Interfacing LM19 temperature sensor with Arduino

Question

community! I have a problem as follows. I've been trying to interface the LM19 temperature sensor using Arduino. I made a function to read and compute the temperature from the sensor according to a formula described in its datasheet. The result was pretty good. However, when I added another function for reading ADC value from an LDR, the result of the temperature computation went crazy!!! Here's my code:

/*
 Author: M. Chenda
 Created on Tue Feb 18, 2020
*/

#define ldr 0       // assign LDR's pin to analog pin A0
#define light 8     // assign LED's pin to digital pin 8
#define lm19 2      // assign LM19's pin to analog pin A2

int ldr_val = 0;    // LDR's ADC reading variable
String state;       // string representing light's state

int lm19_val = 0;   // LM19's ADC reading variable
float Vo = 0.00;    // V output of LM19
float temp = 0.00;  // temperature variable

void setup() {
    pinMode(light, OUTPUT);
    Serial.begin(9600);
    while(!Serial) {;}  // wait for serial port connection
}

void loop() {
    lighting();
    temp_control();
    delay(500);
}

/*  LIGHT CONTROL   */
void lighting() {
    ldr_val = analogRead(ldr);  // read LDR's ADC value (brightness)
    if (ldr_val < 512) {
        digitalWrite(light, LOW);
        state = "off";
    }
    else {
        digitalWrite(light, HIGH);
        state = "on";
    }
    Serial.print("LDR's ADC:  ");
    Serial.println(ldr_val);
}

/*  VALUE MAPPING FUNCTION  */
float mapping(float x, float in_min, float in_max, float out_min, float out_max) {
    return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}

/*  TEMPERATURE MEASUREMENT */
void temp_control() {
    // read LM19's ADC value
    lm19_val = analogRead(lm19);
    // convert the ADC value to voltage (0 to 5V)
    Vo = mapping(lm19_val, 0, 1023, 0, 5);
    // compute temperature value in Celsius
    temp = -1481.96 + sqrt((2.1962*pow(10,6))+((1.8639-Vo)/(3.88*pow(10,-6))));
    Serial.print("      Temperature (Celsius):  ");
    Serial.println(temp);   // print the measured temperature to Serial Terminal
}

What could be the root of this problem?

Answer 1

When you change channels the input section of the ADC requires a bit of time (depending on the source impedance and the difference between the previous and new voltages) for the reading to settle to the new value. Adding a delay or discarding the first reading of the new channel can mitigate that effect.

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Edit: To be useful, the delay would have to be inserted after setting the multiplexer but before initiating the ADC read, so inside the analogRead function. If you look at the source code, there is a commented-out delay.

Maybe this is a FAQ in the Arduino community.

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Don't forget to pay attention to the capacitance loading notes on the temperature sensor if you extend the least, they are not tolerant of capactive loading beyond a couple hundred pF and can oscillate or ring.

Answer 2

The ADC has a built in capacitor at it's input. So, you should allow the capacitor to charge slowly to the actual value. If there is high resistance (LDR), it will take more time to charge.
Since, you are switching the ADC to different channels, you should give sufficient time for the ADC sampling capacitor to charge.

Provide sufficient delay after choosing the channel - OR read the ADC several times, and consider the last couple of values once the values are stable. You can read once, give a delay, and do a second read.

Answer 3

A minor thing but I would suggest changing

#define ldr 0       // assign LDR's pin to analog pin A0
#define light 8     // assign LED's pin to digital pin 8
#define lm19 2      // assign LM19's pin to analog pin A2

to

#define ldr A0       // assign LDR's pin to analog pin A0
#define light 8     // assign LED's pin to digital pin 8
#define lm19 A2      // assign LM19's pin to analog pin A2

I don't expect it will solve your issues (and I can't see any issue with your code) but it is good practice to use the channel numbers.

Further reading on the subject: analogRead(0) or analogRead(A0)