Multiple LM35 sesnors

Using Arduino Sensors
1

Hi guys, bit of newby any advice would be greatly appreciated.
Im trying to run multiple LM35 sensors at once to turn on a cooling fan, but if i add one in parallel the temp becomes very inaccurate, if i add a sensor in a different port it then ignores the original. I have bought a heap of lm35 sensors and are hoping to use them in this project. code as follows.

// Declare variables

float tempC;
int tempPin = 0;
int ledPin = 13;
int fan1 = 5;
void setup ()
{
Serial.begin(9600);
pinMode(ledPin, OUTPUT);
pinMode(fan1, OUTPUT);
}

void loop()
{
tempC = analogRead(tempPin);
tempC = (5.0tempC100.0)/1024.0;
Serial.print((byte)tempC);
if (tempC >25)
{
digitalWrite(ledPin, HIGH);
digitalWrite(fan1, HIGH);
}
else
{
digitalWrite(ledPin, LOW);
digitalWrite(fan1, LOW);
}
delay(3000);

}

2

I believe that this sensor cannot works in parallel with another one. You must use another analog pin. Take care of electric noise.

3

yes i have tried 2 different sensors on different pins but it only reads of the new pin....... thanks for the tip on electrical noise.

4

Maybe this helps.
Just a hobby coder. Untested.
Leo..

float tempC1;
float tempC2;
float tempC3;
int tempPin1 = A0;
int tempPin2 = A1;
int tempPin3 = A2;
int ledPin = 13;
int fan1 = 5;

void setup ()
{
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);
  pinMode(fan1, OUTPUT);
}

void loop()
{
  tempC1 = analogRead(tempPin1);
  tempC1 = (5.0 * tempC * 100.0) / 1024.0;
  tempC2 = analogRead(tempPin2);
  tempC2 = (5.0 * tempC * 100.0) / 1024.0;
  tempC3 = analogRead(tempPin3);
  tempC3 = (5.0 * tempC * 100.0) / 1024.0;

  Serial.print(tempC1, 1); // one decimal place
  Serial.print("  ");
  Serial.print(tempC2, 1);
  Serial.print("  ");
  Serial.println(tempC3, 1);

  if (tempC1 > 25 || tempC2 > 25 || tempC2 > 25) // if 1 or 2 or 3 is >25C
  {
    digitalWrite(ledPin, HIGH);
    digitalWrite(fan1, HIGH);
  }
  else
  {
    digitalWrite(ledPin, LOW);
    digitalWrite(fan1, LOW);
  }
  delay(3000);
}
5

How are you reading the second sensor?

Your code only has one set of variables and read functions for one sensor.

6

npkamen:
How are you reading the second sensor?

Your code only has one set of variables and read functions for one sensor.

I wondered that too, but I wasn't sure if that was OP's final code. But anyway, wawa's code at #3 a couple of hours ago takes care of that issue.

7

JimboZA:
I wondered that too, but I wasn't sure if that was OP's final code. But anyway, wawa's code at #3 a couple of hours ago takes care of that issue.

Yes but the OP won't learn if they just get everything fed to them. This is really a very straightforward problem which should have an obvious solution.

8

Guilty as charged.
There is a lot more to improve though.
An LM35 is a poor performer with simple code like this.
One degree C accuracy/stability at best.
Reading the sensors with 1.1volt Aref enabled, multiple reads, and/or some form of smoothing and calibration is needed to get accuracy and stability <= 0.1 degree C.
Leo..

9

Putting a 1k resistor between LM35 output and analog in pin + 0.1uF between analog in and GND, I was able to get 0.1 degree resolution and good stability with this sketch, maybe I'm lucky or its a fluke, but it works great. I don't have a second LM35 to try a multi sensor version.

// LM35 thermometer, no floats, no delays


byte numSamples = 8;

int8_t fudge = 90; // adjust for calibration 

int kAref = 1090, // analog ref voltage * 1000
    kNumSamples = numSamples * 1000,
    fin = 2000,
    tempC,
    tempF;
    
uint32_t temp,
         start;

void setup()
{
  Serial.begin(9600);
  analogReference(INTERNAL);
  analogRead(0);
  for(int i = 0;i < numSamples;i++)
    temp += analogRead(0);
}
void loop()
{
  if(millis() - start > fin){
    start = millis();
    temp -= (temp / numSamples);
    temp += analogRead(0);
    tempC = temp * kAref / (kNumSamples + fudge);
    tempF = (tempC * 18 + 3200) / 10;
    Serial.print(analogRead(0));
    Serial.print("\t");
    Serial.print(temp);
    Serial.print("\t");
    prntTempC();
    prntTempF();
    Serial.println();
  }
}
    
void prntTempC(){
  Serial.print(tempC / 10);
  Serial.print(".");
  Serial.print(tempC % 10);
  Serial.print("\t");
}
void prntTempF(){  
  Serial.print(tempF / 10);
  Serial.print(".");
  Serial.print(tempF % 10);
  Serial.print("\t");
}
10

Make sure you understand the difference between "precision" and "resolution" - "0.1 degree resolution and good stability" does not mean it was accurate to 0.1 degree - for that you have to work the calibration issue. Good stability and resolution is a good start, but it needs to be calibrated for what you are asking.

11

wow, thanks Wawa, ill try that. As far as reading the other sensors i added another pin for each senor and another tempc. Yes i am very much a newbie, and have recently re-kindled my love electronics and cant wait to get a handle on code writing. Thanks all for you inputs.