Arduino beginner

Erny

Okay after some more searching I found the explanation here on page 115

Arduino - Free Download eBook - pdf

Ladada also confirms it

http://learn.adafruit.com/tmp36-temperature-sensor

The datasheet here also confirms it.

const float get_temperature(){
- const int sensor_voltage = analogRead(TEMP_SENSOR_PIN);
- const float voltage = sensor_voltage * SUPPLY_VOLTAGE / 1024;
- return (voltage * 1000 - 500) / 10;

Hence their example gets a reading of 154 which works out to be 25.19

The issue with yours is that a reading of 900 seems very high.

I would suggest checking your connections, in particular the earth/ground to ensure its making it all the way to the tmp36.

Mark

that kind of helps, but this is what confused me even more, when i tried to work it out on a calculator line 6 seems fine i get a figure that looks correct then as soon as you do line 11 which is supposedly turning the voltage you have got from line 6 to degC  you get a very odd number which seems too high for degC

for example if sensorVal = 900 then its (900/1024.0) * 5.0 = 4.39453125

then line 11 says (4.39453125 - .5) * 100 = 389.453125

that seems wrong to me :/ but like i said not good with math so i could be being stupid

Oops

replying to the thread dumps it above the latest ....

Mark

Erny

Looking at this link Math for Project 3 Love-o-Meter - Arduino Forum

They have the following code.

void loop(){
  int sensorVal = analogRead(sensorPin);
  Serial.print("Sensor Value: ");
  Serial.print(sensorVal);
  //convert the ADC reading to voltage
  float voltage = (sensorVal/1024.0) * 5.0;
  Serial.print(" , Volts: ");
  Serial.print(voltage);
  Serial.print(", degrees C: ");
  //convert the voltage to temp in degrees
  float temperature = (voltage - .5) * 100;
  Serial.println(temperature);

The ADC value is 1024 steps, with 1024 being 5v and 0 being 0 v.

So at line 06 they are dividing the reading to work out the voltage

They have swapped the figures but essentially its 5/1024 * SensorVal/1

(I never was good with that .... but with 5v as the reference you have 4.88758 mV/step)

The datasheet for a tmp36 says it is 10mV/degreeC.

Hence once you know the voltage you should have the temperature.

I cannot understand why they do line 11, as the datasheet doesn't mention any offset voltage

It states that it is 10mV/degC and is 250mV at 25 degC.

There are lots of reasons why I don't use the analogue temp sensors and prefer the Dallas One wire devices.

The comments here  https://www.sparkfun.com/products/10988 support my decision.

Hope this helps.

const int sensorPin = A0;
const float baselineTemp = 20.0;

void setup(){
  Serial.begin(9600);
  for(int pinNumber = 2; pinNumber < 5; pinNumber++){
    pinMode(pinNumber, OUTPUT);
    digitalWrite(pinNumber, LOW);
  }
}

void loop(){
  int sensorVal = analogRead(sensorPin);
  Serial.print("Sensor value: ");
  Serial.print(sensorVal);
  float voltage = (sensorVal/1024.0) * 5.0;
  Serial.print(", Volts: ");
  Serial.print(voltage);
  float temperature = (voltage - .5) * 100;
  Serial.print(", degrees C: ");
  Serial.println(temperature);
  
  digitalWrite(2, LOW);
  digitalWrite(3, LOW);
  digitalWrite(4, LOW);

  if(temperature >= baselineTemp + 2){
    digitalWrite(2, HIGH);
  }
  if(temperature >= baselineTemp + 4){
    digitalWrite(3, HIGH);
  }
  if(temperature >= baselineTemp + 6){
    digitalWrite(4, HIGH);
  }
  
  delay(1);
  
}

that's the code and I don't understand these parts:  float voltage = (sensorVal/1024.0) * 5.0;  float temperature = (voltage - .5) * 100;

  i mean i know what they are there to do but i don't quite understand the maths (i hope i make sense)