Question 11.4: You want to connect a three-position (center-off) toggle swi......
You want to connect a three-position (center-off) toggle switch to your Raspberry Pi, and be able to find the position of the switch in your Python program.
Connect the switch to two GPIO pins, as shown in Figure 11-5, and use the RPi.GPIO library in your Python program to detect the position of the switch.
To make this recipe, you will need:
• Breadboard and jumper wires (see “Prototyping Equipment” on page 380)
• Miniature center-off three-position toggle switch (see “Miscellaneous” on page 382)
The common (center) connection of the switch is connected to ground, and each of the two ends of the switch are connected to a GPIO pin with the internal pull-up resistor enabled.
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 is called switch_2_pos.py:
| import RPi.GPIO as GPIO import time GPIO.setmode(GPIO.BCM) |
Run the program, and as you move the switch from top to center to bottom, the position of the switch will be reported every time it changes:
| $ sudo python switch_3_pos.py up center down |
Discussion
The program sets up two inputs with pull-up resistors enabled. The variable switch_position is used to record the current position of the switch.
Inside the loop, both GPIO inputs are read and the three conditions of the if, elif, and else structure determine the position of the switch, assigning the value to a variable called new_switch_position. If this differs from the previous value, then the switch position is printed.
You will find a wide range of types of toggle switches. Some will be described as DPDT, SPDT, SPST, or SPST, momentary on, and so on. The meaning of these letters is as follows:
• D—Double
• S—Single
• P—Pole
• T—Throw
A DPDT switch is double pole, double throw. The word pole refers to the number of separate switch contacts that are controlled from the one mechanical lever. So, a double pole switch can switch two things on and off independently. A single throw switch can only open or close a single contact (or two contacts if it is double pole). However, a double throw switch can connect the common contact to one of two other contacts.
Figure 11-6 shows the most common types of switch.
See Also
For more information on how if statements work, see Recipe 5.18. For the most basic switch recipe, see Recipe 11.1.
| Table A-3. Prototyping equipment | |
| Description | Suppliers |
| M-M jumper wires | SparkFun: PRT-08431, Adafruit: 759 |
| M-F jumper wires | SparkFun: PRT-09140, Adafruit: 825 |
| F-F jumper wires | SparkFun: PRT-08430, Adafruit: 794 |
| Half-sized breadboard | SparkFun: PRT-09567 Adafruit: 64 |
| Pi Cobbler | Adafruit: 1105 |
| Table A-9. Miscellaneous | |
| 1200mAh LiPo battery | Adafruit: 258 |
| 5V relay | SparkFun: COM-00100 |
| 5V panel meter | SparkFun: TOL-10285 |
| Servo motor | SparkFun: ROB-09065, Adafruit: 1449 |
| 5V 1A power supply | Adafruit: 276 |
| Low power 6V DC motor | Adafruit: 711 |
| 0.1 inch header pins | SparkFun: PRT-00116, Adafruit: 392 |
| 5 V 5-pin unipolar stepper motor | Adafruit: 858 |
| 12 V, 4-pin bipolar stepper motor | Adafruit: 324 |
| Magician chassis with gearmotors | SparkFun: ROB-10825 |
| Tactile push switch | SparkFun: COM-00097, Adafruit: 504 |
| Miniature slide switch | SparkFun: COM-09609, Adafruit: 805 |
| Rotary encoder | Adafruit: 377 |
| 4×3 keypad | SparkFun: COM-08653 |
| Piezo buzzer | SparkFun: COM-07950, Adafruit: 160 |
