Sensy-One CO₂ Add-on

The Sensy-One CO₂ Add-on is a plug-and-play expansion module for the Sensy-One S1 Pro Multi Sense and E1 Pro Multi Sense , equipped with the Sensirion SCD40 sensor. This module measures real CO₂ concentrations with professional NDIR (non-dispersive infrared) technology, instead of estimated values. This gives you accurate insight into the air quality for a healthy indoor climate. The module easily connects to the front of the S1 Pro or E1 Pro via a plug-and-play connector, without soldering or additional tools.

Why a separate CO₂ sensor?

The standard BME688 sensor in the S1 Pro estimates CO₂ levels based on VOC (volatile compounds) measurements, which is suitable for general air quality monitoring. However, the SCD40 measures real CO₂ concentrations with high-quality technology, which is essential for healthy bedrooms, offices, and classrooms. Excessively high CO₂ levels (above 1000 ppm) lead to reduced concentration, fatigue, and poor sleep quality. With the SCD40, you get reliable measurements for optimal ventilation. For the E1 Pro , this add-on is the only way to measure CO₂, as this model does not have a built-in BME688.

Sensirion SCD40 Specifications

The SCD40 is a compact, energy-efficient CO₂ sensor that uses photoacoustic NDIR technology. It measures CO₂ with an accuracy of ±50-100 ppm between 400 and 2000 ppm, delivering professional-quality results. The SCD40 also measures temperature (±0.8°C accuracy) and relative humidity (±6% accuracy). The sensor operates optimally between 0°C and 50°C and has a measurement range of 0-40,000 ppm CO₂. The SCD40 complies with the WELL Building Standard for healthy buildings.

Altitude compensation

A unique feature is automatic altitude compensation . In the S1 Pro, the SCD40 uses pressure readings from the BME688 sensor to correct CO₂ measurements based on altitude and air pressure. The E1 Pro measures air pressure by default and uses the same compensation. This ensures accurate measurements whether you live at sea level or in a high-altitude area. Measurements remain reliable even in changing weather conditions.

Automatic Sensor Tuning

With the S1 Pro , temperature and humidity measurements from both the BME688 and SCD40 are automatically aligned using advanced correction algorithms. With the E1 Pro, the SCD40 provides the primary temperature and humidity measurements. This ensures accurate measurements without manual calibration, even when temperature fluctuates or sensor heating occurs.

Plug-and-Play Installation

The CO₂ Add-on connects to the plug-and-play connector on the front of the S1 Pro or E1 Pro Multi Sense. No soldering, no extra tools – simply plug it in and you're ready to go. The module is automatically recognized by Home Assistant via ESPHome, and the CO₂ readings appear directly on your dashboard alongside other sensor values.

Measurement Interval and Responsiveness

The SCD40 measures CO₂ every 5 seconds by default, but this interval is configurable in ESPHome. The sensor has a response time of approximately 60 seconds to CO₂ changes, which is fast enough for real-time ventilation automation. Power consumption is low (approximately 5 mA on average), which is ideal for continuous monitoring.

Compatibility and Integration

The CO₂ Add-on works with the Sensy-One S1 Pro Multi Sense and the E1 Pro Multi Sense and requires ESPHome firmware. The module integrates seamlessly with Home Assistant and can be used for automations such as ventilation activation when CO₂ levels are high, notifications for poor air quality, or graphs for long-term monitoring.

Calibration and Maintenance

The SCD40 has a built-in automatic self-calibration (ASC) that periodically calibrates the sensor based on the lowest CO₂ value over a 7-day period (assuming regular ventilation to outside air with ~400 ppm CO₂). Manual calibration is possible but rarely necessary. The sensor is maintenance-free and has a lifespan of over 10 years under normal use.

Physical and Dimensions

The CO₂ Add-on is designed to be compact and fits seamlessly onto the front of the S1 Pro or E1 Pro PCB without disrupting the aesthetics.

Usage scenarios

The CO₂ Add-on is ideal for bedrooms (healthy sleep requires CO₂ below 1000 ppm), offices (productivity decreases at high CO₂ levels), classrooms (concentration and learning performance), meeting rooms (CO₂ rises quickly with many people), and nurseries (healthy air for young children). Automations allow you to control fans, window switches, or HVAC systems based on CO₂ levels.

Plus points

• Accurate CO₂ measurements with professional NDIR technology (±50-100 ppm)
• Altitude compensation via BME688 (S1 Pro) or standard air pressure measurement (E1 Pro) for reliable measurements at any air pressure
• Plug-and-play installation without soldering or tools
• Compatible with both S1 Pro and E1 Pro Multi Sense
• Automatic self-calibration for long-term accuracy
• Low power consumption and more than 10 years lifespan
• Also measures temperature and humidity
• Complies with the WELL Building Standard for healthy buildings

Negative points

• Only works with Sensy-One S1 Pro or E1 Pro Multi Sense; not as a standalone sensor
• Requires Home Assistant and ESPHome firmware; limited support for other platforms
• Automatic calibration requires exposure to clean outside air (~400 ppm CO₂)

Frequently Asked Questions

Does the CO₂ Add-on work without the S1 Pro or E1 Pro?

No, the CO₂ Add-on is designed exclusively for use with the Sensy-One S1 Pro Multi Sense or the E1 Pro Multi Sense . The module requires one of these sensors for power, communication with Home Assistant, and altitude compensation.

How do I connect the CO₂ Add-on?

The module connects to the plug-and-play connector on the front of the S1 Pro or E1 Pro. No soldering or tools required – simply click the connector in and the sensor is automatically recognized by Home Assistant. CO₂ measurements appear instantly on your dashboard.
TIP: Watch this video showing the installation.

Do I need to calibrate the CO₂ sensor?

No, the SCD40 has an automatic self-calibration (ASC) that periodically calibrates the sensor based on the lowest CO₂ reading in 7 days. This works well if the room is regularly ventilated with outside air (~400 ppm CO₂). Manual calibration is possible via ESPHome, but is rarely necessary.

What is the difference between the BME688 (S1 Pro) and the SCD40 for CO₂ measurement?

The BME688 in the S1 Pro estimates CO₂ levels based on VOC (volatile compounds) measurements, which is suitable for general air quality. The SCD40 measures real CO₂ concentrations with NDIR technology and offers professional-grade accuracy (±50-100 ppm). For healthy bedrooms, offices, and classrooms, the SCD40 is the better choice. The E1 Pro does not include the BME688, so this add-on is the only way to measure CO₂.

What is altitude compensation and why is it important?

CO₂ measurements are affected by air pressure, which varies with altitude and weather conditions. The SCD40 uses pressure readings from the BME688 (with the S1 Pro) or the standard air pressure measurement (with the E1 Pro) to automatically correct measurements. This ensures accurate CO₂ readings whether you live at sea level or at high altitudes, and in changing weather conditions.

How quickly does the CO₂ sensor respond to changes?

The SCD40 measures every 5 seconds and has a response time of approximately 60 seconds to CO₂ changes. This is fast enough for real-time ventilation automation, for example, switching on a fan when CO₂ rises above 1000 ppm.

Can I use the CO₂ Add-on with Google Home or Alexa?

Not directly. The CO₂ Add-on works exclusively through Home Assistant and ESPHome. You can, however, connect Home Assistant to Google Home or Alexa, giving you indirect access to CO₂ readings through those platforms.

What's the difference between the S1 Pro and E1 Pro with this add-on?

With the CO₂ Add-on, you get the same accurate CO₂ measurements on both sensors. The difference lies mainly in the other features: the S1 Pro has a BME688 for VOC/IAQ measurements and USB-C power, while the E1 Pro offers PoE support and uses an LD2460 mmWave sensor with ceiling and wall modes. Both sensors offer excellent CO₂ monitoring with this add-on installed.