the crocodile clips and now the other LED should light up.
+ 
+## Step 5: Making the lights blink
+
+Now we are going to program the @boardname@ to make the LEDs blink like a railway crossing. For this we are
+going to connect the lights from the previous step to pins 1 and 2 of the @boardname@. 
+
+We can turn on one LED by writing a digital 1 to one pin and a digital 0 to the other
+(digital 1 means 3 Volt, digital 0 means GND). The other LED can be turned on by swapping 0 and 1. 
+Now use the following program to make the lights blink indefinitely.
+
+```block
+basic.forever(() => {
+    pins.digitalWritePin(DigitalPin.P1, 1)
+    pins.digitalWritePin(DigitalPin.P2, 0)
+    basic.pause(300)
+    pins.digitalWritePin(DigitalPin.P1, 0)
+    pins.digitalWritePin(DigitalPin.P2, 1)
+    basic.pause(300)
+})
+```
+
+## Step 6: Programming the railway crossing
+
+In order to make a safe railway crossing, we want the railway crossing to keep the lights 
+flashing for 5 more times after a train is no longer detected.
+This means that our program needs to remember how many flashes are remaining. 
+For this we use a variable called `flashes_remaining`.
+When a train is detected, we set this variable to 5 and after each flash, 
+we subtract 1 from the variable until it reaches 0. Then the flashing stops.
+
+We also want to manually close the railway when button B is pressed.
+First of all, remove the forever block from step 5. Then add the following code:
+
+```block
+let flashes_remaining = 0
+
+input.onButtonPressed(Button.B, () => {
+    flashes_remaining = 5
+})
+
+basic.forever(() => {
+    while (flashes_remaining > 0) {
+        pins.digitalWritePin(DigitalPin.P1, 0)
+        pins.digitalWritePin(DigitalPin.P2, 1)
+        basic.pause(300)
+        pins.digitalWritePin(DigitalPin.P1, 1)
+        pins.digitalWritePin(DigitalPin.P2, 0)
+        basic.pause(300)
+        flashes_remaining += -1
+        pins.digitalWritePin(DigitalPin.P1, 0)
+    }
+})
+```
+
+The lights will now flash for 5 times when button B is pressed.
+You can now go ahead and add blocks to the train detection program from step 3 to start flashing the lights 
+when a train is detected.
+
+## Step 7: Adding the barrier
+
+![Railway crossing with servo](/static/mb/projects/railway-crossing/railway-crossing-with-servo.jpg)
+
+For the barrier you need:
+
+ - An SG90 9g servo, [equipped with crocodile clips](../device/servo)
+ - A straw (preferably white)
+ - Some red tape for decoration
+ 
+### Instructions
+
+ - Glue the servo to the pole
+ - Glue a strip of cardboard over it for stability
+ - Tape the straw to the servo
+ - Add the instructions below at the right location in your program
+ - Connect the servo to the 3V, GND and PIN0 as shown in the simulator
+ 
+#### Open the barrier
+```block 
+pins.servoWritePin(AnalogPin.P0, 90)
+```
+ 
+#### Close the barrier
+```block
+pins.servoWritePin(AnalogPin.P0, 180)
+```
+ 
+Good luck and have fun!
+   
+### ~
+
+## about the author
+This project was contributed by Johan Gorter [@JohanGorter](https://twitter.com/JohanGorter).