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GPIO.output(light[yellow],False); GPIO.output(light[red],True)

time.sleep(2)

GPIO.output(lightl[yellow],True)

time.sleep(0.6)

GPIO.output(light[red],False); GPIO.output(light[yellow],False)

GPIO.output(lihtl[green],True)

except KeyboardInterrupt:

GPIO.cleanup()

3.1.1 How does it work?

The first lines are already known, they import RPi.GPIO libraries for controlling the GPIO ports and the time

for time delays. Then the numbering of the ports GPIO, as in the previous example, is set to BCM.

red = 0; yellow = 1; geuen = 2 These lines define the three variables red, yellow and green

for the three LEDs. Thus the program does not require you to remember numbers or GPIO ports; you can be

easily control the LEDs via its coloring.

Light=[4,18,23] To control the three LEDs, a list is created which contains the GPIO numbers in the

sequence in which the LEDs are installed on the breadboard. Because the GPIO ports surface only in this part

of the program, you can easily rebuild the program to use it with different GPIO ports.

GPIO.setup(light[red], GPIO.OUT, initial=False)

GPIO.setup(light[yellow], GPIO.OUT, initial=False)

GPIO.setup(light[green], GPIO.OUT, initial=True)

One by one, the three GPIO ports used will be initialized as outputs. We will not use the GPIO port numbers

this time, but the list which we have previously defined. Within a list the individual elements are indexed

using numbers starting from 0.

Light[0] is therefore the first element in our case, it is 4. The variables red,

yellow and green contain the numbers 0, 1 and 2, which are required to classify the elements in the list as

indices. That way the GPIO ports can be addressed by the colors used:

•

Light[red] is GPIO port 4 with the red LED.

•

Light[yellow] is GPIO port 18 with the yellow LED.

•

Light[green] is GPIO port 23 with the green LED.

The

GPIO.setupinstructions may include an optional parameter initial, which assigns a logical status to

the GPIO port the moment it is initialized. In this program we already turn on the green LED from start. The

other two LEDs start the program in turned off state.

print (“Ctrl+C exits the program") Brief instructions are displayed on the screen. The program runs

automatically. The key combination

[Ctrl]+[C] is used to quit the program. We will use the query string

try...except to ask the user, whether the program shall quit with [Ctrl]+[C]. The code assigned under

try:is initially executed normally.