This is a list of sensors and their related information.

MH-Z19 CO2 Sensor[edit | edit source]

These CO2 modules cost about $17 USD. They come in 3 models: 0-2000PPM, 0-5000PPM, and 0-10000PPM.

Interface: PWM or UART

See: MH-Z19 Carbon Dioxide Sensor

CCS811 Air Quality Sensor[edit | edit source]

The CCS811 air quality sensor can measure Volatile Organic Compounds (VOCs). The internal microprocessor will calculate the equivalent CO2 value from the VOC measurement. These cost about $8 USD. The device or the library driving it is causing issues with the I2C bus which causes the entire bus to freeze. If you want to measure CO2, I would recommend using the MH-Z19 which actually measures CO2 rather than eCO2.

The datasheet can be found at:

Pin Number Pin Name Description
1 VCC Power supply for the module can typically 3.3V is used
2 GND Ground of the module, connected to ground of the circuit
3 SCL Serial Clock Line, used to provide clock pulse for I2C communication
4 SDA Serial Data Address, used to transfer the data through I2C communication
5 WAK Wake (active low)
6 INT Interrupt (active low)
7 RST Reset (active low)
8 ADD Single address select bit to allow alternate address to be selected

To use the sensor:

  1. Connect Vcc and Ground
  2. For a ESP8266, connect SCL and SDA (I2C) to D1 and D2 respectively.
  3. Connect WAK to ground

Using I2C, the device is on address 0x57.

I2C Freezing Issue[edit | edit source]

The sensor randomly causes the I2C bus to freeze after an hour or so. This issue is discussed at

One proposed solution by Frogmore42 is to add a delay in the Adafruit library:

// in Adafruit_CCS811::read
	Wire.requestFrom((uint8_t)_i2caddr, read_now);

Sensor Broke?[edit | edit source]

The first sensor I tried to use worked for a bit, but the I2C issue cropped up. I then soldered it to a protoboard thinking the issue may be related to the breadboard, but then it completely stopped working. ReadData returned 24 which suggests an issue with the heater?

MQ135 Air Quality Sensor[edit | edit source]

This module will return a high signal when the air quality meets a certain criteria.

DHT 11, DHT22[edit | edit source]

The DHT11 is a digital temperature and humidity sensor. They are notable by their blue perforated plastic package. The DHT11 is created by Aosong Guangzhou Electronics Co. Ltd. Each unit costs around $1.5 USD.

The DHT22, also called AM2302, RHT03, is similar to the DHT11 but offers a slightly wider operating temperature range and higher temperature and humidity resolution. The DHT22 is slightly more expensive at around $3 USD per unit.

The difference between the two modules are outlined in the table below.

Relative Humidity Operating Range 20 ~ 90% 0 ~ 100%
Relative Humidity Accuracy ±5%RH ±2%RH (Max ±5%RH)
Humidity Resolution 1%RH 0.1%RH
Humidity Repeatability ±1%RH ±1%RH
Humidity Hysteresis ±1%RH ±0.3%RH
Long-term Stability ±1%RH/year ±0.5%RH/year
Temperature Operating Range 0 ~ 50°C -40 ~ 80°C
Temperature Accuracy ±2°C ±0.5°C
Temperature Resolution 1°C 0.1°C

BMP085, BMP180, BMP280[edit | edit source]

These BMPs are atmospheric pressure sensors. They use I2C or SPI. The BMP280 appears to have superseded and are readily available on Ebay for around $0.50 and $1 USD each.

The BMP280 I obtained appear on address 0x76. If you are using the Adafruit library, you need to use the BMP280_ADDRESS_ALT address.

MAX30102[edit | edit source]

The MAX30102 is a heart rate monitor and pulse oximeter biosensor. These modules cost about $2 USD on AliExpress.

Using I2C, this has address 0x5A.

HC-SR501 Infrared Motion Sensor[edit | edit source]

The SR501 is an infrared motion sensor. They cost around $1.5 USD each. It has 2 potentiometers that are used to adjust the detection sensitivity and duration of the signal. The middle output pin gives a 3.3V signal when motion is detected and resets back to ground after a period of time.