Script: pushbutton_test

The pushbutton_test script displays whether or not the pushbutton is pressed as values of 1 and 0.  These values are often called states.  With this pushbutton circuit and script, a state of 1 indicates pressed, and 0 indicates not pressed.

• Connect your micro:bit module to your computer with its USB cable.
• In Google Chrome or Microsoft Edge browser, open the micro:bit Python Editor [14]. 
• Set the project name field to pushbutton_test, enter the script, and click Save. 
  (See Save & Edit Scripts [15].)  
• Click Send to micro:bit.
  (See Flash Scripts with Python Editor [16].)

# pushbutton_test

from microbit import *

state = 0
display.off()
pin6.set_pull(pin6.NO_PULL)

while True:
    state = pin6.read_digital()
    print("state = ", state)
    sleep(250)

Tests

• Click the Show serial button to open the serial monitor.
  (See Use the Serial Monitor [17].)
• Verify that while the pushbutton is not pressed, it displays state = 0.
• Verify that when you press and hold the pushbutton, it displays state = 1.

How It Works

• Watch the animation as it repeats, and answer each of the questions below about the differences between pressed and not pressed.  Optionally, view higher res version with pause control: pb-top-button-digital-read.mp4 [18].

1. At the bottom, what value does pin6.read_digital() return that gets stored in the state variable?  
   • Pressed __________, not pressed __________.
2. What is the voltage at the P6 pin?  
   • Pressed __________, not pressed __________.
3. In the schematic, is the 3.3 V supply connected to (or insulated from) the rest of the circuit.
   • Pressed __________, not pressed __________.
4. Which supply voltage do you think the right terminal strip’s (a-e, 25) row is connected to?    
   • Pressed __________, not pressed __________.
5. In the micro:bit, what value does it switch to internally if its pin P6 is set to input?  (Hint: The answer is shown near the USB connection.)
   • Pressed __________, not pressed __________.

(Answers are in the Teacher's Guide.))

Inside the pushbutton_test Script

This script is short, but it contains some important instructions that are unique to the micro:bit module hardware.

To begin, the state = 0 statement just creates a variable named state that will be used inside the while True loop.  

state = 0

The micro:bit module’s P6 pin is part of the module's 5x5 LED display grid.  The display.off() call prevents the LED circuits built into the micro:bit from interfering with the pushbutton circuit you just built on the breadboard.

display.off()

The 10 kΩ resistor connected to the pushbutton is called a pull-down resistor.  More about that later!  P6 also has an internal pull-down resistor enabled by default, and this statement disables it since a 10 kΩ pulldown resistor has been added to the circuit on your breadboard. Many microcontrollers used in

pin6.set_pull(pin6.NO_PULL)

Inside the main loop, when the pushbutton is pressed, the circuit applies 3.3 V to the P6 pin.  The pin6.read_digital() call returns 1 when it detects 3.3 V.  If the button is released, the circuit applies 0 V to the pin, and pin6.read_digital() returns 0 instead.  In state = pin6.read_digital(), the state variable stores that 1 or 0 result.      

while True:
    state = pin6.read_digital()

Next, the print statement displays "state = " followed by the 1 or 0 that just was stored in the state variable.  

    print("state = ", state)

Last in the loop, the sleep(250) call just slows down the display to make it easier to observe the transitions from 0 to 1 and 1 to 0.

    sleep(250)

Try This

The binary (1 or 0) value returned by the pin6.read_digital() method can be used as the argument to turn the LED light on and off in pin13.write_digital(state).  When state is 1 because the button is pressed, pin13.write_digital(state) becomes pin13.write_digital(1).  This sets the P13 LED light to on.  When the state variable is 0 because the button is not pressed, pin13.write_digital(state) becomes pin13.write_digital(0).  This sets the LED light to off.

• Set the project name field to button_light_loopback.  
• Update your script to match the one below, then click Save.
• Click Send to micro:bit.
• Verify that the green light connected to P13 stays off when the button is not pressed.
• Press and hold the button.  Verify that the P13 light stays on while the button is held down.

Did You Know? Input Registers

A light that a microcontroller turns on and off can be considered a binary (2-state) output circuit.  Likewise, a pushbutton circuit can be considered a binary input circuit.  The microcontroller monitors the pushbutton circuit to find out which of two states the button is in: pressed or not pressed.  The microcontroller does this by monitoring the voltage that changes at a certain point in the circuit in response to the pushbutton being pressed.

A pin.read_digital() call sets a micro:bit I/O pin to input.  As an input, an I/O pin monitors the voltage of a circuit that’s connected to it without affecting the circuit.  The circuit behaves the same regardless of whether the I/O pin is connected or not.  

Inside the micro:bit’s microcontroller, there is memory—like a variable—called an input register that stores the voltages measured by I/O pins as 1s and 0s.  Each memory element that stores a 1 or 0 is called a bit.  Likewise, a register bit refers to each memory element that stores a 1 or 0 to indicate the voltage applied to an I/O pin set to input.

When an I/O pin input register bit stores a 1, it means the pin detected a voltage that was above 2.3 V.  If it instead stores a 0, it means the voltage was below 1 V.  If the voltage is in the 1 to 2.3V range, the register just keeps the most recent measurement that was above 2.3 V or below 1 V.  The pin6.digital_read() method returns that binary value (1 or 0) that the P6 input register bit stores.

Your Turn: button_light_blink

The binary (1 or 0) value returend by the pin6.read_digital() method returns can be used in decision making with if statements.  When state is 1 because the button is pressed, code in this example script’s if statement will make the light turn on for 0.25 seconds.  After that, the light is turned off for another 0.25 seconds.  When state is 0, the statements that turn the light on are skipped, and the while True loop just repeatedly turns the P13 light off.  The end result is that the light blinks while the button is pressed and held, and stays off when it is not pressed.

• Set the project name field to button_light_blink.
• Enter the script below into the micro:bit Python Editor [19], then click Save.
• Click Send to micro:bit.
• Verify that the green light connected to P13 stays off when the button is not pressed.
• Press and hold the button.  Verify that the P13 light blinks while the button is held down.

Self-check

• In this activity, you:
  • Built and tested a pushbutton circuit.
  • Wrote scripts that monitored the pushbutton circuit through a micro:bit I/O pin.
  • Wrote scripts that performed different actions that depended on whether or not the pushbutton was pressed.
• Did you successfully build the pushbutton circuit and understand the pushbutton pin map?
• Do you understand how current flows through the pushbutton circuit?
• Were you able to monitor the on/off states of the pushbutton circuit?
• Were you able to manipulate the action of the LED given the state of the pushbutton?

Questions

1. Where might you find pushbuttons?
2. How are pin maps and schematics used?
3. What is the allowable rotation for the pushbutton in relation to the pin map?
4. What value represents the pushbutton when pressed?
5. Name two circuit types that would be considered binary?
6. What is the memory that stores the voltages measured by I/O pins called?
7.  What do you call the memory element storing binary values that indicate whether or not voltage has been applied to a pin.
    

Exercises

1. What call disables the micro:bit’s 5 x 5 LED display grid?
2. What is the function of pin6.read_digital()?
3. Which method is used to dictate the state of the LED?
4. How does digital_read() impact the I/O pins?
5. What is the threshold for determining that voltage has been applied to a pin?
    

Project

Write a script to make the light blink when the button is pressed but without using an if...statement.  Hint: the 1/0 value from the pushbutton state can be used to turn the light on or off, and then sleep.  After that, the script just needs to make sure it turns the light off and sleeps again before repeating the while True() loop.

Questions

1. Answer: keyboard, game controller, power and volume buttons on cell phones.
2. Answer: They are used to show how parts are interconnected and how the pin numbers relate to the physical parts.
3. Answer: 180°
4. Answer: 1
5. Answer: light and pushbutton
6. Answer: input register
7. Answer: register bit

Exercises

1. Answer: display.off()
2. Answer: It returns a call of 0 or 1, depending on whether or not voltage is applied to ping 6.
3. Answer: pinX.write_digital()
4.  Answer: It sets them to input.
5. Answer: 2.3 V

Project

Solution: The trick is to use the 1/0 value of state.

from microbit import *

state = 0
display.off()

while True:
    state = pin6.read_digital()     
    pin13.write_digital(state)
    sleep(100)
    pin13.write_digital(0)
    sleep(100)

Script: continuity_tester.hex

This script will use the CYBERscope to probe the electrical connections when the button is pressed and not pressed.

• Right-click continuity_tester.hex (below), and choose Save Link As...to download.

continuity_tester.hex [20]

• Click the micro:bit Python Editor [19]'s Open button, then select and open continuity_tester.hex. 
• Click Send to micro:bit.  
• Click the three vertical dots  ⋮  by the Send to micro:bit button, and select Disconnect.
• Start the CYBERscope:
  • In a different browser tab, go to cyberscope.parallax.com [21].  
  • Click the CYBERscope's Connect button.
  • In the serial port dialog, select the port with mbed in its name, and then click Connect. 

Tests

Here is the pushbutton’s theory of operation.  That’s the electronics-speak way of saying “how it works.”

• Inside the pushbutton, pins 1 and 4 are always connected to each other.
• Inside the pushutton, pins 2 and 3 are always connected to each other.
• When the pushbutton is not pressed, the 1 and 4 pair of pins IS NOT connected to the 2 and 3 pair.
• When the pushbutton is pressed and held down, the 1 and 4 pair of pins IS connected to the 2 and 3 pair.

Here are continuity tests you can perform to verify the pushbutton’s theory of operation.  First, verify that the 1,4 pair is not connected to the 2,3 pair unless the pushbutton is pressed (and held to get your measurement).

• Start with the alligator clip probes connected as shown in the Circuit diagram:
• Pushbutton pins 3 to 4: red probe to (j, 19) and the black probe to (j, 21)
• Without pressing on the pushbutton, verify that the circuit is open (micro:bit LED displays a circle with a slash).
• Press and hold the pushbutton down, and verify that the circuit is now shorted (micro:bit LED displays a checkmark).
• Repeat the above tests with these continuity probe points:
  • Pushbutton pin 1 to 2: red probe to (a, 19) and the black probe to (a, 21)
  • Pushbutton pin 1 to 4: red probe to (a, 19) and the black probe to (j, 21)
  • Pushbutton pin 3 to 2: red probe to (j, 19) and the black probe to (a, 21)

Next, verify that the 1,4 and 2,3 pairs of pins are interconnected regardless of whether the pushbutton is pressed or not:

• Repeat the continuity tests at these probe points, verifying that the continuity check mark always displays, regardless of whether or not the button is pressed.
  • Pushbutton pin 1 to 4: red probe to (a, 19) and the black probe to (j, 19)
  • Pushbutton pin 2 to 3: red probe to (a, 21) and the black probe to (j, 21)

How the Pushbutton Works

Looking inside the pushbutton, the legs that stick out of both sides of the pushbutton’s body are actually wires that pass all the way through.  One of the wires forms legs 1 and 4, and the other forms legs 2 and 3.  The button has a metal bar attached underneath and a springy material that keeps it floating above the two wires.  When you press the button, the metal bar comes to rest atop the two wires.  

Since legs 1 and 4 are actually part of a single wire, they are always electrically connected, so a micro:bit continuity test will always display the checkmark.  The same applies to legs 2 and 3.  

When the button is not pressed, current cannot conduct between the 1,4 and 2,3 legs, so its connection is called open or an open circuit.  When the button is pressed, current can conduct between the 1,4 and 2,3 legs, and the connection is called closed.  Since this pushbutton is open when it is not pressed, it is called a normally open pushbutton.  Though your kit does not have them, normally closed pushbuttons also exist, where their connection is closed when it is not pressed, and open when it is pressed.

When the button is not pressed, legs 1 and 4 are insulated from 2 and 3.  So probing any of these pairs of pins will result in the micro:bit continuity tester’s LED display showing the not connected X: (4, 3), (4, 2), (1, 2), (1, 3).  When you press and hold the pushbutton, the micro:bit continuity tester displays the connected checkmark instead.

Try This

In addition to checking pushbutton functionality with continuity tests, sometimes the voltage it applies to the micro:bit needs to be checked.  In prototype tests and machine repairs, this is one of the first tests when pressing a button does not produce the desired result.

• Right-click measure_volts_with_cyberscope and select Save link as... to download it.

measure_P6_volts_with_cyberscope.hex [22]

• Go to python.microbit.org/v/2 [23].
• Click Load/Save and then drag the file you just downloaded and drop it on the gray box in the Load section.  
• Click the Flash button to load the script into the micro:bit.
• Click Disconnect.

• Go to cyberscope.parallax.com [24] and click Connect.
• Press and hold the pushbutton for several seconds and verify that the voltage measurement is close to 3.3 V.

Release the pushbutton and verify that the measurement returns to a value close to 0 V.

• In the CYBERscope, click Disconnect.

Did You Know? Active vs. Resting, Pull-down vs. Pull-Up

When this pushbutton is pressed, it is in its active state.  If you think of GND as a “low” voltage, and 3.3 V as a “high” voltage, a pushbutton that sends a 3.3 V high signal when you press it is considered active high.  When a pushbutton is not pressed it is in a 0 V, low resting state.   

The 10 kΩ resistor connected to GND in the circuit below is called a pull-down resistor.  Because the 10 k resistor is connected to ground, it “pulls down” the pushbutton to a GND = 0 V resting state voltage when the button is not pressed.  The circuit also applies that resting state voltage to P6.  On the other hand, when the button is pressed, the circuit applies 3.3 V to P6.  

Without the pull-down resistor, a microcontroller’s I/O pin that’s set to input could “float” in response to nearby electric fields, such as the static electricity that builds up on people as they shuffle along the floor.  That is why a floating input often switches from sensing 0 to 1 and back to 0 for no apparent reason.  It’s also why a resistor that pulls the resting state voltage (either down to GND or up to 3.3 V) is so important.

When the button is pressed, it is in its active state.  P6 becomes connected to 3.3 V through the 220 Ω resistor, and that is called active-high.  A small amount of current also passes through the 10 kΩ resistor, but not through the 220 Ω one.  That’s because as an input, an I/O pin is invisible to the circuit.  It does not supply or draw any voltage or current.  All it does is sense if voltage is above 2.3 V or below 1.0 V.

A pushbutton circuit can also be built with reversed 3.3 V and GND connections.  In other words, the jumper wire could be connected to GND and the 10 kΩ resistor could be connected to 3.3 V.  With the 10 kΩ resistor connected to 3.3 V, it is called a pull-up resistor because the resting state (not-pressed) voltage would be “pulled up” to 3.3 V.  Its active state, while pressed would be 0 V, or active-low.

For prototyping, that 220 Ω resistor protects the microcontroller against a variety of circuit mistakes.  (The 220 Ω resistor maybe could be replaced by a simple wire, but that’s typically for a final design.) The most common scenario is a script that makes a pin an output that sends 3.3 V through a wire to GND = 0 V.  Without that 220 Ω resistor to “resist” the flow of current, some microcontrollers I/O pins can become damaged by trying to supply too much current. 
DO NOT TRY THIS WITH YOUR MICRO:BIT!

Your Turn

There is currently a pushbutton on the breadboard with no voltage, I/O pin, or pull resistor connected.  Create an active-low pushbutton with a pull-up resistor.  Write a script to test the circuit.  Hints:                                                                           

• Use the pushbutton with pull-up schematic in the Did You Know section above.  
• Connect the 220 Ω resistor between the pushbutton’s pin 1 and P9.  
• Connect a 10 kΩ resistor between the pushbutton’s pin 4 and a 3V (+) bus strip socket.  
• Connect a (preferably black) jumper wire between the pushbutton’s pin 3 and a GND (-) bus strip socket.
• Use the micro:bit Python Editor [19] to open the pushbutton_test.hex test script you saved in Build and Test a Pushbutton [25].
• Modify it to test for P9 instead of P6.  That means all instances of pin6 need to be changed to pin9 in the script.
• Click Send to micro:bit to flash the modified script into the micro:bit.
• Use the Serial monitor to verify that pin9.read_digital() returns 1 when the pushbutton is not pressed, and 0 when it is pressed.

Self-check

• In this activity, you:
  • Measured continuity between various combinations of the pushbutton’s pins while it was pressed and not pressed.
  • Looked inside the pushbutton to see how pressing it made the electrical connections change.
  • Examined the functions of the resistors in the pushbutton circuit.
• Do you understand the continuity inside the pushbutton?
• Do you understand how the pull-down and pull-up resistors work inside the pushbutton circuits?
• Were you able to successfully use methods specific to the pushbuttons scripts to manipulate the pushbuttons and LEDs

Questions

1. Which pushbutton pins are always connected to each other?
2. When the pushbutton is pressed, what is the voltage through the circuit and why?
3. What function does a pull down resistor have in a circuit?
4. Describe "active-high."
5. When a pushbutton doesn’t work as expected, what is one of the first things that professionals check?

Exercises

1. In simple terms how would you describe a schematic of a pull-down resistor circuit versus a pull-up resistor circuit?
2. How does display.off() impact pin 6?
3. How does pin6.set_pull(pin6.NO_PULL) work within a pushbutton circuit connected to P6?
    

Project

Imagine that you want a light to blink red when nothing is sensed but to stop when an object is present, much like an automatic door opens when you step on a mat, but is closed the rest of the time. Write a script so that your new pushbutton pull-up circuit flashes the red LED without a pushbutton press and turns off when you press the pushbutton.

Questions

1. Answer: 1 / 4 and 2 / 3
2. Answer: 3.3 V. The two sides of the pushbutton are electrically connected and therefore the circuit is completed.
3. Answer: A pull-down resistor “pulls down” the resting state voltage that the circuit applies to GND = 0 V.
4. Answer: When a pushbutton is pressed and becomes connected to 3.3 V it is called active-high.
5. Answer: The voltage applied to the circuit.

Exercises

1. Answer: It is the reverse.
2. Answer: It disables the display.
3. Answer:  It disables the microbit’s internal pull-down resistor.

Project

Solution:

Hint: Combine scripts from this activity and Build and Test a Pushbutton.

# pushbutton_pullup_with_red_LED

from microbit import *

state = 0
display.off()
pin9.set_pull(pin9.NO_PULL)

while True:
    state = pin9.read_digital()
    print("state = ", state)
    sleep(250)
    
    if state == 1:
        pin15.write_digital(1)
        sleep(250)
    pin15.write_digital(0)
    sleep(250)

Script: pushbutton_test_x2

The pushbutton_test_x2 script displays the states of both pushbuttons in the Serial Monitor.  

• Enter the pushbutton_test_x2 script into the micro:bit Python Editor [19].
• Set the project name field to metric_prefixes_to_values, then click Save.
• Click Send to micro:bit.

# pushbutton_test_x2

from microbit import *

state9 = 0
state6 = 0
display.off()
pin9.set_pull(pin9.NO_PULL)
pin6.set_pull(pin6.NO_PULL)

while True:
    state9 = pin9.read_digital()
    state6 = pin6.read_digital()
    print("state9:", state9, " state6:", state6)
    sleep(250)

Tests

• In the editor, click Show serial.
• Verify that when you do not press the P6 pushbutton, it displays state6 = 0.
• Verify that when you press and hold the P6 pushbutton, it displays state6 = 1.
• Verify that when you do not press the P9 pushbutton, it displays state9 = 0.
• Verify that when you press and hold the P9 pushbutton, it displays state9 = 1.
• When you are done, click Hide serial.

How the pushbutton_test_x2 Script Works

The pushbutton_test_x2 script started as pushbutton_test, and was adapted for two pushbuttons through the following changes: 

• The state variable was replaced by state9 and state6. 
• The pin6.set_pull(pin6.NO_PULL) call was repeated for pin 9: pin9.set_pull(pin9.NO_PULL).
• The single state = pin6.read_digital() statement was replaced with two new versions, one for state6 and pin6, and the other for state9 and pin9. 
• The print statement was expanded so that it shows values for both state9 and state6.    

Try This

The button_light_blink_x2 script is the two-button, two-light version of the button_light_blink script from the Build and Test a Pushbutton activity’s Your Turn section [27].

• Enter the button_light_blink_x2 script into the micro:bit Python Editor [19].
• Set the project name field to button_light_blink_x2, then click Save.
• Click Send to micro:bit.
• Verify that the green light connected to P13 stays off when the button connected to P6  is not pressed.
• Press and hold the button.  Verify that the P13 light blinks while the P6 pushbutton is held down.
• Repeat with the test for control of the P14 light with the P9 pushbutton.

Did You Know? Two-button Control

A pair of pushbuttons can be used to control whether a variable counts up or down.  Take a look at the while True loop below:

while True:
    state9 = pin9.read_digital()
    state6 = pin6.read_digital()

    if state9 is 1:
        index = index + 1
        if index > 2:
            index = 0
    if state6 is 1:
        index = index - 1
        if index < 0:
            index = 2

Each if statement also has a nested if statement that restarts the count when it reaches a certain point.  When up-counting, when the index variable reaches 3, it gets changed back to 0.  When down-counting, when index reaches -1, it gets changed back to 2.  

• Review the LED Lights: Intro to Lists's Script [28] and How It Works [29] sections.
• Think about how you used lists and indexes in Blink Sequencing [10].

If you guessed that counting up or down might work with lists of pins and times, you are right!  In the next Your Turn section, that index value will be used to access the 0, 1, and 2 elements in these pin and time lists (below).  When counting up, the index variable will be used to fetch pin13, pin14, and pin15, then repeat again starting with pin13.  When counting down, the index variable will be used to fetch pin15, pin14, pin13, pin15, pin14, pin13, and so on.

pin = [pin13, pin14, pin15]
time = [200, 200, 200]

Your Turn

With this application, you can hold the P9 pushbutton to make the lights turn on and off in a repeating green-yellow-red pattern.  If you instead hold down the P6 pushbutton, the lights turn on and off in a repeating red-yellow-green pattern.

• Enter the buttons_make_leds_rotate script into the micro:bit Python Editor [19].
• Set the project name field to buttons_make_leds_rotate, then click Save.
• Click Send to micro:bit.
• Verify that the on/off sequence is green-yellow-red-green-yellow-red… while the P9 pushbutton is held down.
• Verify that the on/off sequence is red-yellow-green-red-yellow-green... while the P6 pushbutton is held down.

Self-check

• In this activity, you:
  • Built and tested a second pushbutton circuit.
  • Incorporated it into a light control project.
  • Changed the light control project’s features.
• Does your second pushbutton operate correctly?
• Were you able to create multiple pushbutton projects incorporating multiple LEDs?
• Do you understand how various loop commands can be used to control the functioning of the pushbuttons and LEDs?

Questions

1. What component(s) on your breadboard are used for input and can be used as control devices?
2. What does it mean for an if statement to be nested?

Exercises

1. In basic terms, what happens when statements inside while True and if statements aren’t indented properly?
2. How are list and index used to move through items in a list?
3. How are pushbuttons used to manipulate other components?
    

Projects

1. Modify the Your Turn script so that there are no LEDs lit unless a button is being pressed.
2. Modify the Your Turn script so that the lights stay on for increasingly longer (or shorter) lengths of time depending on which pushbutton is held down. Maintain the LED color change as it is.
3. For this project combine what you know about the zip function (from the LED Lights tutorial), lists, and pushbuttons to create a script that cycles through each LED in increasingly longer time spans before cycling back through the color sequence each time the pushbutton at P9 is pressed (not held down).

Questions

1. Answer: pushbuttons
2. Answer: The if statement is inside another if loop.

Exercises

1. Answer: There will be an error and the script will not run if lines are not properly indented.
2. Answer: The index value can be placed inside a loop with a + / - used to increase or decrease the index value.  Incrementally changing the index value will move through the list items assigned to the corresponding index values.
3. Answer: The pushbutton states are read, 0 or 1, and that information is then passed to other components via write statements to dictate their behavior. Ex: read_digital and write_digital.

Projects

1. Solution.

# rotate_leds_leds_off

from microbit import *

pin = [pin13, pin14, pin15]
time = [200, 200, 200]
index = 0
state9 = 0
state6 = 0

display.off()
pin9.set_pull(pin9.NO_PULL)
pin6.set_pull(pin6.NO_PULL)

while True:
    state9 = pin9.read_digital()
    state6 = pin6.read_digital()

    if state9 is 1:
        index = index + 1
        if index > 2:
            index = 0
        pin[index].write_digital(1)
        sleep(time[index])
        pin[index].write_digital(0)
        
    if state6 is 1:
        index = index - 1
        if index < 0:
            index = 2
        pin[index].write_digital(1)
        sleep(time[index])
        pin[index].write_digital(0)

2. Solution.

#project_solution

from microbit import *

pin = [pin13, pin14, pin15]
time = [1500, 1000, 500]
index = 0
state9 = 0
state6 = 0

display.off()
pin9.set_pull(pin9.NO_PULL)
pin6.set_pull(pin6.NO_PULL)

while True:
    state9 = pin9.read_digital()
    state6 = pin6.read_digital()

    if state9 is 1:
        index = index + 1
        if index > 2:
            index = 0
        pin[index].write_digital(1)
        sleep(time[index])
        pin[index].write_digital(0)
        sleep(time[index])
        
    if state6 is 1:
        index = index - 1
        if index < 0:
            index = 2

        pin[index].write_digital(1)
        sleep(time[index])
        pin[index].write_digital(0)
        sleep(time[index])

3: Solution.

# buttons_with_leds_and_zip

from microbit import *

pin_list = [pin13, pin14, pin15]
time_list = [500, 1000, 1500]
state9 = 0

display.off()
pin9.set_pull(pin9.NO_PULL)

while True:
    state9 = pin9.read_digital()
   
    for pin, time in zip(pin_list, time_list):
        
        if state9 is 1:
            pin.write_digital(1)
            sleep(time)
            pin.write_digital(0)
            sleep(time)

Script: button_led_blink_with_plot

• Right-click button_led_blink_with_plot.hex (below), and choose Save Link As...to download.

button_led_blink_with_plot.hex [31]

• Click the micro:bit Python Editor [19]'s Open button, then select and open button_led_blink_with_plot.hex. 
• Click Send to micro:bit.  
• Click the three vertical dots  ⋮  by the Send to micro:bit button, and select Disconnect.
• Start the CYBERscope:
  • In a different browser tab, go to cyberscope.parallax.com [21].  
  • Click the CYBERscope's Connect button.
  • In the serial port dialog, select the port with mbed in its name, and then click Connect. 

Tests

• Wait until the plotted dots reach (or wrap back around to) somewhere in the 2k to 4k range.
• Press and hold the pushbutton for about six blinks.
• Verify that your plot resembles this.  

NOTE: Before the first pushbutton press, you might notice the red Ch2 voltage trace is plotting irregularly in the 0.5 to 1.5 V range.  That’s because the LED pin is a floating input that has not been set to low or high by pin13.write_digital(0) or pin13.write_digital(1).

Also verify the following:

• While the button is not held down:
  • The black ch0 trace connected to the pushbutton should repeatedly plot points close to 0 V along the bottom of the plot area.  
  • The red ch2 trace connected to the LED should also plot points close to 0 V along the bottom of the plot.  
• While the button is held down:
  • The black ch0 trace should plot points at around 3.3 V.
  • The red ch2 trace should plot the on signals in the 2.5 to 3.3 V neighborhood, and off signals at 0 V, should continue to repeat while the button is pressed and held down.
• In the CYBERscope, click Disconnect.  

How It Works

An oscilloscope is typically a separate piece of equipment.  If we were using one, it wouldn’t be necessary to change the code because the oscilloscope has its own processor for monitoring and displaying measurements.

This script started as button_light_blink from the Build and Test a Pushbutton activity’s Your Turn section [27].  Since the micro:bit is functioning as its own oscilloscope, calls to plot(volts(pin0), "y0", volts(pin2), "y2") are added before and after statements that check the pushbutton and change the LED light on/off states.  

Try This

In electronics, a “start pulse” from one device can cause on/off signals from another device.  Those signals might contain information, like binary communication data, or they might be something like the return waves from an ultrasonic ranging pulse (that measures distance).  To emulate these events, when the script you will use in this activity detects a button press, it will blink the light six times.  

• Right-click button_led_blink_6x_with_plot.hex (below), and choose Save Link As...to download.

button_led_blink_6x_with_plot.hex [32]

• Click the micro:bit Python Editor [19]'s Open button, then select and open button_led_blink_6x_with_plot.hex. 
• Click Send to micro:bit.  
• Click the three vertical dots  ⋮  by the Send to micro:bit button, and select Disconnect.
• Start the CYBERscope:
  • Go back to the CYBERscope tab.
    If you already closed the CYBERscope tab, open a another tab and reopen cyberscope.parallax.com [21].  
  • Click the CYBERscope's Connect button.
  • In the serial port dialog, select the port with mbed in its name, and then click Connect. 

Tests

• Wait until the plotted dots reach (or wraps back around to) somewhere in the 2k to 4k range.
• Press and hold the pushbutton for just a blink or two.
• Verify that your plot resembles the one below.  
• Again, before the first pushbutton press, you might notice the red Ch2 voltage trace is plotting irregularly in the 0.5 to 1.5 V range because P13 is an input that has not been set low or high.
• See how the oscilloscope shows that the button level returned to 0 (not pressed), but the LED continues its on/off pattern for 6 repetitions?

Did You Know: Trigger on Rising or Falling Edge

Sometimes, you have no way of knowing when an event like the button press will happen.  If it happens when the plot is close to the right edge of the screen.  After the signals leave the right side of the screen, they’ll actually wrap around to the left.  

• Repeat your button test, but this time, wait until the points are plotting in the 7k to 9k range before pressing the button for a blink or two.

The CYBERscope trigger feature you experimented with in the Measure Lights with an Oscilloscope activity’s Your Turn section [33] is designed for situations like these because it can keep the plot activity that needs to be analyzed in the right position in the plot area.

The CYBERscope trigger has rising and falling edge settings.  A trigger edge is simply any part of the plot that passes the voltage trigger level.  On the CYBERscope, the default trigger voltage is 1.5 V.  It can be adjusted with a slider to the left of the plot, and is indicated by a turquoise horizontal line.  

With its trigger set to Rise, an oscilloscope waits for two things:

1. The trigger event.  
   • If Trigger is set to Rise, the oscilloscope waits for a pair of data points, one below the trigger voltage, and the next above.  
   • If Trigger is set to Fall, it waits for one point above the trigger voltage, and the next below.
2. Enough points after the trigger event to fill the rest of the plot.

After the oscilloscope has all that data, it positions the plot so that the voltage edge lines up with the trigger time.  The CYBERscope’s default trigger time is 1/5th of the way into the plot.  For example, if the total time axis is 5000 ms, the trigger time would be at 1000 ms.  The time trigger is indicated by a vertical purple line, and can be adjusted by a slider below the plot.

Your Turn: Set the Trigger

• Set Time/Div to 500 ms.
• Set Trigger to Rise.
• Press the button down for at least one blink, then wait 10 seconds.
• Did the plot self-adjust to show you this?
• Try to predict how the plot will change if you changed the Trigger setting to Fall and repeated the button press.

• Test your predictions by setting the Trigger to Fall and repeating the button press for at least one blink.  
• Drag the trigger slider right so that the vertical purple line lines up with 2000 ms.  
• Repeat the button press for at least one blink.

• When you are done, click the CYBERscope's Disconnect button.

Self-check

• In this activity, you:
  • Measured the pushbutton and LED voltages with an oscilloscope
  • Learned how the plotted data relates to what’s actually happening with the circuit voltages
  • Studied start pulses, which are used in electronics to initiate communication and other events
  • Used oscilloscope trigger settings to align that start pulse with a certain time to make the plot easier to view.
  • Were you able to test the signal activity in the pushbutton and LED circuits?
• Do you understand the oscilloscope graph and how to manipulate the view of the information?
• Do you understand start pulses?

Questions

1. Where can you find oscilloscopes used in real-world applications?
2. What types of information might be contained in a start pulse?
3. What happens to the oscilloscope graphs if the plotted line(s) runs off the right side?
4. What is a trigger ‘edge’ and the default trigger voltage on the CYBERscope?

Exercises

1. Why is the black trace line connected to the pushbutton at 0 V when the pushbutton is not pressed?
2. Why is the red trace line connected to the LED at 0 V when the pushbutton is not pressed?

Projects

1. Adjust the volts on the graph so that the trigger edge will be 2 V. What change do you notice?
2. Change the Your Turn program to evaluate the signal for the yellow LED. What do you observe?
3. Modify the Your Turn script so that the LED stays on for 750 ms and off for 250 ms. What do you notice about the graph?

Questions

1. Answer: They can be used to test signals in inventions and robots. They are also used for testing machinery signals and comparing them to repair manuals.
2. Answer: The information in a start pulse might contain binary communication data or return waves from an ultrasonic ranging pulse. 
3. Answer: It wraps around back to the left side.
4. Answer: A trigger ‘edge’ is the part of the plot where it passes the voltage trigger level. The default trigger voltage is 1.5 V.

Exercises

1. Answer: The black trace line connected to the pushbutton is at 0 V when the button is not pressed because there is no current flowing through the circuit.
2. Answer. The red trace line connected to the LED is at 0 V because a pressed pushbutton activates the circuit allowing current to flow.

Projects

1. Solution. The trigger event happens at 2 V instead.
2. Solution. The red trace line is slightly under 3.0 V.  A little less than for the green LED.
3. Solution. The on peak is three times wider than the plot for the off portion. The height doesn’t change.