Overview

The AHT20 (and its predecessor AHT10) is a digital MEMS temperature and humidity sensor manufactured by ASAIR (Aosong Electronics). Equipped with a custom ASIC signal processing chip, a capacitive MEMS humidity sensing element, and an on-chip temperature sensor, the AHT20 delivers factory-calibrated digital measurements over an $I^2C$ bus.

Providing higher accuracy ($\pm 0.3^\circ\text{C}$ temperature, $\pm 2\%\text{ RH}$ relative humidity), lower power consumption, and standard SMD packaging at a low cost, the AHT20 has emerged as a popular replacement for older legacy sensors like the DHT11 and DHT22 in indoor air-quality monitors, smart thermostats, and ESPHome weather stations.

Quick reference

Pinout

Breakout modules feature a standard 4-pin 0.1" (2.54 mm) header or Qwiic / STEMMA QT connector:

Specifications

$I^2C$ Communication & Protocol

The AHT20 uses a fixed 7-bit $I^2C$ address of 0x38.

1. Initialization: Upon power-up ($V_{CC} > 2.0\text{V}$), wait 40 ms. Read status byte. If bit 3 (0x08) is 0, send initialization command 0xBE 0x08 0x00 and wait 10 ms.
2. Trigger Measurement: Send 3-byte command sequence 0xAC 0x33 0x00.
3. Read Data: Wait 80 ms for measurement completion. Read 7 bytes from 0x38:
   • Byte 0: Status byte (bit 7 = Busy flag: 0 ready, 1 measuring).
   • Bytes 1–3: 20-bit Raw Humidity data ($S_{RH}$).
   • Bytes 3–5: 20-bit Raw Temperature data ($S_{T}$).
   • Byte 6: CRC-8 checksum.

Conversion Formulas

$\text{Relative Humidity (\% RH)} = \left( \frac{S_{RH}}{2^{20}} \right) \times 100\%$

$\text{Temperature } (^\circ\text{C}) = \left( \frac{S_{T}}{2^{20}} \right) \times 200 - 50$

Wiring

Example

#include <Wire.h>
#include <Adafruit_AHTX0.h>

Adafruit_AHTX0 aht;

void setup() {
  Serial.begin(9600);
  while (!Serial) delay(10);

  Serial.println("Initializing AHT20 Sensor...");
  if (!aht.begin()) {
    Serial.println("Could not find AHT20 sensor! Check wiring.");
    while (1) delay(10);
  }
  Serial.println("AHT20 initialized successfully.");
}

void loop() {
  sensors_event_t humidity, temp;
  aht.getEvent(&humidity, &temp);

  Serial.print("Temperature: ");
  Serial.print(temp.temperature);
  Serial.print(" °C | Humidity: ");
  Serial.print(humidity.relative_humidity);
  Serial.println(" %RH");

  delay(2000);
}

Common mistakes

• Not waiting 80 ms after measurement trigger: Reading bytes before the busy bit (Status Bit 7) drops to 0 returns stale or zeroed 20-bit sensor payloads.
• Conflating AHT10 and AHT20 initialization commands: AHT10 uses 0xE1 0x08 0x00 while AHT20 requires 0xBE 0x08 0x00. Modern libraries auto-detect and send the correct byte sequence.
• Condensation on MEMS element: Exposure to liquid water drops shifts capacitive readings temporarily. Allow the sensor 24 hours in dry ambient air to recover.

Notes

• AHT20 vs SHT31: AHT20 offers comparable accuracy to Sensirion's SHT30/SHT31 at a lower unit cost, making it ideal for high-volume IoT nodes.