Arduino Library for the SparkFun Qwiic Formaldehyde Sensor (SFA40)
The SparkFun Qwiic Formaldehyde Sensor puts the Sensirion SFA40 miniature electrochemical formaldehyde (HCHO) sensor on a Qwiic-enabled breakout, making indoor air-quality measurement as simple as plugging in a cable. The SFA40 is highly selective to formaldehyde with very low cross-sensitivity to common indoor gases such as ethanol, and is factory-calibrated to report a digital concentration directly in parts per billion (ppb).
This library provides an easy-to-use interface to the SFA40 over I2C, built on the SparkFun Toolkit. It handles the Sensirion command protocol for you — including CRC validation of every reading — and returns calibrated formaldehyde, relative humidity, and temperature values.
The library exposes the full SFA40 command set:
- Calibrated formaldehyde concentration in ppb (measurement range 0–2000 ppb)
- On-board relative humidity and temperature (used internally to compensate the formaldehyde signal)
- Continuous measurement start / stop control
- Sensor warm-up status reporting (ready / within specifications)
- Unique 48-bit serial number readout (matches the laser marking on the sensor)
- Built-in self-test for verifying the sensor's electrical connections (e.g. after soldering)
- Soft reset via the I2C general call
- Automatic CRC-8 validation of every received data word
Note
The SFA40 needs to warm up after power-on. For the first minute the sensor is not ready and the formaldehyde output is forced to 0. Between roughly 1 and 10 minutes the sensor is ready but not yet within specifications. After about 10 minutes the formaldehyde reading is within specifications. See the status helpers below and Example 04.
The sensor connects via I2C using either a Qwiic connector or by using the plated through-hole headers. The SFA40 uses a single fixed 7-bit I2C address of 0x5D.
| Pin / Header | Use | Notes |
|---|---|---|
| Qwiic / I2C | Power + communication | Standard 3.3V Qwiic connection |
The SFA40 supports I2C standard mode (100 kHz) and fast mode (400 kHz).
Install through the Arduino Library Manager by searching for SparkFun SFA40, or download this repository as a ZIP and add it via Sketch > Include Library > Add .ZIP Library. This library depends on the SparkFun Toolkit, which the Library Manager will offer to install alongside it.
The I2C interface to the sensor is provided by the SfeSFA40ArdI2C class. Declare a sensor object:
#include <SparkFun_SFA40.h>
// Declare our sensor object
SfeSFA40ArdI2C mySensor;In setup(), start I2C and call begin(). begin() confirms the device is present by reading and validating its serial number, and leaves the sensor in idle state:
Wire.begin();
while (mySensor.begin() == false)
{
Serial.println("SFA40 not connected, check your wiring!");
delay(1000);
}To read measurement data, bring the sensor into continuous measurement mode with startMeasurement(). The sensor then updates a new data point internally about every 0.5 seconds:
mySensor.startMeasurement();Call readMeasurement() to fetch a fresh data point (this validates the CRC of every value), then read the converted values:
if (mySensor.readMeasurement() == ksfTkErrOk)
{
Serial.print("HCHO (ppb): ");
Serial.print(mySensor.getHCHO(), 1);
Serial.print(" Humidity (%RH): ");
Serial.print(mySensor.getHumidity(), 1);
Serial.print(" Temperature (C): ");
Serial.println(mySensor.getTemperature(), 1);
}getTemperatureF() returns the temperature in degrees Fahrenheit. The raw, unconverted tick values are also available via getHCHORaw(), getHumidityRaw(), and getTemperatureRaw().
Reading once per second is plenty — reading faster than the sensor's ~0.5 s internal update simply returns the most recent data point again.
Each measurement carries a status that reports the sensor's readiness. Use it to decide when the formaldehyde value can be trusted:
if (!mySensor.isReady())
Serial.println("Sensor warming up..."); // first ~1 minute
else if (!mySensor.isWithinSpecifications())
Serial.println("Stabilizing, not yet in spec"); // first ~10 minutes
else
Serial.println("Formaldehyde reading is valid");getStatus() returns the raw status byte if you need it.
Most library methods return a SparkFun Toolkit error code (ksfTkErrOk on success, a negative value on failure). For the data-reading commands, this also covers CRC validation. A corrupted reading returns an error rather than bad data. For simple sketches you can ignore the return value:
mySensor.readMeasurement(); // assume good data
float hcho = mySensor.getHCHO();For robust applications, check it:
if (mySensor.readMeasurement() != ksfTkErrOk)
{
// Handle the communication / CRC error
}
else
{
// The cached values hold a valid reading
}Every SFA40 has a unique 48-bit serial number that matches the value laser-marked on the sensor cap. It must be read while the sensor is idle (no measurement running):
uint64_t serialNumber = 0;
mySensor.getSerialNumber(serialNumber);The self-test checks the integrity of the electrical connections to the sensor cell and is recommended once after the sensor is soldered to a PCB. It runs for 5–6 minutes; poll for the result until it is no longer 0xFFFF (still running). A result of 0x0000 means the test passed:
mySensor.startSelfTest();
uint16_t result = 0xFFFF;
while (result == 0xFFFF)
{
delay(10000);
mySensor.readSelfTestResult(result);
}
Serial.println(result == 0x0000 ? "Self-test passed" : "Self-test failed");reset() returns the sensor to its power-on state. The SFA40 reset is issued as an I2C general call, so it affects all devices on the bus that respond to a general-call reset:
mySensor.reset();The library ships with a set of examples that build from the basics to more advanced features:
- Example 01 - Basic Reading — the minimal sketch to read formaldehyde, humidity, and temperature
- Example 02 - Serial Number — read the sensor's unique 48-bit serial number
- Example 03 - Self-test — run the built-in self-test and report the result
- Example 04 - Sensor Status — warm-up-aware reading that only trusts the formaldehyde value once it is within specifications
API documentation is generated with Doxygen and published to GitHub Pages from the main branch.
- SparkFun Qwiic Formaldehyde Sensor - SFA40
If you would like to contribute to this library, please report issues and submit pull requests against the GitHub repository.
This product is open source! Please see LICENSE.md for more information.
- Your friends at SparkFun