YOU LEARN HERE...

how to use the micro:bit as a data acquisition device that records and stores readings and how to transfer stored data to a PC..

USE OF THE ANALOG INPUT

To capture a physical quantity and process it digitally, you need a sensor that converts the physical value into a voltage. Normally, this is an analog voltage that can take any value within a certain range. To use it in a computer system it has to be converted into a number (integer) by an analog-to-digital converter (ADC). The micro:bit pin P0 can be configured as input of an ADC that converts voltages 0..3.3V to numbers in the 10-bit range 0..1023. The command pin0.read_analog() performs the conversion and returns the number.

EXAMPLES

Program: [► Online Editor]

from microbit import *

t = 0
while True:
    v = pin0.read_analog()
    print("t = %4.1f v = %d" %(t, v)) 
    t += 0.1
    sleep(100)

In the print() function you use an elegant formatting specification, so that the values are automatically rounded to the desired number of digits. For example the format string %4.1f says that you want to output a field of length 4 with a decimal number (float) that is rounded to 1 decimal place. %d is the format string for an integer. The values themselves then follow the string in a parenthesis with a preceding % sign.

In a typical measurement equipment you want to record a series of measurements and transfer them later to a computer for further data processing and visualization. Such a system is also called a data logger, because you log the acquired data in a log file. To do this you simply have to open a text file with open('data.log') and store the data strings s with f.write(s). For convenience, the fields t and v are separated by a semicolon. Don't forget to add a new line character \n to separate the records line-by-line. You start the data recording for 10 s by clicking the button A.

Program: [► Online Editor]

from microbit import *

display.show(Image.SQUARE_SMALL) 
while not button_a.was_pressed():
    sleep(10)
display.show(Image.SQUARE) 

T = 10
with open('data.log', 'w') as f:
    t = 0
    while t < T:
        v = pin0.read_analog()
        f.write("%4.1f; %d\n" %(t, v)) 
        t += 0.1
        sleep(100)
display.show(Image.NO)  

To transfer the data to a PC you can use the module mbm of TigerJython and extract the file with the command extract() from the micro:bit and copy it to the directory where the program is located. Since this is a Python program that runs on the PC, you have to click the green Run button and not the black Download/Execute button.

from mbm import *

extract('data.log')

Use any text editor and open the log file to check the data. You may visualize them with a spreadsheet program or another graphics tool (such as TigerJython's GPanel).

MEMO

A data logger converts the measurement data into a digital number and stores it in a file, usually by adding a time stamp.

EXERCISES