Question 11.2: You want to turn something on and off with a push switch so ......

Record the last state of the button and invert that value each time the button is pressed.

The following example toggles an LED on and off as you press the switch.

To make this recipe, you will need:

• Breadboard and jumper wires (see “Prototyping Equipment” on page 380)

• Tactile push switch (see “Miscellaneous” on page 382)

• LED (see “Opto-Electronics” on page 381)

• 470Ω resistor (see “Resistors and Capacitors” on page 380)

Figure 11-3 shows how to connect a tactile push switch and LED using a breadboard and jumper wires.

As well as the male-to-female jumper wires connecting the Raspberry Pi to the breadboard, you will also need one male-to-male jumper wire or solid core wire.

Open an editor (nano or IDLE) and paste in the following code. As with all the program examples in this book, you can also download the program from the Code section of the Raspberry Pi Cookbook website, where it’s called switch_on_off.py:

Discussion

The variable led_state contains the current state of the LED (True for On and False for Off). Whenever the button is pressed, the following line is run:

The not command inverts the value of led_state, so if led_state is True, it becomes False and vice versa.

The variable old_input_state is used to remember the button position so that a button press is defined as occurring only when the input state changes from being True (switch not pressed) to False (switch pressed).

See Also

You will find that sometimes when you press the button, it does not seem to toggle the LED. This is because of switch bounce. You can find other techniques for avoiding switch bounce in Recipe 11.5.