Test the QTI Sensors Using Micro:bit’s LEDs
Now it is time to test the QTI sensors using the micro:bot LEDs for visual feedback.
- First you will need to add the qti, cyberbot, and intbits modules to your micro:bit. This requires the cyberbot library 0.7.0 or later. The Add Modules to your micro:bit tutorial demonstrates this process for both the online Python and user-installed Mu editors. Follow this tutorial to add these modules to your micro:bit.
As shown with the Mu editor you will see the modules appear on your micro:bit after successfully copying each one over.
- Enter and run this script into your editor.
# QTI_Display_Detections.py # Test program uses micro:bit LEDs to display QTI states from cyberbot import * # import modules from qti import * from intbits import * # cyberbot library 0.7.0 or later bot(22).tone(500, 1000) # start beep while True: # main loop pattern = qti(7, 4).read() # store 1/0 detect states in pattern for x in range(0,4): # loop 4x count LED columns with x z = bit.get(pattern, x) # get 1s or 0s from pattern for y in range (0,5): # use y to visit each row display.set_pixel(x, y, z*9) # LED on 9 or off 0 in x-col y-row
- Position your robot over the black line on your track. The QTI sensors directly over the line will cause the corresponding column of micro:bit LEDs to illuminate.
- Move the robot around slightly to test each QTI sensor Place each QTI sensor over black and then white to verify that the LEDs turn on and off, as shown in the video below.
If the sensors don't see the line, try the following adjustments:
- Double-check all of your wiring connections to make sure the sensors are properly connected to power, ground, and each I/O pin.
- Try replacing the jumper wire connecting to 5 V with a 220 ohm resistor. This makes the QTIs less bright, which might be overwhelming with a shiny line or shiny paper.
- Try removing the nylon spacers from one of the sensor's posts. If this improves performance, remove it from the rest of the posts.
How It Works - Display QTI States with LEDs
This statement copies line detection information for all four QTI sensors into a single int variable:
pattern = qti(7, 4).read()
The qti(7, 4).read() call returns the states of QTI sensors connected to sockets P7 through P4. These states are stored as binary 1 and 0 digits in the int variable named pattern. If a digit is 1, it indicates a black or non-reflective surface. If it’s 0, it indicates a white or reflective surface.
Suppose the two middle QTI sensors are over a black line so that they are both 1’s. The pattern variable’s value would be 0b0110.
After storing the QTI states, a loop uses the qti module’s check_bit method to successively check binary digits in pattern and store them in the z variable. The for x in range(0, 4) loop starts at the rightmost binary digit in pattern and works its way left by increasing the x variable’s value with each repetition: 0, 1, 2, 3. As an argument in the check_bit method, x selects which binary digit in pattern to return, and that binary 1/0 return value gets stored in the z int variable.
for x in range(0,4): # loop 4 times through qti value z = qti.check_bit(pattern, x) # qti.check_bit is 1 or 0 for each x index
With the example 0b0110, this is how z evaluates at each loop repetition:
A nested loop turns all micro:bit LEDs in certain columns on or off to indicate which QTIs are over a black or white surface:
for y in range (0,5): # set each x column of LEDs on when z is 1 display.set_pixel(x, y, z*9)
The third argument in display.set_pixel(x, y, z*9) sets the brightness (from 0 to 9). If z is 0, then the brightness is z * 9 = 0 (off). If z is 1 then the brightness of z * 9 = 9 (max brightness). Instead of just one LED, the for y in range (0,5) loop makes display.set_pixel set all LEDs in a given x column to on or off in response to a z value of 1 or 0. For more info on display, try the examples in the LED Matrix tutorial.