2. Chapter Button and leds

Usually, there are three essential parts in a complete automatic control device: INPUT, OUTPUT, and CONTROL. In last section, the LED module was the output part and RPI was the control part. In practical applications, we not only make LEDs flash, but also make a device sense the surrounding environment, receive instructions and then take the appropriate action such as turn on LEDs, make a buzzer beep and so on.

../../../_images/button-led-flow-chart.png

Next, we will build a simple control system to control an LED through a push button switch.

2.1. Project Push Button Switch & LED

In the project, we will control the LED state through a Push Button Switch. When the button is pressed, our LED will turn ON, and when it is released, the LED will turn OFF. This describes a Momentary Switch.

2.1.1. Component List

Raspberry Pi

(Recommended: Raspberry Pi 5 / 4B / 3B+ / 3B)

(Compatible: 3A+ / 2B / 1B+ / 1A+ / Zero W / Zero)

raspberrypi5

LED x1

red-led

Breadboard x1

breadborad-830

GPIO Extension Board & Ribbon Cable

extension-board

Push Button

Switch x1

button-small

Resistor 10KΩ x2

res-10k

Resistor 220Ω x1

res-220R

Jumper (some)

jumper-wire

Note

Please Note: In the code “button” represents switch action.

2.1.2. Component knowledge

2.1.2.1. Push Button Switch

This type of Push Button Switch has 4 pins (2 Pole Switch). Two pins on the left are connected, and both left and right sides are the same per the illustration:

../../../_images/button-small-img-sch.jpg

When the button on the switch is pressed, the circuit is completed (your project is Powered ON).

2.1.3. Circuit

  1. Schematic diagram

    ../../../_images/button-led-sch1.jpg

    Note

    R3 is used to limit current to protect GPIO 18, if you set it to output HIGH level by mistake.

  2. Hardware connection

../../../_images/button-led-hdc1.jpg

Note

There are two kinds of push button switch in this kit.
The smaller push button switches are contained in a plastic bag.

Note

Youtube video: https://www.youtube.com/watch?v=eOUoSkcqdfU

Hint

If you need any support, please feel free to contact us via: support@freenove.com

This is how it works. When button switch is released:

../../../_images/button-led-released.png

This is how it works. When button switch is released:

../../../_images/button-led-pressed.png

2.1.4. Code

This project is designed for learning how to use Push Button Switch to control an LED. We first need to read the state of switch, and then determine whether to turn the LED ON in accordance to the state of the switch.

2.1.4.1. Python Code ButtonLED

First, observe the project result, then learn about the code in detail. Remember in code “button” = switch function

Hint

If you have any concerns, please contact us via: support@freenove.com

  1. Use cd command to enter 02.1.1_ButtonLED directory of Python code.

$ cd ~/Freenove_Kit/Code/Python_GPIOZero_Code/02.1.1_ButtonLED

  1. Use Python command to execute btnLED.py.

$ python ButtonLED.py

Then the Terminal window continues to show the characters “led off…”, press the switch button and the LED turns ON and then Terminal window shows “led on…”. Release the button, then LED turns OFF and then the terminal window text “led off…” appears. You can press “Ctrl+C” at any time to terminate the program.

The following is the program code:

 1#!/usr/bin/env python3
 2########################################################################
 3# Filename    : ButtonLED.py
 4# Description : Control led with button.
 5# Author      : www.freenove.com
 6# modification: 2023/05/11
 7########################################################################
 8from gpiozero import LED, Button
 9
10led = LED(17)       # define LED pin according to BCM Numbering
11button = Button(18) # define Button pin according to BCM Numbering
12
13def loop():
14    while True:
15        if button.is_pressed:  # if button is pressed
16            led.on()        # turn on led
17            print("Button is pressed, led turned on >>>") # print information on terminal 
18        else : # if button is relessed
19            led.off() # turn off led 
20            print("Button is released, led turned off <<<")    
21
22if __name__ == '__main__':     # Program entrance
23    print ('Program is starting...')
24    try:
25        loop()
26    except KeyboardInterrupt:  # Press ctrl-c to end the program.
27        print("Ending program")

Import the Button class that controls Button from the gpiozero library.

from gpiozero import LED, Button

Define GPIO17 as the LED control pin and GPIO18 as the button control pin. The button is set to the input mode with a pull-up resistor by default.

led = LED(17)       # define LED pin according to BCM Numbering
button = Button(18) # define Button pin according to BCM Numbering

The loop continuously determines whether the key is pressed. When the button is pressed, the variable button.is_pressed has a value of 1 and the LED lights up. Otherwise, the LED will be off.

1def loop():
2    while True:
3        if button.is_pressed:  # if button is pressed
4            led.on()        # turn on led
5            print("Button is pressed, led turned on >>>") # print information on terminal 
6        else : # if button is relessed
7            led.off() # turn off led 
8            print("Button is released, led turned off <<<")

For more information about GPIOZero, please refer to the link below:

https://gpiozero.readthedocs.io/en/stable/

For more information about the methods used by the Button class in the GPIO Zero library,please refer to:

https://gpiozero.readthedocs.io/en/stable/api_input.html#button

2.2. Project MINI Table Lamp

We will also use a Push Button Switch, LED and RPi to make a MINI Table Lamp but this will function differently: Press the button, the LED will turn ON, and pressing the button again, the LED turns OFF. The ON switch action is no longer momentary (like a door bell) but remains ON without needing to continually press on the Button Switch.

First, let us learn something about the push button switch.

2.2.1. Debounce a Push Button Switch

When a Momentary Push Button Switch is pressed, it will not change from one state to another state immediately. Due to tiny mechanical vibrations, there will be a short period of continuous buffeting before it stabilizes in a new state too fast for Humans to detect but not for computer microcontrollers. The same is true when the push button switch is released. This unwanted phenomenon is known as “bounce”.

../../../_images/Button_switch.png

Therefore, if we can directly detect the state of the Push Button Switch, there are multiple pressing and releasing actions in one pressing cycle. This buffeting will mislead the high-speed operation of the microcontroller to cause many false decisions. Therefore, we need to eliminate the impact of buffeting. Our solution: to judge the state of the button multiple times. Only when the button state is stable (consistent) over a period of time, can it indicate that the button is actually in the ON state (being pressed).

This project needs the same components and circuits as we used in the previous section.

2.2.2. Code

In this project, we still detect the state of Push Button Switch to control an LED. Here we need to define a variable to define the state of LED. When the button switch is pressed once, the state of LED will be changed once. This will allow the circuit to act as a virtual table lamp.

2.2.2.1. Python Code Tablelamp

First, observe the project result, and then learn about the code in detail.

Hint

If you have any concerns, please contact us via: support@freenove.com

  1. Use cd command to enter 02.2.1_Tablelamp directory of Python code

$ cd ~/Freenove_Kit/Code/Python_GPIOZero_Code/02.2.1_Tablelamp

  1. Use python command to execute python code “Tablelamp.py”.

$ python Tablelamp.py

When the program is executed, pressing the Button Switch once turns the LED ON. Pressing the Button Switch again turns the LED OFF.

In GPIO Zero, you assign the when_pressed and when_released properties to set up callbacks on those actions.

Once it detects that the button is pressed, it executes the specified function onButtonPressed().In the onButtonPressed function, the led. toggle () function reverses the state of the LED device.If it’s on, turn it off; If it’s off, turn it on.Each time the key is pressed, the state of the LED will change once.

To explicitly close a connection to a pin, you can manually call the close() method on a device object:

For more information about the methods used by the Button class in the GPIO Zero library,please refer to: https://gpiozero.readthedocs.io/en/stable/api_input.html#button