HTU21D | GUILCOR SENSORS

HTU21D
Temperature sensors

Digital temperature and humidity sensor offering reliable performance for HVAC and consumer electronics.

Measurement range
-40 / 125°C

Temperature accuracy
+/- 0,3°K

Humidity accuracy
+/- 2,0 % Rh

Resolution (Rh and T°)
12 and 14 bits

Power supply voltage
1,4 → 3,6V

Conversion time
5ms / 85ms

Typical current
500µA(active)

Price
Low

What is a HTU21D sensor ?

The HTU21D is a digital relative humidity and temperature sensor with a calibrated and compensated I²C output.

It is often compared to the SHT21 from Sensirion, with which it shares a nearly identical design and protocol.

It is a reliable, stable, and cost-effective sensor, ideal for IoT, weather, home automation, or HVAC applications.

Operating principle

The sensor combines:

a polymer capacitive cell sensitive to humidity,
an integrated temperature sensor (for RH compensation),
a 14-bit ADC converter, and a calibration EEPROM.

The measurements are compensated and converted into physical values from the raw data using standardized formulas:

RH(%) = −6 + 125 × (S_RH / 65536)

T(°C) = −46,85 + 175,72 × (S_T / 65536)

Temperature / Humidity Curve (examples)

Raw measurement (hex)	Temperature (°C)	Humidity (%)
0x0000	−46,8	0,0
0x8000	+41,0	62,5
0xFFFF	+128,9	119,0

💡 The formulas are linear and factory calibrated.

Principle diagram (I²C connection)

+3.3V │ [HTU21D] │ SDA ─────────┐ │ SCL ─────────┤── [I²C Microcontroller │ GND

💡 Fixed I²C address: 0x40.

Application areas

🌦️ Weather stations and IoT sensors

🏠 Home automation and HVAC

🧠 Low-power embedded applications

🧪 Surface temperature/humidity measurement

🕹️ DIY environmental sensors (Arduino, ESP32, Raspberry Pi)

Should I choose a HTU21D sensor ?

Strengths points

💰 Excellent value for money
→ Precise, stable, and very affordable — ideal for IoT projects or serial sensors.
🧠 Factory calibrated and compensated→ No software calibration required.
⚙️ Simple interface
→ Standard I²C protocol, compatible with the majority of microcontrollers.
📉 Low energy consumption
→ Less than 1 µA in standby, perfect for battery power.

Weaknesses points

🌬️ Sensitive to condensation and dust
→ Not protected by a hydrophobic coating (provide a filter if needed).
📡 One single I²C address
→ It is impossible to connect multiple without a multiplexer.
🌡️ Lower accuracy than modern sensors (SHT3x, MCP9808)
→ Very good for general measurements, but not for fine metrology.

Useful information

Here is some useful information regarding the HTU21D sensors.

Main I²C commandes

Command (hex)	Function
0xE3	Température measurement (hold master)
0xF3	Température measurement (no hold)
0xE5	Humidity measurement (hold master)
0xF5	Humidity measurement (no hold)
0xE6	Write user register
0xE7	Read user register
0xFE	Reset logiciel

I²C Reading Example (pseudo-code)

// Temperature lecture
i2c_start();
i2c_write(0x40 << 1);
i2c_write(0xE3);
delay(85);
i2c_start();
i2c_write((0x40 << 1) | 1);
MSB = i2c_read_ack();
LSB = i2c_read_ack();
CRC = i2c_read_nack();
i2c_stop();

S_T = (MSB << 8) | LSB;
temp = -46.85 + 175.72 * (S_T / 65536.0);

✅ Exemple :

S_T = 35 000 → T = 46,8 °C

S_RH = 33 000 → RH = 56,0 %

Typical integration diagram

3.3V ───────────┐ │ [HTU21D] │ SDA ───────┐ │ SCL ───────┤── MCU (Arduino, ESP32, STM32…) │ GND

💡 Provide pull-up resistors of 10 kΩ on SDA/SCL if not included on the module.

User register bits

Bit	Name	Description
7	Resolution	0: RH 12b / T 14b ; 1: RH 8b / T 12b
2	Heater	1 = ON (deactivates humidity)
1	Battery	0 = OK ; 1 = low battery
0	Reserved	—