The MHZ19C carbon dioxide gas sensor, referred to as the sensor, is a general-purpose intelligent sensor that uses the non-dispersive infrared (NDIR) principle to detect the presence of CO2 in the air. Features built-in temperature compensation and offers serial output and PWM output for ease of use. This high-performance sensor combines mature infrared absorption gas detection technology with sophisticated optical and circuit design. Product Parameters: Detection Gas: Carbon Dioxide Power Supply Voltage: DC 5.0 ± 0.1V Average Current: <40mA (5V power supply) Peak Current: 125mA (5V power supply) Interface Level: 3.3V (compatible with 5V) Measuring Range: 400-5000PPM (optional) Output Signal: Serial (UART TTL level 3.3V) Warm-up Time: 1 minute Response Time: T90 < 120s Working Temperature: -10-50 degrees Celsius Working Humidity: 0-95%RH (non-condensing) Pin Description: Vin: Power In GND: Power Supply GND PWM: PWM HD: HD zero calibration (low level is effective for more than 7 seconds) Rx: UART RXD TTL level data input Tx: UART TXD TTL level data output Output Method: PWM Output Measurement Range: 400-2000ppm CO2 Concentration Output: 400-2000ppm Period: 1004ms ± 5 High-Level Output at the Beginning of the Cycle: ~2ms (theoretical value) Central Period: 1000ms ± 5 Low-Level Output at the End of the Period: ~2ms (theoretical value) Calculation Formula for Obtaining Current CO2 Concentration Value with PWM: Cppm = 2000X (TH - 2ms) / (TH + TL - 4ms) Cppm: CO2 concentration value calculated with the unit of ppm TH: The time when the output is high level in an output cycle TL: The time when the output is low level in an output cycle