This chapter presents how to use sensors with micro: bit to sense the physical environment. It has some built-in sensors such as accelerometers, compasses, temperatures, lights, and touch. You can use them without attaching any external components to your micro:bit.
12-1. Using Built-In Accelerometer
Problem
You want to get the acceleration values in the left and right direction (x-axis).
Solution
In the Toolbox, click on the Basic category. Then click and drag the show number block over, and place it inside the forever block.
In the Toolbox, click on the Input category. Then click and drag the acceleration (mg) block over, and place it inside the placeholder of the show number block. By default, the acceleration (mg) block outputs the acceleration values in the x-axis.
In the Toolbox, click on the Basic category. Then click and drag the pause (ms) block over, and place it underneath the show number block .
- Once completed, your code should look like this (Figure 12-1 ).
Figure 12-1.Full code listing
How It Works
The micro:bit has an on-board three-axis accelerometer chip that can be used to measure the acceleration . The accelerometer is internally connected to the micro:bit’s I2C bus. It measures the acceleration or movement along the three axes: x and y axes (the horizontal panes) and the z axes (the vertical pane), which it experiences relative to free fall. This is most commonly called the G-force. With the micro:bit’s accelerometer, you will get acceleration values in mG (milliG).
When you place the micro:bit board on the surface of the earth, it will measure acceleration due to the earth’s gravity, straight upward of g~9.81 m/s2. The micro:bit accelerometer can measure accelerations between +2g and −2g. This range is suitable to use with a wide range of applications.
x - Outputs the acceleration values in the x-axis. Put your micro:bit on a level table with the screen pointing up. Initially, x=0, y=0, and z=-1023. Now, tilt your micro:bit board from the left to right or the right to left. Your micro:bit will display values ranging from −1023 to +1023.
y - Outputs the acceleration values in the y-axis. Put your micro:bit on a level table with the screen pointing up. Initially, x=0, y=0, and z=-1023. Now, tilt your micro:bit board forward and backward. Your micro:bit will display values ranging from −1023 to +1023.
z - Outputs the acceleration values in the z-axis. Put your micro:bit on a level table with the screen pointing up. Initially, x=0, y=0, and z=-1023. Now, move your micro:bit up and down. Your micro:bit will display values ranging from −1023 to +1023.
strength - Outputs combined force in all directions (x, y, and z) also known as the overall acceleration. The overall acceleration can be calculated by the Pythagorean theorem . The formula uses the acceleration along the x, y, and z axes as shown below.
Displaying overall acceleration
Watch this great video located at https://youtu.be/byngcwjO51U to learn how the accelerometer on micro:bit works .
12-2. Using Gestures
Problem
You want to display a random number from 1 to 6, when you shake your micro:bit .
Solution
In the Toolbox, click on the Input category and then click on the on shake block.
In the Toolbox, click on the Basic category. Then click and drag the show number block over, and place it inside the on shake block.
In the Toolbox, click on the Math category. Then click and drag the pick random block over, and place it inside the placeholder of the show number block.
In the pick random block, type 1 for the minimum and 6 for the maximum value.
- Once completed, your code should look like that in Figure 12-3 .
Figure 12-3.Full code listing
How It Works
Shake
Logo up
Logo down
Screen up
Screen down
Tilt left
Tilt right
Free fall
3g
6g
8g
Accelerometer gestures
12-3. Using Compass
Problem
You want to find which direction on a compass the micro:bit is facing.
Solution
In the Toolbox, click on the Variables category. Next, click on the Make a Variable button. In the New variable name window, type degrees. Then click on the Ok button.
In the Toolbox, click on the Input category. Then click and drag the compass heading block over, and place it inside the placeholder of the of the set degrees to block.
In the Toolbox, click on the Logic category. Next, click and drag the if-then-else block over, and place it underneath the set degrees to block. Then add more else if branches as shown in Figure 12-5 . Use the show arrow block to display different directions.
- The conditional statements for if, else if, and else are as follows:
If degrees < 45, then show arrow north
If degrees < 135, then show arrow east
If degrees < 255, then show arrow south
If degrees < 315, then show arrow west
- Else, show arrow north
Figure 12-5.Full code listing
How It Works
The dedicated magnetometer chip located on the back of your micro:bit measures the compass heading from 0 to 359 degrees. If the compass is not ready, it returns −1003. The micro:bit compass is based on the NXP/Freescale MAG3110, which is a three-axis magnetometer sensor that can be accessed through the I2C bus. The compass can also act as a metal detector.
North: 315–44 degree
East: 45–134 degrees
South: 135–224 degrees
West: 225–314 degrees
12-4. Calibrating the Compass
Problem
You want to calibrate the built-in compass .
Solution
- In the Toolbox, click on the Input category. Then click and drag the calibrate compass block over, and place it inside the on start block (Figure 12-6 ).
Figure 12-6.Full code listing
How It Works
Before using the compass, you should calibrate it to ensure correct readings. It is also wise to calibrate the compass each time you use it in a new location.
In some situations, when the compass needs to be calibrated, the micro:bit will automatically prompt the user to calibrate it. However, the calibration sequence can also be manually started with the calibrate compass block .
You can place the calibrate compass block at any point in your code, when you need to calibrate the compass. Sometimes the compass may not work even after calibration. It can give spurious results, so it shouldn’t be relied on fully for navigation.
12-5. Using Built-In Temperature Sensor
Problem
You want to read the air temperature surrounding your micro:bit in Celsius .
Solution
In the Toolbox, click on the Basic category. Then click and drag the show number block over, and place it inside the forever block.
In the Toolbox, click on the Input category. Then click and drag the temperature block over, and place it inside the placeholder of the show number block.
In the Toolbox, click on the Basic category. Then click and drag the pause (ms) block over, and place it underneath the show number block.
- Once completed, your code should look like the following (Figure 12-7 ).
Figure 12-7.Full code listing
How It Works
The micro:bit doesn’t have a dedicated temperature sensor. Instead, the temperature block outputs the temperature of the micro:bit’s main CPU. The temperature is a good approximation of the air temperature where your micro:bit is kept and known as ambient temperature .
In the above solution under Recipe 12-5, the temperature block outputs the CPU temperature in Celsius. The forever and the show number blocks are used to continually update and display the temperature values on the micro:bit’s LED screen.
Fahrenheit = ((Celsius x 9) / 5) + 32
Converting Celsius to Fahrenheit
12-6. Using Built-In Light Sensor
Problem
You want to find the light level around your micro:bit .
Solution
In the Toolbox, click on the Variables category. Next, click on the Make a Variable button. In the New variable name window, type reading. Then click on the Ok button.
In the Toolbox, click on the Input category. Then click and drag the light level block over, and place it inside the placeholder of the set reading to block.
In the Toolbox, click on the Led category. Then click and drag the plot bar graph of block over, and place it underneath the set reading to block.
In the Toolbox, click on the Variables category. Next, click and drag the variable named reading over and place it inside the first placeholder of the plot bar graph of block . Then type 255 in the second value box.
- Once completed, your code should look like the following (Figure 12-9 ).
Figure 12-9.Full code listing
How It Works
The micro:bit doesn’t have a dedicated light sensor. Instead, when you shine light on the front of your micro:bit, it measures the capacitance across a number of LEDs on the front of the board . Then these values are averaged together and give you a number between 0 and 255. The 0 indicates darkness and the 255 indicates bright light. The plot bar graph block is used to display a vertical bar graph based on the light level.
12-7. Using Touch Pins
Problem
You want to display the happy icon when you touch the pin 0 .
Solution
In the Toolbox, click on the Input category and then click on the on pin P0 pressed block.
In the Toolbox, click on the Basic category. Now, click and drag the show icon block over, and place it inside the on pin P0 pressed block. Then choose the happy icon from the drop-down list.
- Once completed, your code should look like that shown in Figure 12-10 .
Figure 12-10.Full code listing
How It Works
Micro:bit board has three specialized pins in the edge connector with large pads, known as touch pins. They are pins 0, 1, and 2. These pins can be used to build touch-sensitive applications based on the analog input. The large connector pads allow you to touch them with your fingertips to change the capacitance of the internal circuit .