AHT20 with ESP32

Specifications

Power and logic levels

3.3 V: safe to connect directly

ESP32 pins, including those on a display board's connectors, work at 3.3 V and are not 5 V tolerant. Modules differ, so if in doubt, measure the signal before you connect it.

Wiring

Arduino code

#include <Wire.h>

const int SDA_PIN = 8, SCL_PIN = 9;
const uint8_t ADDR = 0x38;

void command(uint8_t a, uint8_t b, uint8_t c) {
  Wire.beginTransmission(ADDR);
  Wire.write(a); Wire.write(b); Wire.write(c);
  Wire.endTransmission();
}

uint8_t crc8(const uint8_t *b, int n) {   // polynomial 0x31, start 0xFF
  uint8_t c = 0xFF;
  while (n--) {
    c ^= *b++;
    for (int i = 0; i < 8; i++) c = (c & 0x80) ? (c << 1) ^ 0x31 : c << 1;
  }
  return c;
}

void setup() {
  Serial.begin(115200);
  Wire.begin(SDA_PIN, SCL_PIN);
  delay(40);
  command(0xBE, 0x08, 0x00);              // calibrate
  delay(10);
}

void loop() {
  command(0xAC, 0x33, 0x00);              // start measuring
  delay(80);
  uint8_t b[7];
  Wire.requestFrom(ADDR, (uint8_t)7);
  for (int i = 0; i < 7; i++) b[i] = Wire.read();
  if ((b[0] & 0x80) || crc8(b, 6) != b[6]) {   // busy or damaged
    Serial.println("No valid reading");
  } else {
    uint32_t rh = ((uint32_t)b[1] << 12) | (b[2] << 4) | (b[3] >> 4);
    uint32_t t = ((uint32_t)(b[3] & 0x0F) << 16) | (b[4] << 8) | b[5];
    Serial.printf("%.2f C  %.1f %%\n", t * 200.0 / 1048576 - 50, rh * 100.0 / 1048576);
  }
  delay(2000);
}

No extra library needed. Compiled for ESP32-S3 with arduino-esp32 3.x. Not yet tested by us on a physical sensor. Change the pins to match your board.

On a display

Read the value in your sketch and pass it to a label, or send it over MQTT and show it with an MQTT value in the editor.

Boards with a ready connector

Only boards whose connectors we have surveyed. Others may still work on free GPIOs.

Good to know

• The address 0x38 cannot be changed.
• Accuracy is looser away from room temperature.
• Keep it away from warm parts.
• Avoid condensation.

What it is good for

• Room climate displays
• Replacing a DHT22
• Sharing an I2C connector

FAQ

Sources

• https://www.mouser.se/new/seeed-studio/seeed-grove-aht20-i2c-sensor
• https://st.robu.in/product/smartelex-aht20-temperature-humidity-sensor

Related parts

• DHT22 / AM2302
• SHT31 / SHT3x