217 lines
5.9 KiB
Markdown
217 lines
5.9 KiB
Markdown
# 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
|
|
|
|
|
|
|
|
## 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
|
|
|
|
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.
|
|
|
|
|
|
|
|
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
|
|
|
|
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! |