[solved] Serial communication between arduino and python fails in script?

Projects General Guidance
1

Hello! This is my first time posting on this forum.
I am testing out serial communication with various applications, starting with python. I have the following Arduino Code which listens at my port /dev/ttyACM0 for bytes to respond to by turning on/off LEDs.

#define redLED      7
#define greenLED    6
#define blueLED     5

int data;

void setup() {
	pinMode(redLED, OUTPUT);
	pinMode(greenLED, OUTPUT);
	pinMode(blueLED, OUTPUT);
	pinMode(buzzer, OUTPUT);	
	Serial.begin(9600);
}

void loop() {
	if (Serial.available() > 0) {
		data = Serial.read();
		switch(data) {
			case 'r': 
				digitalWrite(redLED, HIGH);
				break; 
			case 'g': 
				digitalWrite(greenLED, HIGH);
				break;
			case 'b': 
				digitalWrite(blueLED, HIGH);
				break;
			default:
				digitalWrite(redLED, LOW);
				digitalWrite(greenLED, LOW);
				digitalWrite(blueLED, LOW);
				break;
		}
	}
}

I then used the python library pySerial to write to the serial port . I run the following commads in an interactive Python session and they work perfectly (the LEDs turn on and off as expected):

from serial import Serial
arduino = Serial('/dev/ttyACM0');
arduino.baudrate = 9600;
arduino.write(b'rgb') # red, then green, then blue,
arduino.write(b'x')     # turn them off
arduino.close()

However, when I put the above commands into a script and run the script python3 ./script.py, it does not work. The RX and TX pins flash but the LEDs do not glow. Is there any explanation for this?

2

Try 'char data;'

The usual debugging approach is to have the Arduino print out what it receives.

Finally, if your code is working properly, how will you see the result? These steps should execute in about four milliseconds total, from start to finish.

arduino.write(b'rgb') # red, then green, then blue,
arduino.write(b'x')     # turn them off
3

Thank you for the response. You were right - that was stupid. So I slightly modified the lines in the script:

from serial import Serial
arduino = Serial('/dev/ttyACM0');
arduino.baudrate = 9600;
arduino.write(b'r')  # turn red LED on
arduino.close()

Now the red LED should always remain on even when the script exists. However when I run the script, it doesn't turn on.I tried this with the other LEDs but no luck. I also tried sticking in some
sleep() lines and even an input() line to slow down the script before it exists but none worked. Is this some issue with the transmission of data?

4

Yes, you need to allow time for the Serial port on the Arduino to open.

from serial import Serial
arduino = Serial('/dev/ttyACM0');
arduino.baudrate = 9600;
time.sleep(2);#allow time for serial port to open
arduino.write(b'r')  # turn red LED on
arduino.close()
5

Many thanks! That worked.