NodeMCU with DHT22 and Blynk reading 255 every 20 seconds

Need Help With My Project
1

Hi all,

this evening I encountered something I couldn’t fix, and now that I’ve fixed it, I don’t understand the problem.

The code below is a very simple integration of a DHT22 sensor on a NodeMCU writing a humidity and a temperature value to Blynk every 5 seconds.

For visual feedback and debugging I temporarily added a physical LED to the NodeMCU. At first I set the interval to 2 seconds, and I noticed that about every 20 seconds there were three spikes in the reading for both temp and humidity: 255 (the temp and hum were read as byte), normal, 255, normal, 255, normal, and then normal for 20 seconds, three spikes, normal for 20 sec, etc.

I then changed the interval to 5 seconds, which led to a single spike in the reading, still about every 20sec.

I changed the type for the readings from byte, to int (reading 2,147,483,647), to float (reading NaN), which changed the values but not the pattern of a misreading every 20 seconds.

Then I commented out all the stuff relating to the LED code, and that solved the problem.

But why?

Edit: after I added some code for a soil moisture sensor the problem returned, and it remained after I commented out the code that I added.

#define BLYNK_PRINT Serial

// Include DHT22 sensor library
#include <DHT.h>
#include <DHT_U.h>

// Include Blynk libraries
#include <ESP8266WiFi.h>
#include <BlynkSimpleEsp8266.h>

// Get Auth Token in the Blynk App Project Settings
char auth[] = "*removed*";

// Your WiFi credentials.
// Set password to "" for open networks.
char ssid[] = "*removed*";
char pass[] = "*removed*";

//Define variables
DHT dhtA(5, DHT22);
BlynkTimer timer;
//int ledState = LOW;
//int lastLedState = HIGH;

void setup()
{
  // Debug console
  Serial.begin(9600);

  Blynk.begin(auth, ssid, pass);
  dhtA.begin();

  timer.setInterval(5000L, climateRoutine);

  //pinMode(4, OUTPUT);

}

void loop()
{
  Blynk.run();
  timer.run();
}



void climateRoutine() {
  float h1 = dhtA.readHumidity();
  float t1 = dhtA.readTemperature();
  Blynk.virtualWrite(V0, h1);
  Blynk.virtualWrite(V1, t1);

//  //Write an LED every time data is sent, remove in final build
//  ledState = digitalRead(4);
//  if (ledState != lastLedState) {
//    if (ledState == HIGH) {
//      digitalWrite (4, LOW);
//      lastLedState = HIGH;
//    } else {
//      digitalWrite (4, HIGH);
//      lastLedState = LOW;
//    }
//  }
}
2

Edit: apparently for some reason the fix stopped working, every 20 seconds the reading drops out to NaN for the DHT22 readings.

3

The DHT sensors are very poor, and don’t like being read too frequently. You also need to catch the NaN errors, as is done in the example here:

https://examples.blynk.cc/?board=ESP8266&shield=ESP8266%20WiFi&example=More%2FDHT11

Also, why do you have two DHT libraries?
I have read that some versions of the DHT libraries are better than others at giving good results.

Pete.

1 Like
4

Hi Pete,

thanks for the reply, I added the part about catching the NaN readings. A supplementary question though: that only works for float’s right? With int and byte it does give a reading, but that reading is off the chart. So how to fix it for other types?

About the libraries: no idea. When I add the DHT sensor library by Adafruit they both get added to the sketch.

5

You can filter on the hardware with t > -100 and t < 100 or filter in the graph settings.