Merge branch 'master' of https://github.com/Microsoft/pxt-microbit

docs/projects/timing-gates.md

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+# Timing gates
+
+In ths project, we will build a timing gate, a system that can measure the speed of a car.
+
+The timing gate is made of two sensors that can detect the moving car.
+When the car goes through the gate, it triggers each sensor and the micro:bit 
+records the times. The speed is then computed by dividing the distance between the sensors 
+by the time between each sensor trigger.
+
+### ~hint
+
+This lesson explains the principles of timing gates using household materials. To build high performance gates, 
+you will need better sensors such as [Hall Effect sensors](https://en.wikipedia.org/wiki/Hall_effect_sensor).
+
+### ~
+
+## Materials
+
+* Carboard
+* Aluminum fail
+* Double-side tape (carpet tape)
+* 4 crocodile clips
+* A micro:bit board and USB cable
+
+![](/static/mb/projects/timing-gates/materials.jpg "Materials")
+
+## blocks
+
+```cards
+basic.showLeds(`
+        . . . . .
+        . . . . .
+        . . # . .
+        . . . . .
+        . . . . .
+        `)
+input.onPinPressed(TouchPin.P0, () => {})
+let t = 0
+input.runningTime()
+t - 1
+control.eventTimestamp();
+basic.showNumber(0)
+```
+
+## Building the gate
+
+The sensor is made by tapping two strips of fail on the cardboard as close as possible.
+
+Add two strips of double-sided tape on the cardboard. Remove the protective film.
+
+![](/static/mb/projects/timing-gates/tape.jpg "Double sided tape")
+
+Lay the Aluminum foil on the double-sided tape. Press firmly on the tape to get a good bonding of the foil.
+
+![](/static/mb/projects/timing-gates/stickfoil.jpg "Foil sensor")
+
+Strip the out foil around and between the tape strips. Make sure both foil strips don't touch each other.
+
+![](/static/mb/projects/timing-gates/spreadfoil.jpg "Foil taped")
+
+Connect a crocodile strip to each foil strip.
+
+![](/static/mb/projects/timing-gates/connectsensor.jpg "Connecting sensor")
+
+Connect the crocodile plugs to the ``GND`` and ``P0`` pins on the micro:bit.
+
+![](/static/mb/projects/timing-gates/connectcrocs.jpg "Connecting the micro:bit")
+
+The gate is ready to use! Your circuit should look like the picture below:
+
+![](/static/mb/projects/timing-gates/sensordone.jpg "A single gate")
+
+
+## Detecting the car with code
+
+The micro:bit provides an event [on pin pressed](/reference/pins/on-pin-pressed) 
+that is raised when a circuit between ``GND`` and a pin is detected. The circuit conductor could be a wire or even your body!
+We will attach a foil to the bottom of the car. When it passes over the gate, it connect both foil strips, close the circuit and trigger the event. 
+
+Open the [code editor](/) and start a new project and add the following blocks. Notice that we are using pin ``P0`` here. 
+
+```blocks
+basic.showLeds(`
+        . . . . .
+        . . . . .
+        . . # . .
+        . . . . .
+        . . . . .
+        `)
+input.onPinPressed(TouchPin.P0, () => {
+    basic.showLeds(`
+        # . . . .
+        # . . . .
+        # . . . .
+        # . . . .
+        # . . . .
+        `)
+})
+```
+
+Testing the code with our finger, we see a LED column turn on when pressing both strips.
+
+https://youtu.be/zi_-NAmdDpY
+
+## Upgrading the car
+
+In this lesson, we picked a random toy car and tapped foil to the bottom. 
+As the car goes through the gate, it will connect both sides of the gate and trigger it. Make sure to add enough foil to get a good connection on the ground.
+
+![](/static/mb/projects/timing-gates/carfoil.jpg "Attaching foil to the car")
+
+By moving the car (slowly) through the gate, you will see that it triggers the ``on pin pressed`` event.
+
+https://youtu.be/M3DIUvDPlIA
+
+### ~hint
+
+It does not work always! Sometimes the foil does not touch long enough both strip to be detected. This is due to the poor quality of our sensor.
+To fix this, you would have to consider using better sensors based on IR or Hall effect.
+
+### ~
+
+## Adding the second gate
+
+Repeat the same process with tape and foil to build the first gate.
+
+![](/static/mb/projects/timing-gates/sensor2.jpg "Double foil sensors")
+
+Connect the crocodile plugs to the ``GND`` and ``P1`` pins on the micro:bit.
+
+![](/static/mb/projects/timing-gates/sensormicrobit2.jpg "Sensor and microbit")
+
+## Detecting the second gate
+
+Since the second gate is connected to pin ``P1``, we add a second [on pin pressed](/reference/pins/on-pin-pressed) event
+that display 2 columns of LEDs.
+
+```blocks
+basic.showLeds(`
+        . . . . .
+        . . . . .
+        . . # . .
+        . . . . .
+        . . . . .
+        `)
+input.onPinPressed(TouchPin.P0, () => {
+    basic.showLeds(`
+        # . . . .
+        # . . . .
+        # . . . .
+        # . . . .
+        # . . . .
+        `)
+})
+input.onPinPressed(TouchPin.P1, () => {
+    basic.showLeds(`
+        # . . . #
+        # . . . #
+        # . . . #
+        # . . . #
+        # . . . #
+        `)
+})
+```
+Strolling the car over both gates, you can see how the first gate triggers then the second.
+
+https://youtu.be/N4bWQcu6yWs
+
+## Computing time
+
+The micro:bit has a clock that measures time precisely. It measures how many seconds the micro:bit has been on. 
+We will record the time where each gate is tripped in variables ``t0`` and ``t1``. 
+We take the different between ``t1`` and ``t0`` to compute the duration between the gates.
+
+```blocks
+let t0 = 0;
+let t1 = 0;
+basic.showLeds(`
+        . . . . .
+        . . . . .
+        . . # . .
+        . . . . .
+        . . . . .
+        `)
+input.onPinPressed(TouchPin.P0, () => {
+    t0 = control.eventTimestamp();
+    basic.showLeds(`
+        # . . . .
+        # . . . .
+        # . . . .
+        # . . . .
+        # . . . .
+        `)
+})
+input.onPinPressed(TouchPin.P1, () => {
+    t1 = control.eventTimestamp();
+    basic.showLeds(`
+        # . . . #
+        # . . . #
+        # . . . #
+        # . . . #
+        # . . . #
+        `)
+    let d = t1 - t0
+    basic.showNumber(d)
+})
+```
+
+https://youtu.be/piyym_ux1EM
+
+## Computing velocity
+
+Measure the distance between the gates and apply Newton's laws to compute the velocity of the car.
+
+    v = d / t
+
+We'll let you try to code this one on your own!