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# Timing gates
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### ~avatar
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2016-10-04 00:18:20 +02:00
This project explains the principles of timing gates using household materials.
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### ~
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## Timing gates
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The two gates are connected to the micro:bit and can detect a car passing through.
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.
The micro:bit records the time in a variable ``t0``.
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.
The micro:bit records the time in a variable ``t1``.
The rest is a bit of math and physics. The time taken to cross the gates is computed as the difference of ``t1 - t0``.
By dividing the distance between the gates by the duration, we get the speed of the car!
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## Materials
* Carboard
* Aluminum fail
* Double-side tape (carpet tape)
* 4 crocodile clips
* A micro:bit board and USB cable
## 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.
Lay the Aluminum foil on the double-sided tape. Press firmly on the tape to get a good bonding of the foil.
Strip the out foil around and between the tape strips. Make sure both foil strips don't touch each other.
Connect a crocodile strip to each foil strip.
Connect the crocodile plugs to the ``GND`` and ``P0`` pins on the micro:bit.
The gate is ready to use! Your circuit should look like the picture below:
## Detecting the car with code
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The micro:bit provides an event [on pin pressed ](/reference/input/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!
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.
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.
Connect the crocodile plugs to the ``GND`` and ``P1`` pins on the micro:bit.
## 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/input/on-pin-pressed ) event
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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!
