ESP8266 Community Forum

- Wed Jan 06, 2016 4:39 pm #38080 Hi, i use an ESP8266-12 with DHT22 and the adafruit DHT-Lib. Also I'm using the Test-INO with Arduino 1.65. i have connected DHT PIN1 -> ESP-VCC, DHT PIN2 -> ESP-GPIO02, GRD-> GRD and DHT-PIN1 -> DHT-PIN2 with 10k resistor. I activated the Debugging Mode in DHT.h. If i start the module, i get the following error msg:

Failed to read from DHT sensor!
Timeout waiting for start signal high pulse.

Can anybody help me?

Here is my coding:
// Example testing sketch for various DHT humidity/temperature sensors
// Written by ladyada, public domain

#include "DHT.h"

#define DHTPIN 2 // what digital pin we're connected to

// Uncomment whatever type you're using!
//#define DHTTYPE DHT11 // DHT 11
#define DHTTYPE DHT22 // DHT 22 (AM2302), AM2321
//#define DHTTYPE DHT21 // DHT 21 (AM2301)

// Connect pin 1 (on the left) of the sensor to +5V
// NOTE: If using a board with 3.3V logic like an Arduino Due connect pin 1
// to 3.3V instead of 5V!
// Connect pin 2 of the sensor to whatever your DHTPIN is
// Connect pin 4 (on the right) of the sensor to GROUND
// Connect a 10K resistor from pin 2 (data) to pin 1 (power) of the sensor

// Initialize DHT sensor.
// Note that older versions of this library took an optional third parameter to
// tweak the timings for faster processors. This parameter is no longer needed
// as the current DHT reading algorithm adjusts itself to work on faster procs.
DHT dht(DHTPIN, DHTTYPE);

void setup() {
Serial.begin(115200);
delay(20000);
Serial.println("DHTxx begin:!");
dht.begin();
}

void loop() {
// Wait a few seconds between measurements.
delay(3000);

// Reading temperature or humidity takes about 250 milliseconds!
// Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
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!");
return;
}

// Compute heat index in Fahrenheit (the default)
float hif = dht.computeHeatIndex(f, h);
// Compute heat index in Celsius (isFahreheit = false)
float hic = dht.computeHeatIndex(t, h, false);

Serial.print("Humidity: ");
Serial.print(h);
Serial.print(" %\t");
Serial.print("Temperature: ");
Serial.print(t);
Serial.print(" *C ");
Serial.print(f);
Serial.print(" *F\t");
Serial.print("Heat index: ");
Serial.print(hic);
Serial.print(" *C ");
Serial.print(hif);
Serial.println(" *F");
}

- Sat Jan 09, 2016 5:59 pm #38366 Hi, The sketch works fine for me with a DHT11 or DHT22 connected to a ESP8266. The pin numbers for the DHT-22 are viewed from the front side of the device. This is the side with the multiple opens holes showing. So pin 1 would be on your left side as viewed from the front. Also, when using the DHT22 with the ESP8266, the voltage on pin 1 of the DHT22 must not be greater than 3.3 Volts or you will damage the ESP8266 GPIO PIN 2. I suggest you check your wiring.

Vince

- Thu Jan 14, 2016 4:59 pm #38799 Hi!
I had the same problem.
I think the power from the usb port is not enough for both esp and dht-22 (dht-11works fine while connected to usb power).
I solved the problem by connecting the DHT-22 to the external power source .
Now it works like a charm.

- Wed Jan 20, 2016 7:50 am #39291

Hi!
I had the same problem.
I think the power from the usb port is not enough for both esp and dht-22 (dht-11works fine while connected to usb power).
I solved the problem by connecting the DHT-22 to the external power source .
Now it works like a charm.

I don't have any problem in powering the NodeMCU and DHT22 from my computers USB.