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SHT4x temperature and humidity sensor

Sensirion's tiny I²C sensor for temperature (±0.2 °C typ.) and relative humidity (±1.8 % RH typ.). Needs only 100 nF and the I²C pull-ups.

What it does#

The SHT40 measures temperature and relative humidity and answers over I²C. Supply 1.08–3.6 V, so it runs straight from 3V3 or a LiPo-fed 3.3 V rail. The catalog’s SHT40-AD1B answers at address 0x44; other variants use 0x45 or 0x46.

Example circuit: U1 on 3V3 with C1 (100 nF), SDA/SCL with 4.7 kΩ pull-ups, and a Qwiic connector so the bus can go on to other sensors.
U1 on 3V3 with C1 (100 nF), SDA/SCL with 4.7 kΩ pull-ups, and a Qwiic connector so the bus can go on to other sensors. Open the sht40_i2c example from this page in PCB Canvas: “Insert into my schematic”.

Placement is the hard part#

A humidity sensor measures the air around it. On a board with a microcontroller, a regulator and a radio, that air is warm.

  • Put the sensor at the board edge, ideally on a tongue cut free by slots, away from the regulator and module.
  • No ground pour under or around it; thin traces only.
  • Measure briefly and sleep: self-heating of the board shows up as wrong readings.

Pitfalls#

  • DFN-4 1.5 mm: not something to hand-solder with an iron; use hot air or JLCPCB.
  • Do not wash the board after soldering with the sensor on; keep the opening free of flux and conformal coating.

Footprint#

Sensirion DFN-4 1.5 × 1.5 mm without central pad (C2909890, extended).