Sensor not working: Blynk in combination with DHT21 (Si7021 by Itead Sonoff TH16)


I’m trying to get the Sonoff TH16 to work with Blynk, but the sensor is not functioning. See below.

//#include <ESP8266mDNS.h>
#include <WiFiUdp.h>
//#include <ArduinoOTA.h>
#include <ESP8266WiFi.h>
#include <DHT.h>                // DHT thermocouple/ humidity sensor
#include <BlynkSimpleEsp8266.h>

// Pins for blynk
#define VPIN V1               // For Relay
#define VPIN_T V2             // For temperature data
#define VPIN_H V3             // For humidity data
#define VPIN_HIC V4             // For temperature in farenheit
#define VPIN_tag V20          // For all relays in a tag
//DHT setup
#define DHTPIN 14   // what digital pin we're connected to
#define DHTTYPE DHT21   // DHT 21 (AM2301) SI7021 itead with change in DHT.ccp
// On/off callbacks, PINS
boolean verwarmingState = 0;
boolean switchReset = true;   // Flag indicating that the hardware button has been released
const int buttonPin = 0;      // Pin for hardware momentary switch. On when grounded
const int relayPin =  12;// Relay switching pin. Relay is pin 12 xxGPIO5 is D1 the number of the Relay pin on the NodeMCU (this one chosen for testing)
const int ledPin =  13;// On / Off indicator LED. Onboard LED is 13 on Sonoffthe number of the LED pin (D4 op NodeMCU)
// variables will change
int buttonState = LOW;          // variable for reading the pushbutton status
int lastButtonState = HIGH;     // previous state of the button
int relayState = LOW;
int relayStatus = 0;
int ledState = LOW;
int volts = 0;
int valueForBridge;
long lastTime = 0;              // the last time the output pin was toggled
long debounce = 200;            // the debounce time, increase if the output flickers
int requestedTemp = 0;
// Define DHT, Blynk timer
BlynkTimer timer;             // Blynk timer
DHT dht(DHTPIN, DHTTYPE);     // Define DHT
// Your WiFi credentials.
// Set password to "" for open networks.
const char* ssid     = "xx";
const char* password = "xxx";
char auth[] = "xxxx";

WidgetLED led1(V8); // Widget Relay LED

BLYNK_WRITE(V1) { // Widget relay button
  relayStatus = param.asInt();
  if (relayStatus == 1) {
    relayState = HIGH;
  else {
    relayState = LOW;
} // end - BLYNK_WRITE(V1)

void pushButton() {
  buttonState = digitalRead(buttonPin);
  if (buttonState == HIGH && lastButtonState == LOW && millis() - lastTime > debounce) {
    if (relayState == HIGH) {
      relayState = LOW;
      Blynk.virtualWrite(V1, 0);
    else {
      relayState = HIGH;
      Blynk.virtualWrite(V1, 1);
    lastTime = millis();
  digitalWrite(relayPin, relayState); // Relay open/close - digital pin 12

  lastButtonState = buttonState;
} //end - pushButton()

void ledStatus() { // Widget LED - relay status
  ledState = digitalRead(relayPin);
  if (ledState == HIGH)
  requestedTemp = param.asInt();
  Serial.print("requested Temp: ");
void checkTemp() {
  // Wait a few seconds between measurements.
  float h = dht.readHumidity();
  // Read temperature as Celsius (the default)
  float t = dht.readTemperature();
  // Read temperature as Fahrenheit (isFahrenheit = true)
  float f = dht.readTemperature(true);
  // Check if any reads failed and exit early (to try again).
  if (isnan(h) || isnan(t) || isnan(f)) {
    Serial.println("Failed to read from DHT sensor!");
  // Compute heat index in Celsius (isFahreheit = false)
  float hic = dht.computeHeatIndex(t, h, false);

  Serial.print("Humidity: ");
  Serial.print(" %\t");
  Serial.print("Temperature: ");
  Serial.print(" *C ");
  Serial.print(" *F\t");
  Serial.print("Heat index: ");
  Serial.println(" *C ");
  Blynk.virtualWrite(VPIN_T, t);
  Blynk.virtualWrite(VPIN_H, h);
  Blynk.virtualWrite(VPIN_HIC, hic);
void setup() {
  Serial.println("DHT21 (si7021 itead) test!");

  Blynk.begin(auth, ssid, password);
  while (Blynk.connect() == false) {
    Serial.println("Connection Failed! Rebooting...");
  pinMode(buttonPin, INPUT);  // on/off button
  pinMode(relayPin, OUTPUT);  // relay
  pinMode(ledPin, OUTPUT);    // led
  digitalWrite(ledPin, LOW);  // always on
  //timer.setInterval(1000, ButtonCheck);
  timer.setInterval(2000, checkTemp);
  // DHT beginning
void loop() {;

I’m trying to get a Si7021 sensor from Itead(Sonoff) to get to work in Blynk. The sensor works fine as long as I don’t include Blynk connect code in the sketch.
Hardware: Sonoff TH16 (ESP8266EX) upload via USB to Serial FTDI232
Blynk library 0.5.4 in Arduino IDE
DHT library modified (in DHT.cpp replaced delay(20); with delayMicroseconds(500); //time to set dataline low)

I tested many variations and found out that:

  • When flashed without Blynk.begin(auth, ssid, password); sensor works fine
  • When flashed with Blynk.begin(auth, ssid, password); sensor stops, no more readings possible
  • When this occurs: re-flashing sketch without Blynk (“Tools/Erase flash: Sketch Only”) doesn’t get sensor to work.
  • Re-flashing (sketch without Blynk) with “Tools/Erase flash: All Flash Contents” sensor starts working again.

I’ve tried two different Si7021 sensors, same result.
Both sensors work perfectly with the original Sonoff firmware.

Could there be a conflict in the flash memory (???). Any suggestions for a solution?


In the first place I’d delete the delay(2000); from checkTemp().


@IBK, OK, taken out of the checkTemp() function. Did not change the problematic behaviour.


Update: I could get it to work with a different DHT library: TroykaDHT.h (Thanks Igor Dementiev!)

To use this library with the Si7021 sensor from Itead-Sonoff one has to change the TroykaDHT.cpp file as follows:
line 34 delay(18); to be replaced by delayMicroseconds(500);