PC/CP320 Physical Computing

Raspberry Pi Python PWM

Objectives

  1. To become familiar with Raspberry Pi GPIO pins and BOARD and BCM numbering schemes
  2. To learn nano editor commands
  3. To learn about Pulse Width Modulation channels on the Raspberry Pi

Preparation

Equipment

Procedure

caution

The Rapsberry Pi GPIO pins can easily be damaged with improper handling. For this lab, you will start by only connecting an oscilloscope to a GPIO pin.

  1. Testing the Serial adapter driver installation

    Connect the TTL-serial cable via the USB cable to the computer.
    Open the Device Manager and check to see what COM port the adapter appeared on.

  2. Open putty, and set it for serial on the COM port just noted and set the baud rate to 115200.

  3. Any time you connect the cobbler, make sure you have it in the correct orientation!!!

    Connect the TTL-serial cable to the Raspberry Pi as follows:
    • Don't connect the red wire.
    • Connect the black wire to GROUND.
    • Connect the white wire to TxD. Use a white wire for the connection!
    • Connect the green wire to RxD. Use a green wire for the connection!

  4. Plug in the Pi, and you should see the boot process in the serial console and get a login prompt as expected.

    Show this to the lab instructor.

  5. Here are some common commands for linux. Note that the cursor up and down keys take you through a history of previously used commands.
    Common linux commands
    Command Description Example
    ls list files in directory ls *.py
    cp copy a file cp test.py newtest.py
    mv move (or rename) a file mv oldtest.py newtest.py
    sudo execute program as superuser sudo python3 test.py
    mount mount external (flash) drive mount /mnt/usb
    umount unmount external (flash) drive umount /mnt/usb
    shutdown shutdown computer properly sudo shutdown -h now


    Here are some common commands for nano.
    Common nano commands
    Command Description
    CTRL-K cut line(s)
    CTRL-U uncut line(s) (i.e. paste)
    CTRL-X save and quit
    CTRL-O save without quitting
  6. Using Pulse Width Modulation on GPIO18


    Broadcom pin GPIO18 has the ability to produce asquare wave output using Pulse Width Modulation or PWM. Once started, a square wave will be produced without the program having to toggle the pin explicitly. To set it up, after the pin is set to output
    p = GPIO.PWM(12, 60)
    will set up the pin for a 60Hz waveform.
    p.start(50)
    will turn it ON with a 50% duty cycle and
    p.stop()
    will turn it OFF.

    p.ChangeDutyCycle(20)
    will change the duty cycle to 20% without stopping it and
    p.ChangeFrequency(100)
    will change the frequency to 100Hz without stopping.

    Download pwm_test_2018.py

  7. Now connect the LED and resistor to the pin, and run the program to see how changing the duty cycle changes the brightness.

  8. Change the frequency to see how low it can go before you see the LED flickering.

  9. Change the duty cycles to see how small a difference in brightness you can detect.

  10. Remove (or comment out) the command to stop the pwm at the end of the program. Run the program and see what happens when the program stops. (You may need to remove the cleanup line as well.)

  11. As before, modify the program to use the same pin GPIO18 but using the Broadcom numbering scheme.


    Demonstrate the output using the scope and LED and the effect of changing the duty cycle to the lab instructor.

  12. Using the other Pulse Width Modulation channel.

    Look up the location of PWM1.

  13. Copy the test program to a new one and change it to work with this new pin.
    Now connect an LED and resistor to this pin, and run the program to show that it works correctly.

  14. Create a program which combines the two, so that the duty cycles are opposites for the two LEDS. (So if one is 10%, the other will be 90%, and so on.)

  15. Change the duty cycles to see how small a difference in brightness you can detect.

  16. Modify the program so that the duty cycle increases from 0 to 100% over 10 seconds for one, with the other having the opposite duty cycle, as in the question above.


    Demonstrate the output to the lab instructor.

  17. Proper shutdown

    Shut down the pi using
    sudo shutdown -h now
    When shutdown is complete, you can disconnect power and put everything away.

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