I wanted to add an LCD screen to my CO2 sensor, so I bought a white LCD 1602 with an I2C controller. The I2C controller needs to be soldered to the LCD, but my basic soldering skills were sufficient for the task.

I also wanted to place it in a box, so I purchased this plastic enclosure but I cannot recommend it. It required a lot of glue from a glue gun to install the LCD and the ESP. I also had to use the soldering iron to create space for the ESP and a hole for the USB connector. I installed the plastic buttons but they are only decorative.

I made room for the sensors inside the box, but finally left them outside because they are more precise that way.

The ESP32-WROOM-32 was too large for the enclosure, so I used a ESP32-C3 Supermini with an expansion board. This is a really amazing board with a 32-bit RISC-V 160MHz microcontroller, WiFi, Bluetooh, I2C and UART. It’s not as powerful as the ESP32-WROOM-32 with a dual core 32-bit Xtensa 240Mhz, but it’s more than capable to control the sensors and the LCD.

This is the ESPHome configuration, which includes a switch to control the LCD backlight and a clock synchronizing the time with Home Assistant:

esphome:
  name: co2sensor
  friendly_name: CO2 Sensor

esp32:
  board: esp32-c3-devkitm-1
  framework:
    type: arduino

logger:
  level: ERROR

api:
  password: ""

ota:
  platform: esphome
  password: ""

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password
  reboot_timeout: 90s
  ap:
    ssid: ${friendly_name} Hotspot
    password: !secret wifi_password

captive_portal:

web_server:
  port: 80

debug:

time:
  - platform: homeassistant
    id: homeassistant_time
    timezone: "Europe/Madrid"

uart:
  rx_pin: GPIO20
  tx_pin: GPIO21
  baud_rate: 9600

i2c:
  sda: GPIO8
  scl: GPIO9

sensor:
  - platform: uptime
    name: Uptime
    filters:
      - lambda: return x / 60.0;
    unit_of_measurement: minutes

  - platform: wifi_signal
    name: "WiFi signal sensor"
    update_interval: 60s

  - platform: dht
    model: AM2302
    pin: GPIO10
    temperature:
      id: "dht22_temperature"
      name: "DHT22 Temperature"
    humidity:
      id: "dht22_humidity"
      name: "DHT22 Humidity"
    update_interval: 50s

  - platform: mhz19
    co2:
      id: "mhz19_co2"
      name: "MH-Z19 CO2"
    temperature:
      id: "mhz19_temperature"
      name: "MH-Z19 Temperature"
    update_interval: 60s
    automatic_baseline_calibration: false

display:
  - platform: lcd_pcf8574
    dimensions: 16x2
    address: 0x27
    update_interval: 1s
    lambda: |-
        auto time =  id(homeassistant_time).now();
        it.printf(11, 0, "%02d:%02d", time.hour, time.minute);
        it.printf(11, 1, "%02d/%02d", time.day_of_month, time.month);
        auto co2 = id(mhz19_co2).state;
        if (!isnan(co2)) {
          it.printf(0, 0, "%.0fppm", co2);
        }
        it.printf(0, 1, "%.1fC", id(dht22_temperature).state);
        it.printf(6, 1, "%.0f%%", id(dht22_humidity).state);

switch:
  - platform: gpio
    pin: GPIO7
    name: "LED Backlight"
    id: led_backlight
    restore_mode: ALWAYS_OFF

And the full BOM:

• ESP32-C3 Supermini
• Expansion board
• MH-Z19B CO2 sensor
• AM2302 temperature and humidity sensor 
• White LCD 1602 with an I2C controller
• 10cm female-female Dupont cables
• Plastic enclosure

To keep a healthy environment at home or at the workplace, one of the important things to control is the carbon dioxide (CO2) level.

It’s measured in ppm (parts per million), indicating how many parts of CO2 there are in one million parts of air. As a reference:

• Less than 1000 ppm are healthy levels
• Between 1000 ppm and 2000 ppm, we need to reduce the CO2 levels
• Levels greater than 2000 ppm are associated with headaches, sleepiness, poor concentration, loss of attention…

To reduce the CO2 level, we need to ventilate the room. It can be manually done (opening the windows) or it can be automated with a ventilation system.

To measure it we need a proper CO2 sensor, and one of the most reliables sensors is the MH-Z19B. It is not cheap for the Aliexpress standards (it costs around 20 EUR), but other cheap sensors announced as “air quality” sensors or “eCO2” sensors are not really measuring the CO2 level (i.e. the MQ135).

I bought this MH-Z19B from Aliexpress and hooked it to an ESP32-WROOM-32 board. This board is going to be also purposed as a temperature and humidity sensor, so I also attached a DHT22 sensor. I bought this DHT22 sensor but it is not an original one, and the measures do not seem very correct, so I ordered again an original AM2302 (=DHT22). The MH-Z19B includes a temperature sensor, but it’s mainly used for calibration and it lacks precision, as it does not report decimals. I’m also using an expansion board to simplify the connections.

• The jumper in the expansion board needs to be set to 5V (because both of these sensors need 5V).
• Connected VCC and GND of both sensors to the expansion board
• Connected the RX and TX of the MH-Z19B to the TX and RX (GPIO1 and GPIO3) of the ESP
• Connected GPIO16 to the DAT of the DHT22

Finally, I installed ESPHome to the board with this configuration:

substitutions:
  devicename: co2sensor
  friendly_name: CO2 sensor

esphome:
  name: ${devicename}
  friendly_name: ${friendly_name}
  platform: ESP32
  board: nodemcu-32s

logger:

api:
  password: ""

ota:
  password: ""

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password
  reboot_timeout: 90s
  ap:
    ssid: ${friendly_name} Hotspot
    password: !secret wifi_password

captive_portal:

web_server:
  port: 80

debug:

time:
  - platform: homeassistant
    id: homeassistant_time

uart:
  rx_pin: GPIO3
  tx_pin: GPIO1
  baud_rate: 9600

sensor:
  - platform: uptime
    name: Uptime
    filters:
      - lambda: return x / 60.0;
    unit_of_measurement: minutes

  - platform: wifi_signal
    name: "WiFi signal sensor"
    update_interval: 60s

  - platform: dht
    model: DHT22
    pin: GPIO16
    temperature:
      id: "dht22_temperature"
      name: "DHT22 Temperature"
    humidity:
      id: "dht22_humidity"
      name: "DHT22 Humidity"
    update_interval: 60s

  - platform: mhz19
    co2:
      id: "mhz19_co2"
      name: "MH-Z19 CO2"
    temperature:
      id: "mhz19_temperature"
      name: "MH-Z19 Temperature"
    update_interval: 60s
    automatic_baseline_calibration: false

And It’s working nicely, this is the ESPHome web interface:

Keeping the CO2 levels under control is helping me create a healthier workplace environment and improve performance.