Question 14.5: You want to use PyFirmata, but over the serial connection (R......

Use a level converter to connect the RXD pin of the Raspberry Pi to the Tx pin of the Arduino, and the TXD pin of the Raspberry Pi to the Rx pin of the Arduino.

To make this recipe, you need:

• Arduino Uno (see “Modules” on page 381)
• Breadboard and jumper wires (see “Prototyping Equipment” on page 380)
• 270Ω and 470Ω resistors (see “Resistors and Capacitors” on page 380) or a four-way bidirectional level converter (see “Modules” on page 381)

If you are using the level converter module, then connect the breadboard as shown in Figure 14-7.

If, on the other hand, you are using the pair of resistors, then connect the breadboard as shown in Figure 14-8.

The Arduino Rx input is fine with just 3.3V from the Raspberry Pi TXD pin; however, the 5V coming from the Arduino Tx pin must be dropped to the 3V expected by the Raspberry Pi.

You will need to set up PyFirmata—see Recipe 14.3. The Arduino side of the project remains exactly the same as Recipe 14.4, where USB is used instead of the serial connection. There is one change that you need to make to the Python program running on the Raspberry Pi—change the device name from /dev/ttyACM0 to /dev/ttyAMA0, as the serial port has a different device name from the USB interface.

The following Python script makes the LED blink at a rate of about 1 Hz. 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 http://www.rasp berrypicookbook.com, where it is called ardu_flash_ser.py.

Discussion

The level conversion is necessary because the Raspberry Pi serial port connections, RXD and TXD, operate at 3.3V, whereas the Arduino Uno operates at 5V. While it is OK for the 5V Arduino to use a 3V signal, the reverse is not true and a 5V signal connected to the 3V RXD pin is likely to damage the Raspberry Pi.

See Also

You can also easily adapt the other examples that use PyFirmata (Recipes 14.6 through 14.9) to use a serial rather than USB connection, simply by changing the device name in the Python program.