Sensor node!

/*
 * serial_usb_simple_arduino - For communicating over USB serial. Send it a '1' (character one)
 * and it will make the builtin LED start blinking every one second. Send it a '0'
 * (character zero) and it will make it stop blinking.
 *
 * Each time it receives one of the commands, it sends back an 'A' for acknowledge.
 * But send it a commmand it doesn't recognize and it sends back an 'E' for error.
 */

bool blinking = false;
bool led_on = false;
int target_time;

void setup() {
  Serial.begin(115200);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB
  }
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  char c;

  if (Serial.available() > 0) {
    c = Serial.read();
    switch (c) {
    case '0':
      // stop blinking
      blinking = false;
      if (led_on) {
        digitalWrite(LED_BUILTIN, LOW);
      }
      Serial.write("A", 1);
      break;
    case '1':
      // start blinking
      if (blinking == false) {
        blinking = true;
        digitalWrite(LED_BUILTIN, HIGH);
        led_on = true;
        target_time = millis() + 100; // turn off in 1 tenth of a second (100 milliseconds)
      }
      Serial.write("A", 1);
      break;
    default:
      Serial.write("E", 1);
      break;
    }
  } else if (blinking) {
    if (millis() >= target_time) {
      if (led_on) {
        digitalWrite(LED_BUILTIN, LOW);
        led_on = false;
        target_time = millis() + 100; // turn on in 1 tenth of a second (100 milliseconds)
      } else {
        digitalWrite(LED_BUILTIN, HIGH);
        led_on = true;
        target_time = millis() + 100; // turn off in 1 tenth of a second (100 milliseconds)
      }
    }
  }
}

/*
 * serial_usb_simple - Demo that communicates over USB using serial I/O
 * from a Raspberry Pi to an Arduino.
 *
 * To show that it work, this writes a '1' to the Arduino which then
 * blinks the builtin LED on and off. The Arduino also sends back an 'A'
 * to acknowledge that it got the message. This does a read() to get
 * the 'A', demonstrating that reading also works. Two seconds later,
 * this writes a '0' to the Arduino which then stops the blinking.
 * The Arduino again sends back an 'A' to acknowledge that it got the
 * message and this reads the 'A'.
 *
 * This was tested between a Raspberry Pi 3B (running Raspbian) and
 * an Arduino Mega 2560 and also between an NVIDIA Jetson TX1 (running
 * Ubuntu) and the same Arduino.
*/

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// for the serial I/O
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>
#include <linux/serial.h>
#include <sys/ioctl.h>
#include <sys/stat.h>

int open_serial_port(char *name)
{
	int fd, bits;
	struct termios term;
	struct serial_struct kernel_serial_settings;

	if ((fd = open(name, O_RDWR | O_NONBLOCK | O_NOCTTY )) == -1) {
		fprintf(stderr, "open(%s) failed: %s\n", name, strerror(errno));
		return -1;
	}

	/*
	 * Reset the Arduino's line. This is key to getting the write to work.
	 * Without it, the first few writes don't work.
	 * Clear DTR, wait one second, flush input, then set DTR.
	 * Without this, the first write fails.
 	*/
	if (ioctl(fd, TIOCMGET, &bits) < 0) {
		close(fd);
		perror("ioctl(TCIOMGET)");
		return -1;
	}
	bits &= ~(TIOCM_DTR | TIOCM_RTS);
	if (ioctl(fd, TIOCMSET, &bits) < 0) {
		close(fd);
		perror("ioctl(TIOCMSET)");
		return -1;
	}
	sleep(1);
	tcflush(fd, TCIFLUSH);
	bits &= TIOCM_DTR;
	if (ioctl(fd, TIOCMSET, &bits) < 0) {
		close(fd);
		perror("ioctl(TIOCMSET)");
		return -1;
	}

	memset(&term, 0, sizeof(term));
	term.c_iflag = IGNBRK | IGNPAR;
	term.c_cflag = CS8 | CREAD | HUPCL | CLOCAL;
	cfsetospeed(&term, B115200);
	cfsetispeed(&term, B115200);
	if (tcsetattr(fd, TCSANOW, &term) < 0) {
		perror("tcsetattr()");
		return -1;
	}
	if (ioctl(fd, TIOCGSERIAL, &kernel_serial_settings) == 0) {
		kernel_serial_settings.flags |= ASYNC_LOW_LATENCY;
		ioctl(fd, TIOCSSERIAL, &kernel_serial_settings);
	}
	tcflush(fd, TCIFLUSH);

	return fd;
}

int read_from_serial(int fd, char *buf)
{
	int n;

	while (1) {
		if ((n = read(fd, buf, 1)) == -1) {
			if (errno != EAGAIN) {
				fprintf(stderr, "read() failed: (%d) %s\n", errno, strerror(errno));
				return -1;
			}
			// errno == EAGAIN, loop around and read again
		} else {
			if (n == 1) {
				return n; // stop reading
			}
			// read 0 bytes, loop around an read again
		}
	}
}

int main(int argc, char **argv)
{
	int fd;
	ssize_t n;
	char buf[10];

	if ((fd = open_serial_port("/dev/ttyACM0")) == -1) {
		exit(EXIT_FAILURE);
	}

	while (1) {

		printf("Telling the Arduino to start blinking...\n");

		if ((n = write(fd, "1", 1)) == -1) {
			fprintf(stderr, "write() failed: %s\n", strerror(errno));
			exit(EXIT_FAILURE);
		}

		// read to get the acknowledgement from the Arduino

		if (read_from_serial(fd, buf) == -1) {
			exit(EXIT_FAILURE);
		}
		buf[1] = '\0';
		printf("Arduino sent back '%s'\n", buf);

		sleep(2);

		printf("Telling the Arduino to stop blinking...\n");

		if ((n = write(fd, "0", 1)) == -1) {
			fprintf(stderr, "write() failed: %s\n", strerror(errno));
			exit(EXIT_FAILURE);
		}

		// read to get the acknowledgement from the Arduino

		if (read_from_serial(fd, buf) == -1) {
			exit(EXIT_FAILURE);
		}
		buf[1] = '\0';
		printf("Arduino sent back '%s'\n", buf);

		sleep(2);
	}

	return EXIT_SUCCESS;
}

"""
serial_usb_simple.py  Demo that communicates over USB using serial I/O
from a Raspberry Pi to an Arduino.

To show that it work, this writes a '1' to the Arduino which then
blinks the builtin LED on and off. The Arduino also sends back an 'A'
to acknowledge that it got the message. This does a read() to get
the 'A', demonstrating that reading also works. Two seconds later,
this writes a '0' to the Arduino which then stops the blinking.
The Arduino again sends back an 'A' to acknowledge that it got the
message and this reads the 'A'.

This was tested between a Raspberry Pi 3B (running Raspbian) and
an Arduino Mega 2560 and also between an NVIDIA Jetson TX1 (running
Ubuntu) and the same Arduino.
"""

import serial
import time

ser = serial.Serial("/dev/ttyACM0", 115200, timeout=1)

# Reset the Arduino's line. This is key to getting the write to work.
# Without it, the first few writes don't work.
# Clear DTR, wait one second, flush input, then set DTR.
# Without this, the first write fails.
# This trick was learned from:
# https://github.com/miguelasd688/4-legged-robot-model

ser.setDTR(False)
time.sleep(1)
ser.flushInput()
ser.setDTR(True)
time.sleep(2)

while True:

	print('Telling the Arduino to start blinking...')
	ser.write(b'1')

	# read to get the acknowledgement from the Arduino
	while True:
		ack = ser.read()
		if ack == b'A':
			break
	print('Arduino sent back %s' % ack)

	time.sleep(2)

	print('Telling the Arduino to stop blinking...')
	ser.write(b'0')

	# read to get the acknowledgement from the Arduino
	while True:
		ack = ser.read()
		if ack == b'A':
			break
	print('Arduino sent back %s' % ack)

	time.sleep(2)