Arduino sends different numbers over serial to Raspberry Pi than Linux/Mac

Question

I've set up an arduino uno to monitor room temperature and communicate this to my Raspberry Pi (old model B) over USB (2.0 cable) using pySerial to read the data on the raspberry pi side.

The code I am using on the arduino [see Update 1] is:

LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

void setup() {
  Serial.begin(9600);
  pinMode(13,OUTPUT);
}

void loop() {
  int valT = analogRead(0);
  float mV = (valT / 1024.0) * 5000;
  float temperature = (mV - 500)  / 10;
  lcd.setCursor(0, 1);
  lcd.print(temperature);
  Serial.println(temperature);

  delay(5000); 
} 

as per example tutorial. On the raspberry pi side I am reading the data in the interpreter using the following commands:

import serial
ser = serial.Serial('/dev/ttyACM0',9600)
while : 1
  ser.readline()

When I run this on the pi I receive values of around 25/26 degrees:

26.17

26.66

26.66

26.66

26.66

which seems a bit high. When I connect it to linux/mac desktop I receive temperatures of 19/20 degrees which seems correct:

19.34

19.82

19.82

19.34

20.31

I don't understand why the pi is printing numbers that are substantially higher. I've recompiled the arduino code on the pi and repeated the experiments and I get the same result. I've sent static numbers over the serial connection e.g. (Serial.println(13.77)) and I can read that accurately on the pi.

Any ideas? Am I missing something obvious?

UPDATE 1

I think it is some way related to the USB port of the raspberry pi. I'm not moving the board so I'm pretty sure its the same temperature. I installed an LCD screen on the board to output the numbers without going via the serial port. When I plug it into the pi I get 28.61 and my linux desktop it displays 20.31.

UPDATE 2

I tried with a new (latest) Model B raspberry pi and it outputs the same temperature as the Mac/Linux desktop. Its something to do with the old raspberry pi.

Answer 1

I had the same issue with the old Model B raspberry Pi, but also with the new Raspberry Pi, and in my case the problem was solved by powering the Arduino independently through the power connector rather than relying on the USB for power.

In my case I had the Arduino connecting alternately to my Mac/Linux laptop and to the Pi. The output values on the laptop were slightly low (at first I thought they were correct, but once I resolved the problem & compared the values to a thermometer I realised they were a little low). Weirdly, when connecting by USB to the Pi, the values were way too high (as the OP found). So it was a voltage problem.

Answer 2

Given that the Arduino code operates independently from whatever system is connected to it the most probable cause is, that you are moving the Arduino/breadboard between locations. The system you are using to read the serial port can not be at fault. Provided, of course, both use the same baudrate settings etc. Are you using the Arduino IDE serial-port interface? Or a commandline tool like cu or picocom?

This is very likely an environmental problem.

Probably the Arduino is standing in a sun-lit and/or poorly ventilated area when near the Pi and in the shadows when near the desktop computer.