Attempting to monitor serial output for a string to trigger function, apparently matching string is output but the function doesn't trigger

Question

I'm working on my second Arduino project, and am somewhat stuck.

I'm running a PIR module from my Arduino Uno with the following sketch:

/* 
 * PIR Sensor test
 */
 int pirPin = 6;
 int minSecsBetweenEmails = 60; // 60 seconds
 long lastSend = -minSecsBetweenEmails * 1000l;
 void setup() 
 {
 pinMode(pirPin, INPUT);
 Serial.begin(9600);
 }
 void loop() {
 long now = millis();
 if (digitalRead(pirPin) == HIGH)
 {
 if (now > (lastSend + minSecsBetweenEmails * 1000l))
 {
 Serial.println("MOVEMENT");
 lastSend = now;
 }
 else
 {
 Serial.println("Too soon");
 }
 }
 delay(500);
 }

I'm monitoring the COM3 port using the following in JupyterLab with PySerial.

import time
import serial
import smtplib
to = '[email protected]'
GMAIL_USER = '[email protected]'
GMAIL_PASS = '**********'
SUBJECT = 'MOTION pir'
TEXT = 'PIR sensor detected motion'
ser = serial.Serial('COM3', 9600)
def send_email():
 print("Sending Email")
 smtpserver = smtplib.smtp("smtp.gmail.com",587)
 smtpserver.ehlo()
 smtpserver.starttls()
 smtpserver.ehlo
 smtpserver.login(GMAIL_USER, GMAIL_PASS)
 header = str('To:' + TO + '\n' + 'From: ' + GMAIL_USER)
 header = header + '\n' + 'Subject:' + SUBJECT + '\n'
 print (header)
 msg - header + '\n' + TEXT + ' \n\n'
 smtpserver.sendmail(GMAIL_USER, TO, msg)
 smtpserver.close()
while True:
 message = ser.readline()
 print(message.decode('utf-8'))
 if message[0] == 'MOVEMENT' :
 send_email()
 time.sleep(0.5)

I believe my problem is with the ser.readline() function. I've been reading the docs, they seem to say I need to provide some kind of endpoint or timeout for the function to provide for EOF, but my goal was to have the

if message [0] == 'MOVEMENT' : 

continuously evaluate the output lines from COM3...

Any input at all would be very much appreciated.

EDIT: Thanks to all three of you for your help! By changing the Python as shown below I was able to fulfill the if message[0] == 'MOVEMENT' :statement

Python should have been:

import time
import serial
import smtplib
to = '[email protected]'
GMAIL_USER = '[email protected]'
GMAIL_PASS = '**********'
SUBJECT = 'MOTION pir'
TEXT = 'PIR sensor detected motion'
ser = serial.Serial('COM3', 9600)
def send_email():
 print("Sending Email")
 smtpserver = smtplib.smtp("smtp.gmail.com",587)
 smtpserver.ehlo()
 smtpserver.starttls()
 smtpserver.ehlo
 smtpserver.login(GMAIL_USER, GMAIL_PASS)
 header = str('To:' + TO + '\n' + 'From: ' + GMAIL_USER)
 header = header + '\n' + 'Subject:' + SUBJECT + '\n'
 print (header)
 msg - header + '\n' + TEXT + ' \n\n'
 smtpserver.sendmail(GMAIL_USER, TO, msg)
 smtpserver.close()
while True:
 message = ser.readline()
 print(message)
 if message == b'MOVEMENT\r\n' :
 send_email()
 time.sleep(0.5)

• Edgar Bonet: Use the serial console to see what the Arduino is sending. If it is sending what you expect, then it is working properly, your problem lies in your Python code, and your question does not belong to this site. If it is not sending what you expect, then remove all the Python/SMTP stuff from your question, and make it focused on the Arduino sketch.
• KIIV: The readline usually reads line ending characters too. Are you sure it's not the case?
• KIIV: And are you sure you are not comparing first character from the message with whole string 'MOVEMENT'?
• Edgar Bonet: Note that your timing test will fail when millis() rolls over. You should rather write if (now - lastSend > minSecsBetweenEmails * 1000l), which is rollover-safe.
• Dylan Shroll-Tews: Edgar, thanks for the heads-up about the millis() rollover. I guess I misunderstood the breadth of this specific StackExchange community, I was under the impression that since I'm dealing with a specific Arduino implementation it would be relevant here, but I guess not. The Uno's output is exactly what I expect, so I'm thinking it's more of a Python issue. KIIV, as to your first comment, I've tried to add the line ending characters to the string since I'm getting the correct output from the Uno, but no luck yet.

Answer 1

You're confusing readlines() with readline(). The former returns an array of lines, the latter a single line. You need to drop the [0] since you're not working with an array of lines, and you need to include the line endings that the Arduino sends:

while True:
 message = ser.readline()
 print(message.decode('utf-8'))
 if message == 'MOVEMENT\r\n' :
 send_email()
 time.sleep(0.5)

• Dylan Shroll-Tews: Thanks Majenko. You're correct on my confusion between those functions. I've attempted to add the line ending characters but have yet to fulfill the if statement, so I'm going to continue investigating where I screwed up.