MikroE Air Quality Click Board on i.MX7D

This tutorial shows how to use the Air Quality Click board on the i.MX 7Dual SABRE-SD and its mikroBUS compatible socket.

According to the MikroE description, the Air quality click is a simple solution for detecting a variety of gases that impact air quality. Composed by a MQ-135 sensor, this click board is designed to run on a 5V power supply and communicates with the target microcontroller over the AN pin on the mikroBUS line.

The mikroBUS AN pin in i.MX 7Dual SABRE-SD is configured to the ACD0 - channel 0 (ACD0_IN0), an analog-to-digital converter pin enabled by default in the imx7-sdb.dts file.

To read the ACD0_IN0 value, boot the board and enter the command line below:

$ cat /sys/bus/iio/devices/iio\:device0/in_voltage0_raw

The value from this command is the results of the formula:

Vread = Vin * 4095 / Vref

where:

• Vread is the digital value actually read;
• Vin is the analog input voltage we want;
• 4095 is 2^12 bits (4096) words of word-addressable memory;
• Vref is the ADC voltage reference, which is 1.8V on i.MX 7D SoCs.

To simplify the formula, you can read the in_voltage_scale:

$ cat /sys/bus/iio/devices/iio\:device0/in_voltage_scale

This value means Vref / 4095. In this case, 1800 mV / 4095 = 0.4395.

So Vin can be calculated just by:

Vin = Vread * 0.4395

You can use the follow script to get a continuous application of this formula, used to measure the polluting gases concentration in the air in ppm:

# /bin/sh

while :
do
        num=$(cat /sys/bus/iio/devices/iio\:device0/in_voltage0_raw)
        echo "$(($num * 1800 / 4095)) ppm"
        sleep 1
done

The polluting gases concentration is detected by the MQ-135 sensor, which has a layer made of tin dioxide (SnO2) that has lower conductivity in clean air than in polluting gases presence. The following video shows a demonstration of ppm variance caused by the lighter gas and smoke: