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