**Location A for Model A or B (purchased before July 2014)** With the Pi-Stop fitted in the correct position you can now power up your Raspberry Pi. ###Run Scratch GPIO### If you are still in the Raspberry Pi terminal, start the desktop environment using: `startx ` Open Scratch GPIO from the desktop using the Scratch GPIO icon (we do not need the *ScratchGPIO 7 Plus*): **Scratch GPIO 7** is the standard version, while **Scratch GPIO 7 plus** provides additional support for several add-on boards. > > **NOTE:** If you are using **X-Forwarding**, you can run Scratch GPIO with the following commands: > sudo cp ~/.Xauthority ~root/ sudo /opt/scratchgpio7/./scratchgpio7.sh ###Get ready### In this example we will use the following blocks, all of which are located in the **control** section: We will use one or more of the following: - **broadcast** blocks - **wait** blocks - **when I receive** blocks - **forever** block > > **NOTE:** For more information on the **broadcast**, **wait** and **forever** blocks see > [**Explore and Challenge Scratch GPIO: Pi-Stop First Steps**](ExploreScratchGPIO-PiStopFirstSteps.md) which introduced them for the first time. ###Designing our traffic sequence### Before we go any further we should take some time to work out what the correct sequence of lights a traffic light should show. This way we can design our program and then test to see if it behaves as we wanted. ####The STOP Sequence#### **QUESTION:** When a traffic light is ***GREEN (GO)*** and starts to change, what lights will be lit as it changes to ***STOP*** the traffic?

**WORKSHEET:** On your worksheet colour in the correct traffic light sequence, starting with ***GREEN*** (as shown above) and ending with the light showing ***STOP***. **WORKSHEET:** On the broadcast blocks fill in the required commands to switch **on** or **off** the appropriate LEDs (by switching **on** or **off** the pins), ####The GO Sequence#### **QUESTION:** When a traffic light is ***RED (STOP)*** and changes to ***GO***, what is the normal sequence of lights? Again, fill in the required **broadcast** commands.

**WORKSHEET:** On your worksheet colour in the correct traffic light sequence, starting with ***RED*** and ending with the light showing GO. **WORKSHEET:** Again, complete the required broadcast blocks to switch each of the lights on or off. *Great work!* *You can now create each of the **broadcast** blocks in Scratch and click on them to test each step.* ###Putting the blocks together### We will now create a new **broadcast** group of blocks to recreate the required LED sequence.

####The broadcast STOP Block#### Start the group with a **when I receive** block called **STOP**: Add our first light change: Add a wait block (so the lights do not change instantly): Add our second light change: ####Broadcast GO Block#### Simply do the same with the other light changes, but call this **when I receive** block **GO**: ***Excellent!*** *Now you can test these blocks by clicking on them directly and see if we have our correct traffic light sequences!* **WORKSHEET:** Tick the checkbox marked **"I've created the Pi-Stop STOP and GO sequences"**

##The Final Program - Changing Lights## At the moment our program will not do anything unless we click on it. Just like we did in the previous guide ([**Explore and Challenge Scratch GPIO: Pi-Stop First Steps**](ExploreScratchGPIO-PiStopFirstSteps.md)) we will use a **forever** block to repeat our sequence. We will also add a **when green flag clicked** block to kick off our sequence (allowing you to use the small flag in the top right to **start**, and the red circle to **stop**. For our traffic lights we want them to start by changing to **GO** (by broadcasting "GO") then wait for some time (for example 10 seconds) and then change to **STOP** (by broadcasting "STOP"). Putting the blocks together we end up with: When you run the program (by clicking on the **Green Flag**) you will see each part is highlighted with a white outline while it runs through the **forever** loop and runs each of the **broadcast** group blocks **GO** and **STOP**.

**WORKSHEET:** Tick the checkbox marked **"I've created the Pi-Stop the full Traffic Light Sequence"**

Remember to save your completed work by selecting **File** and **Save As...** from the menu. ###Behold the Changing Lights!### ##Try your own projects...## Now you have your very own traffic light you can use in your own projects! *Below are some ideas, or you can move onto the next guide:* [**Explore and Challenge Scratch GPIO: Pi-Stop Reaction Game**](ExploreScratchGPIO-PiStopReactionGame.md). ###Start a race### **IDEA:** Fed up with unfair starts when starting a race? Why not start your races with your own starting lights! Use the Pi-Stop to start your **Scalextric** (TM) or **Hot Wheels** (TM) races, ensuring everyone gets a fair chance. **QUESTION:** Can you create a Formula 1 style starting lights (where each light lights up and they all go off, the race starts when the lights are off). **QUESTION:** Can you change the program to make the light change with a random wait time (making it harder to go before they change)? *Hint: See the next guide ([**Explore and Challenge Scratch GPIO: Pi-Stop Reaction Game**](ExploreScratchGPIO-PiStopReactionGame.md)) which makes use of random delays.* **WORKSHEET:** Tick the checkbox marked **"Extended Task: Created a racing start light control"**

###Control multiple Traffic Lights### **IDEA:** Perhaps your **LEGO City** (TM) needs some traffic control.

**QUESTION:** Can you extend the traffic light sequence to use a second **Pi-Stop** fitted in **Location B** so you can control traffic at a junction?

**Location B for Model A+, B+ or Raspberry Pi 2 (purchased after July 2014)** **Location B for Model A or B (purchased before July 2014)** *If you want to try this, please ask for an extra Pi-Stop to use in your project.* *Hint: You will need to create new **broadcast** blocks using the 2nd set of pins. Also, think about giving drivers time to react before the other set of lights turns GREEN!*

**WORKSHEET:** Tick the checkbox marked **"Extended Task: Controlled two Pi-Stops at the same time!"** *The next guide is:* [**Explore and Challenge Scratch GPIO: Pi-Stop Reaction Game**](ExploreScratchGPIO-PiStopReactionGame.md).