pxt-calliope/docs/projects/timing-gates.md

# 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!