Build a Model Planetary Lander with micro:bit

This project follows the Engineering Design Process. Confirm with your teacher if this is acceptable for your project, and review the steps before you begin.

In this procedure, you will connect the ultrasonic sensor to your micro:bit and test your code before you build your model planetary lander. This will make it easier to identify any problems with your circuit or code. Once you have everything working, you will build and test a lander designed to protect your micro:bit.

Assemble Your Circuit

Connect the ultrasonic distance sensor's pins to the micro:bit's pins as follows and as shown in Figure 2. Note that Figure 2 shows single wires connecting the pins to make the connections easier to see, but you will need to connect male-female jumper wires to the ultrasonic sensor's pins, and then use alligator clips to connect them to the micro:bit's pins, as shown in Figure 3.

1. Connect the ultrasonic sensor's Vcc pin to the micro:bit's 3V pin. This connection provides power to the sensor from the micro:bit.
2. Connect the ultrasonic sensor's Trig pin to micro:bit pin 0. This connection sends a signal, or "trigger," from the micro:bit to activate the sensor.
3. Connect the ultrasonic sensor's Echo pin to micro:bit pin 1. This connection allows the micro:bit to receive a signal back from the sensor.
4. Connect the ultrasonic sensor's Gnd pin to the GND pin on the micro:bit. Electrical circuits require a "common ground" to form a closed circuit and allow electricity to flow properly. 
5. Make sure none of the exposed metal ends of the jumper wires or alligator clips bump into each other. This can create a short circuit, prevent your sensor from working, and possibly damage your sensor or micro:bit.

Image Credit: Ben Finio / Science Buddies

Figure 2. Connections between the micro:bit and ultrasonic sensor.

Image Credit: Ben Finio / Science Buddies

Figure 3. Jumper wires and alligator clips connecting the micro:bit to the ultrasonic sensor.

Write and Test Your Code

Note: leave your micro:bit plugged into your computer with the USB cable while you test your code. That way, your micro:bit will get its power through the USB cable and will not drain the batteries. The batteries can drain quickly and prevent your sensor from working properly, so save them for actual drop tests of your lander.

1. If this is your first time using a micro:bit, follow the Getting Started instructions on the micro:bit website to learn how to use your micro:bit and connect it to your computer. 
2. Decide what programming language you will use for your micro:bit:
   1. If you are new to coding, we recommend using Microsoft MakeCode to program your micro:bit. MakeCode is a graphical programming language where you make computer programs using colored blocks of code instead of a text editor. The example code in this procedure uses MakeCode. We recommend following a few of the MakeCode tutorials before you proceed.
   2. You can also program your micro:bit in Python, JavaScript, or Scratch. You may prefer this approach if you already have programming experience in one of these languages. See the Let's Code page for more details.
3. Create a program like the one in Figure 4. This program will take a reading from the ultrasonic sensor and display it in your web browser while your micro:bit is connected to your computer.
   1. Click Variables in the blocks menu, then Make a Variable..., and create a new variable called distance.
   2. Click Extensions in the blocks menu. Search for "sonar" on the new page that opens. Click on the Sonar extension to add it to your project. If you cannot find the Sonar extension, go to step 4.
   3. Click on Variables in the blocks menu. Drag out a set distance to 0 block and snap it into the forever loop.
   4. Click on Sonar in the blocks menu. Drag out the block so it replaces the 0 in the set distance to 0 block. Change the drop-down menus as follows:
      1. ping trig P0
      2. echo P1
      3. unit cm
   5. Click on Advanced in the blocks menu.
      1. Click on Serial. 
      2. Drag out a serial write value "x" = 0 block and snap it below your set distance to block.
      3. Replace the text "x" with "distance (cm)".
      4. Click on Variables in the blocks menu. Drag out the distance variable and use it to replace the 0.
4. Alternatively, if you cannot find the Sonar extension, you can write the code to interface with the ultrasonic sensor yourself, as shown in Figure 5. Many of the blocks required to make this program are available under Advanced / Pins or Advanced / Control (you may need to select More to find the blocks).

Image Credit: Ben Finio / Science Buddies

Figure 4. Makecode program to read the ultrasonic sensor using the built-in Sonar extension.

Image Credit: Ben Finio / Science Buddies

Figure 5. Custom program to read the sensor without using the Sonar extension.

5. Place your ultrasonic sensor flat on the table so it is facing upward.
6. Download the code to your micro:bit.
7. Click Show data Device in Makecode. This will open the serial monitor, which should display both a graph and a numerical value for the distance reading in centimeters.
8. Hold your hand directly over the sensor and move it up and down. You should see the distance values change. If the values do not change, or do they do not seem realistic:
   1. Make sure you have connections to the correct pins as shown in Figures 2 and 3.
   2. Make sure you do not have any short circuits and that the metal parts of the alligator clips are not bumping into each other.
   3. Make sure you have changed the drop-down menus in your code to use the correct pins, as shown in either Figure 4 or 5 depending on which version of the program you wrote. 
9. Once your sensor is working, modify your code to trigger an output based on the distance. This is an open-ended part of the project and you could program multiple outputs or actions at different distances. Figure 6 shows simple example code that uses an if else block to play a buzzing sound when the sensor detects a distance less than 20 cm. Test your code to make sure it works before you continue. 
   1. Note: the micro:bit might not have enough power to light up lots of LEDs and use the ultrasonic sensor at the same time. Try starting with just one or two LEDs, test to make sure the sensor still works properly, and then gradually add more. 

Image Credit: Ben Finio / Science Buddies

Figure 6. Example code to play a tone below a certain threshold distance.

Build and Test Your Lander

Now that you have your circuit and code working, it is time to build and test a lander. If you have ever done an egg drop project, this is similar - except instead of trying to protect an egg, you need to protect your valuable payload (your micro:bit)! Here are some things to consider for your design:

1. Your lander needs to hold your micro:bit, ultrasonic sensor, cables, and 2xAA battery pack without letting them fall out.
2. Your lander should probably have features that slow it down during descent, like parachutes or streamers.
3. Your lander should also have features that help absorb some of the impact when it hits the ground.
4. You will need to make sure your ultrasonic sensor can face downward, with an unobstructed "view" of the ground.
5. Figures 7 and 8 show an example lander with the following features, but your design can be different:
   1. A cardboard box holds the payload.
   2. Two circular holes are cut in the bottom of the cardboard box so the ultrasonic sensor can face downward.
   3. A plastic bag acts as a parachute and is tied to the four corners of the cardboard box with string.
   4. Cotton balls glued to the bottom of the box act as shock absorbers for the landing. 
6. To avoid damage to your micro:bit during initial testing, put another object, like a toy or small rock, in your lander to use as a payload.
7. Test your lander starting with low drop distances (just holding it out in front of you). Watch how hard it hits the ground, and look out for other problems, like parachutes not deploying. Can you think of ways to improve your lander?
8. Test your lander from higher drop heights, for example, by standing on a chair or table (with adult supervision).
9. Once you are ready to test with your micro:bit, move on to the next section.

Image Credit: Ben Finio / Science Buddies

Figure 7. Top view of cardboard box lander with plastic bag parachute.

Image Credit: Ben Finio / Science Buddies

Figure 8. Bottom view of lander with cotton ball shock absorbers and holes cut for the ultrasonic sensor.

Put It All Together

1. Unplug your micro:bit from your computer. 
2. Plug the 2xAA battery pack into the micro:bit.
3. Move your hand back and forth in front of your ultrasonic sensor to make sure your circuit still works. If your circuit does not work properly:
   1. Make sure you are using fresh batteries.
   2. Make sure you have not created any short circuits when moving the micro:bit and cables around.
4. Pack your electronics into your lander (Figure 9).
   1. Remember to make sure the ultrasonic sensor has an unobstructed downward view.
   2. Take care not to create short circuits when bundling the cables up to fit them in your lander. If needed, you can wrap the exposed metal parts in tape to help prevent short circuits. You can also use twist ties or zip ties to help organize your cables.

Image Credit: Ben Finio / Science Buddies

Figure 9. micro:bit, battery pack, and ultrasonic sensor packed into the cardboard box lander. The ultrasonic sensor is facing downward through holes cut in the box.

5. Test your lander by holding it and lowering it slowly toward the ground. Does the output that you programmed trigger properly? 
6. Carefully drop-test your lander. Start from a low height and drop onto a soft surface like carpet, a couch, or a pillow. Does your output seem to trigger at the correct height? If it seems like your lander falls too fast and the output triggers too late:
   1. Try increasing the distance at which you trigger the output to give the micro:bit more time to a react (there is a limit to how fast it can take sensor readings).
   2. Remove any unnecessary code to speed up your program (for example, serial write blocks do not do anything useful when the micro:bit is not connected to your computer).
7. Gradually try increasing your drop height and testing your lander again. To avoid damage to your micro:bit, avoid dropping onto hard surfaces or from very high locations. 
8. What other features can you add to your lander? See the Variations section for more ideas.