DS18S20 serial output => 3.31 ?

Other Hardware Sensors
1

Hi,

I am new to this and hope somebody can point out my error!

I play around with the DS18S20 and the serial monitor shows 3.31 - when I touch the sensor it goes up 4.06 or so.

How can it display the real temp in Celsius?

Wiring: http://bildr.org/2011/07/ds18b20-arduino/

(I had no 4.7K and put 2 10K parallel, actual value is below 5K)

Code:

  #include <OneWire.h>

int DS18S20_Pin = 2; //DS18S20 Signal pin on digital 2

//Temperature chip i/o
OneWire ds(DS18S20_Pin); // on digital pin 2

void setup(void) {
 Serial.begin(9600);
}

void loop(void) {
 float temperature = getTemp();
 Serial.println(temperature);
 
 delay(100); //just here to slow down the output so it is easier to read
 
}


float getTemp(){
 //returns the temperature from one DS18S20 in DEG Celsius

 byte data[12];
 byte addr[8];

 if ( !ds.search(addr)) {
   //no more sensors on chain, reset search
   ds.reset_search();
   return -1000;
 }

 if ( OneWire::crc8( addr, 7) != addr[7]) {
   Serial.println("CRC is not valid!");
   return -1000;
 }

 if ( addr[0] != 0x10 && addr[0] != 0x28) {
   Serial.print("Device is not recognized");
   return -1000;
 }

 ds.reset();
 ds.select(addr);
 ds.write(0x44,1); // start conversion, with parasite power on at the end

 byte present = ds.reset();
 ds.select(addr);  
 ds.write(0xBE); // Read Scratchpad

 
 for (int i = 0; i < 9; i++) { // we need 9 bytes
  data[i] = ds.read();
 }
 
 ds.reset_search();
 
 byte MSB = data[1];
 byte LSB = data[0];

 float tempRead = ((MSB << 8) | LSB); //using two's compliment
 float TemperatureSum = tempRead / 16;
 
 return TemperatureSum;
 
}
2

When I change from:

float TemperatureSum = tempRead / 16;

to:

float TemperatureSum = tempRead / 2;

I get a meaningful output in 0.50ºC steps.

I think that was the error, or?

Actually there is a more accurate code on the same page, but with errors: http://bildr.org/2011/07/ds18b20-arduino/

#include <OneWire.h>

int DS18S20_Pin = 2; //DS18S20 Signal pin on digital 2

//Temperature chip i/o
OneWire ds(DS18S20_Pin); // on digital pin 2

void setup(void) {
Serial.begin(9600);
}

void loop(void) {
float temperature = getTemp();
Serial.println(temperature);

delay(100); //just here to slow down the output so it is easier to read

}


float getTemp(){
//returns the temperature from one DS18S20 in DEG Celsius

byte data[12];
byte addr[8];



if ( !ds.search(addr)) {
//no more sensors on chain, reset search
   ds.reset_search();
return -1000;
}

if ( OneWire::crc8( addr, 7) != addr[7]) {
Serial.print("CRC is not valid!\n");
return -1000;
}

if ( addr[0] != 0x10 && addr[0] != 0x28) {
Serial.print("Device is not recognized");
return -1000;
}

ds.reset();
ds.select(addr);
ds.write(0x44,1); // start conversion, with parasite power on at the end

byte present = ds.reset();
ds.select(addr);  
ds.write(0xBE); // Read Scratchpad


for (int i = 0; i < 9; i++) { // we need 9 bytes
  data[i] = ds.read();
}

ds.reset_search();

byte MSB = data[1];
byte LSB = data[0];
float CONT_REMAIN = data[6];
float CONT_PER_C = data[7];

float tempRead = ((MSB << 8) | LSB); //using two's compliment
TemperatureSum = tempRead/2-0.25+(CONT_PER_C-CONT_REMAIN)/CONT_PER_C;

return TemperatureSum;

It said: I had to change the code at the end, a bracket was left off, as was the tempSum type

I can see that one } is missing, but where to place it, and how to set the tempSum type?

So typical, people ask first before using some common sense... pathetic... This works perfectly:

float TemperatureSum = tempRead/2-0.25+(CONT_PER_C-CONT_REMAIN)/CONT_PER_C;

return TemperatureSum;
}

3

The circuit diagram you link to shows the DS18S20 wired in non-parasitic mode but your code requests the temperature in parasitic mode. If the code and the wiring aren't consistent the readings you get may not be reliable.

Pete