(Solved)DHT21 wrong value

Using Arduino Sensors
1

I was testing DHT21 sensor in Uno for a project, which printed weird value so i changed.
With no success so i went to another. Again, no difference.
I do not want to believe that we have 4 sensors, and all of them is faulty,
while printing exact same wrong values.

This is my wiring, no resistors as I was told 21 comes internal.

20240809_171815
20240809_1718151920×2560 315 KB

#include <dht.h>
dht DHT;

#define DHT21_PIN 4

float hum;
float temp;

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

void loop() {
  int chk = DHT.read21(DHT21_PIN);

  hum = DHT.humidity;
  temp = DHT.temperature;
  Serial.print("Humidity: ");
  Serial.print(hum);
  Serial.print(" %, Temp: ");
  Serial.print(temp);
  Serial.println(" Celsius");
  delay(2000);
}

I used DHTlib from Mr.Tillaart, also tried DHTsensorlibrary of Adafruit,
which seems flawlessly working with others,
and this is what I get.

17:12:34.772 -> Humidity: 49.90 %, Temp: -101.90 Celsius
17:12:36.776 -> Humidity: 49.90 %, Temp: -101.90 Celsius
17:12:38.803 -> Humidity: 49.90 %, Temp: -102.00 Celsius
17:12:40.817 -> Humidity: 49.90 %, Temp: -102.00 Celsius
17:12:42.850 -> Humidity: 49.90 %, Temp: -101.90 Celsius
17:12:44.846 -> Humidity: 49.90 %, Temp: -101.90 Celsius

What's with my sensor and what should i try next?

2

The examples show a call to DHT.begin() in setup.

3

It might be a wiring problem. Try connecting the DHT21 cables directly to the Arduino board.

Also,I suggest trying with the Adafruit library.

4

Tried and neither switching to Adafruit or direct plugging worked.
Also, should DHT.begin moved to somewhere else then?
New code, an Adafruit example trimmed of captions.

#include "DHT.h"

#define DHTPIN 2


#define DHTTYPE DHT21  // DHT 21 (AM2301)

DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(9600);
  Serial.println(F("DHTxx test!"));

  dht.begin();
}

void loop() {
  delay(2000);

  float h = dht.readHumidity();
  float t = dht.readTemperature();

  if (isnan(h) || isnan(t)) {
    Serial.println(F("Failed to read from DHT sensor!"));
    return;
  }


  Serial.print(F("Humidity: "));
  Serial.print(h);
  Serial.print(F("%  Temperature: "));
  Serial.print(t);
  Serial.println(F("°C "));
}


11:51:23.239 -> DHTxx test!
11:51:25.262 -> Humidity: 1.00%  Temperature: 1.00°C 
11:51:27.277 -> Humidity: 49.90%  Temperature: -3273.80°C 
11:51:29.267 -> Humidity: 49.90%  Temperature: -3273.80°C 
11:51:31.325 -> Humidity: 49.90%  Temperature: -3273.80°C 
11:51:33.325 -> Humidity: 49.90%  Temperature: -3273.80°C
5

Try extending the delay in loop to 3 seconds as a test. The example states "wait a few seconds".

6

I have two different datasheets for that sensor and both show a typical circuit with an external pull-up.
Try a 4.7K

7

I extended the delay to 3 and 5 secs to no joy.

16:56:44.190 -> Humidity: 49.90%  Temperature: -3276.10°C 
16:56:47.215 -> Humidity: 49.90%  Temperature: -3276.10°C 
16:56:50.216 -> Humidity: 49.90%  Temperature: -3276.10°C 

16:58:22.158 -> Humidity: 49.90%  Temperature: -3276.20°C 
16:58:27.178 -> Humidity: 49.90%  Temperature: -3276.20°C

Pull-up needs some time as i have none in possession, but i have 10k coming up in Monday so maybe that'll be enough.

8

You could try using the internal pullup resistor, just add the following to setup:

pinMode(DHTPIN, INPUT_PULLUP);
9

Still no joy, I'm starting to think if i got a wrong batch.

#include "DHT.h"

#define DHTPIN 7     
#define DHTTYPE DHT21   

DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(9600);
  Serial.println(F("DHTxx test!"));

  dht.begin();
  
  pinMode(DHTPIN, INPUT_PULLUP);
}

void loop() {
  delay(2000);

  float h = dht.readHumidity();
  float t = dht.readTemperature();

  if (isnan(h) || isnan(t)) {
    Serial.println(F("Failed to read from DHT sensor!"));
    return;
  }


  Serial.print(F("Humidity: "));
  Serial.print(h);
  Serial.print(F("%  Temperature: "));
  Serial.print(t);
  Serial.println(F("°C "));
}

09:47:58.804 -> DHTxx test!
09:48:00.852 -> Humidity: 49.90%  Temperature: -3274.60°C 
09:48:02.852 -> Humidity: 49.90%  Temperature: -3274.60°C 
09:48:04.925 -> Humidity: 49.90%  Temperature: -3274.60°C

Only the External pullup are left to try (when it arrives).

10

Update: tried heating it, jumped straight above 0 deg celcius till 37 deg or so.
Still no change on humidity.
Ordering another sensor incase it really was bad batch.

11

Final update: Indeed it was bad batch, code is now working as intended.
I appriciate everyone who answered this clueless question.