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