Smart systems and connected devices

A smart crossroads with the micro:bit - from the LED screen to two traffic lights that talk to each other

Now we are going one step further.

We won’t just make a program that runs on one micro:bit any more. We will build smart devices that can control real lights, sound, sensors, motors and other parts of our models.

We will learn how one micro:bit can:

• switch on a real LED;
• control a traffic light;
• notice what is happening around it;
• send a message to another micro:bit;
• receive a message and react to it;
• work together with another device as part of a bigger system.

Imagine a traffic light at a crossroads. It isn’t just a set of lights. It has to know when the other direction is red, when pedestrians may cross and how to send a clear message to the people who are waiting.

That is an example of a smart system.

A smart system has several important parts:

Input ⇒ the program decides ⇒ output ⇒ communication with other devices

For example, at our crossroads today:

• A button or a radio message
• The micro:bit program decides what state the crossroads is in
• The traffic light switches between red, yellow and green
• The LED screen shows the signal for pedestrians
• A sound sends an extra message to people with poor eyesight

From today, we will think like a team building a real device:

• What should our device do?
• What information does it need?
• How will it show the result?
• What happens when it works together with another device?
• How will we check that it doesn’t make a mistake?

Today we start with a smart crossroads – our first project in which our micro:bits will light up, communicate and work together.

A smart crossroads with the micro:bit

Today we will build a small smart crossroads.

First we will show a traffic light on the micro:bit’s built-in LED screen. Then we will move the same logic to our 3D-printed traffic light model with real LEDs. After that we will connect two traffic lights by radio, add a pedestrian light on the LED screen and, finally, add sound to help people with poor eyesight.

Our crossroads won’t be a real traffic system. It is an educational model that helps us learn:

• how a program controls a light;
• how the micro:bit sends a signal to external LEDs;
• how two micro:bits exchange messages;
• how one device can show information in several ways: with light, a picture and sound.

The most important rule of our crossroads: two directions must never have a green light at the same time.

Our mission

By the end of the workshop, each pair will have:

• two 3D-printed traffic lights;
• two micro:bits;
• two directions at the crossroads;
• external red, yellow and green LEDs on each model;
• the micro:bit LED screen showing a pedestrian light;
• radio messages that keep both traffic lights in step;
• a short sound signal when pedestrians get the signal to cross.

Our system goes through several states

For our educational model we use a simple rule:

• Vehicles green ⇒ pedestrians in the same direction walk.
• Vehicles green flashing ⇒ pedestrians in the same direction stand.
• Vehicles yellow ⇒ pedestrians in the same direction stand.
• Vehicles red + yellow ⇒ pedestrians in the same direction stand.
• Vehicles red ⇒ pedestrians in the same direction stand.

In real towns and cities, traffic lights can have more rules, special times for pedestrians, sensors for cars and extra safety phases. Today we are building a simpler model so that we can understand the basic logic.

What we use

Each pair gets:

• 2 × micro:bit V2 – The programmable “brains” of the traffic lights
• 2 × 3D-printed traffic light – Models with real LEDs
• 2 × mini breadboard – A place for the resistors and connections
• 2 × battery pack – Power for the micro:bit after programming
• Crocodile clips – Connect the micro:bit, the breadboard and the traffic light model
• 6 × resistor – Each LED needs its own resistor
• Wires from the model – Red, yellow, green and a shared black wire

On the micro:bit V2 we will also use the built-in LED screen and the built-in speaker.

Connecting the traffic light safely

Our 3D-printed models have four wires:

• Red: Connected to the red LED
• Yellow: Connected to the yellow LED
• Green: Connected to the green LED
• Black: The shared GND wire for all the LEDs

Each LED must have its own resistor. The red, yellow and green LEDs don’t share one resistor. The resistors aren’t inside the 3D-printed model. We put them on the mini breadboard.

The layout we have agreed on is:

• micro:bit P0 ⇒ resistor ⇒ red wire of the traffic light
• micro:bit P1 ⇒ resistor ⇒ yellow wire of the traffic light
• micro:bit P2 ⇒ resistor ⇒ green wire of the traffic light
• micro:bit GND ⇒ shared black wire of the traffic light

A micro:bit digital pin can send the state 1 to switch a signal on or 0 to switch it off. In MakeCode we use this to switch external LEDs on and off.

The rule before connecting

1. The battery pack must be switched off.
2. We connect the wires and the resistors.
3. We check the connections.
4. The leader checks the circuit.
5. Only then do we switch on the power and test.

We never connect an LED straight to a pin without a resistor.

Exercise 1: A traffic light on the LED screen

Before the real model, we will make a traffic light on the micro:bit’s built-in LED screen.

The micro:bit has a small screen of 5 × 5 LEDs. In MakeCode we can use the show leds block and draw our own pictures. The LED dots that are lit make a symbol, a picture or a message.

A traffic light you control by hand

Let’s make three simple pictures:

• Button A = red: STOP
• Button B = yellow: GET READY
• Buttons A+B = green: GO

We can use these symbols:

• Red: X
• Yellow: !
• Green: ↑

Or we can draw our own pictures with the show leds block.

What are we learning here?

• Button = input
• LED screen = output

When we press a button, the micro:bit receives information. Then the program decides which picture to show on the LED screen.

Quiz 1: The LED screen and events

Exercise 2: The green light flashes

Now we will add an important warning.

Before the green light changes to yellow, the green light will flash for a short time. The flashing tells drivers that the state is about to change.

The flashing plan:

• Switch green on (1)
• Wait
• Switch green off (1)
• Wait
• Switch green on (2)
• Wait
• Switch green off (2)
• Show yellow
• Wait a little longer

To start with, let green flash two or three times.

In MakeCode we will use:

• pause to wait,
• repeat to flash a set number of times,
• forever to keep repeating the whole traffic light cycle.

• pause says how long we wait.
• repeat says how many times we repeat the same action.
• forever says that we keep repeating the whole cycle.

Extra challenge

Change the waiting time when the light flashes:

• 100 ms = a tenth of a second
• 1000 ms = one second

Which kind of flashing looks better: faster or slower?

Quiz 2: Flashing and time

Exercise 3: A real 3D-printed traffic light

Now we move the logic from the LED screen to the real model.

The micro:bit won’t just show a symbol on its own screen any more. Now it will use its pins to control real LEDs in the 3D-printed traffic light.

Connecting

First disconnect the micro:bit from the computer and from the batteries (if you are using them).

Then connect:

• P0 ⇒ resistor ⇒ red wire
• P1 ⇒ resistor ⇒ yellow wire
• P2 ⇒ resistor ⇒ green wire
• GND ⇒ black wire

After connecting, check:

• The black wire is on GND.
• The red LED has its own resistor.
• The yellow LED has its own resistor.
• The green LED has its own resistor.
• The crocodile clips aren’t touching each other.
• The wires aren’t pulled too tight.
• The leader has checked the connections.

Colour test

Before the automatic traffic light, we check each colour separately:

• Button A ⇒ switch on only the red light
• Button B ⇒ switch on only the yellow light
• Buttons A+B ⇒ switch on only the green light

When we switch one colour on, the other two should be off.

If pressing button A switches on the green LED instead of the red one, we don’t change the whole program straight away. First we check:

• whether the red wire really is connected to P0;
• whether the wire has been mixed up with another colour;
• whether the resistor is in the right place;
• whether the shared black wire is on GND.

Moving the automatic cycle across

After testing, we use the pins instead of the LED screen:

• Red:
  • P0 = 1
  • P1 = 0
  • P2 = 0
• Yellow:
  • P0 = 0
  • P1 = 1
  • P2 = 0
• Green:
  • P0 = 0
  • P1 = 0
  • P2 = 1

To make the green light flash, we repeat:

• P2 = 1
• wait
• P2 = 0
• wait

This way the same program idea controls a real light, not just a picture on the screen.

Open the code

Quiz 3: External LEDs

Exercise 4: Two traffic lights talk by radio

Now each pair has two micro:bits and two traffic lights:

• Traffic light A controls one direction of the road.
• Traffic light B controls the other direction.

The traffic lights need to know when the other direction has green.

The micro:bits can send short radio messages without any wire between them.

Radio group

For two micro:bits to talk to each other, they must be in the same radio group.

Pair 1 ⇒ radio group = 41
Pair 2 ⇒ radio group = 42
Pair 3 ⇒ radio group = 43
Pair 4 ⇒ radio group = 44

Only micro:bits with the same group number should react to each other. A radio group can have a value from 0 to 255.

First we test the radio without the traffic lights

If the second micro:bit shows the number 1, the radio link works.

Only then do we connect the radio message to the real traffic lights.

Agreeing on the messages

Both micro:bits need to know what each number means:

• 0 = Both traffic lights are red
• 1 = Traffic light A is red and yellow, traffic light B is red
• 2 = Traffic light A is green, traffic light B is red
• 3 = Traffic light A flashes green, traffic light B is red
• 4 = Traffic light A is yellow, traffic light B is red
• 5 = Traffic light A is red, traffic light B is red and yellow
• 6 = Traffic light A is red, traffic light B is green
• 7 = Traffic light A is red, traffic light B flashes green
• 8 = Traffic light A is red, traffic light B is yellow

The number 2 doesn’t mean green on its own. We agreed that message 2 means this state.

Notice: after message 4 comes the phase in which both traffic lights are red. It doesn’t have its own number because it is the same as the state for message 0, so we use the number 0 again.

The most important test

For each radio message, we check both models:

• If traffic light A shows green,
  • traffic light B must show red.
• If traffic light B shows green,
  • traffic light A must show red.

No message may give a green light on both traffic lights at the same time.

From now on, each micro:bit is programmed separately.

The micro:bit in charge (the main one)   Traffic light A:

Open the code

The micro:bit that listens   Traffic light B:

Open the code

Quiz 4: The radio link

Exercise 5: A pedestrian light on the LED screen

The external 3D-printed model shows the lights for vehicles. We use the micro:bit’s built-in LED screen as the light for pedestrians.

We will make two pictures:

• A person standing ⇒ pedestrians wait
• A person walking ⇒ pedestrians can cross

Pedestrian standing

You can draw a picture like this in the show leds block:

. . # . .
. # # # .
. . # . .
. # . # .
. # . # .

Pedestrian walking

You can draw a picture like this:

. . # . .
# # # # .
. . # . .
. # . # .
# . . # #

It doesn’t have to look exactly the same. What matters is that you can tell the difference between a person standing and a person walking.

The pedestrian rule

On one micro:bit:

• If vehicles have green:
  • the pedestrian walks
• If vehicles have flashing green:
  • the pedestrian stands
• If vehicles have yellow:
  • the pedestrian stands
• If vehicles have red:
  • the pedestrian stands

This way one micro:bit controls two different outputs:

• External LEDs ⇒ the traffic light for vehicles
• LED screen ⇒ the light for pedestrians

If you look at the rule above, you will see that the pedestrian only walks when the green light is on all the time. That means we can add this behaviour to our earlier code (from exercise 4) as a call to the function pjesak (Croatian for “pedestrian”), and only in the part of the code that deals with the green light (the zeleno parameter):

• If green is on (value 1):
  • The pedestrian walks
• Otherwise (value 0 or 2):
  • The pedestrian stands.

The function’s parameter is called hoda (“walks”): true means the pedestrian walks, and false means the pedestrian stands.

Quiz 5: The pedestrian light

Exercise 6: Sound to help pedestrians

Now we add sound.

A question to think about:

The MakeCode block play tone can play a tone of a certain pitch and length. We set the tone with a frequency and a time in milliseconds. We will use the in background option, so that playing the sound doesn’t get in the way of the traffic light working normally.

A good device can send a message in several ways: with light, a picture and sound.

Task: Add suitable sounds to the pjesak function.

Final challenge: Show your crossroads

Each pair shows one full traffic cycle.

Before you show it, check:

• Both traffic lights use the same radio group within the pair.
• Different pairs use different radio groups.
• Each LED has its own resistor.
• No crocodile clips are touching each other.
• The LED screen can be seen.
• At no moment is there a green light on both traffic lights.

Check that everything works as shown in the traffic light state table.

What did we learn?

• The micro:bit can control its built-in LED screen and real external LEDs.
• The micro:bit’s buttons can be inputs, and the LED screen and external LEDs can be outputs.
• A green light can flash by switching the LED on and off again and again.
• pause sets how long the program waits.
• repeat repeats an action a set number of times, and forever keeps repeating the program.
• Each external LED needs its own resistor.
• The shared black wire of our traffic light is connected to GND.
• Pins P0, P1 and P2 can control the red, yellow and green LEDs.
• Two micro:bits can exchange radio messages when they use the same radio group.
• Different pairs use different radio groups so that their traffic lights don’t get in each other’s way.
• Radio messages must have a meaning that has been agreed in advance.
• A pedestrian light can be shown on the micro:bit’s LED screen.
• In our model, pedestrians walk when vehicles in the same direction have a steady green light.
• Sound can help people with poor eyesight find out that the state has changed.
• Two directions at a crossroads must never have a green light at the same time.

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