Ozone 2 Click Board™

Featuring an accurate 22-bit ADC and a high-quality O₃ sensor protected by a stainless mesh, the Ozone 2 Click Board™ can be used in a range of applications used to measure or detect O₃ gas concentrations in the atmosphere, for small testing equipment development, various warning systems for air quality / ozone pollution, and other similar applications that are used for the ozone concentration detection and monitoring.

How Does The Ozone 2 Click Board™ Work?

The main sensor component of the Ozone 2 Click Board™ is the MQ131 ozone (O₃) gas sensor from Winsen Sensor, which uses the SnO2 (tin-oxide) alloy, which decreases its resistance while exposed to the O₃ gas. The greater the O₃ concentration is, the more conductive this material becomes. This can be utilized to obtain the O₃ concentration readings. The sensor itself contains a small heating element, connected to 5V power supply. It needs to be preheated for 48h before it can perform as specified. The sensitivity of the sensor is given as the ratio between the resistance in air and the resistance in the O₃ gas concentration of 50ppm, which is ≥ 3 (R_O/R_S ≥ 3). The sensor is protected against particles and mechanical damage by a stainless mesh, however, exposing to excessive moisture and corrosive gases can damage the inner structure.

The measuring circuit consists of the MQ131 sensor, a power source and a load resistor (RL) between the output pin and GND. The sensor with its internal resistance forms a voltage divider with the load resistor. The RL is designed as a variable resistor, allowing the output voltage to be trimmed to the desired value. The calibration should be performed in controlled conditions, as the resistance of the sensor is affected by both the ambient temperature and humidity. The sensor can be used to measure relative O₃ concentration change without an accurate calibrating, which is useful for building applications that can be used as warning systems. The middle tap of the sensor-RL voltage divider is routed to an SMD jumper labelled as ADC SEL. This jumper can be used to redirect the measuring voltage either to the ADC for sampling or to the AN pin, so it can be used in an external circuitry (external ADC or some other form of measurement signal conditioning).

The MCP3551, a 22bit sigma-delta ADC from Microchip is used to sample the output of the sensor when selected by the ADC SEL jumper. This ADC converts the input voltage with a very high resolution of 22 bits and low noise, to a digital data, which can be obtained via the SPI interface of the Ozone 2 Click Board™. This ADC uses the reference voltage which is the same as the power supply voltage, and in this case, it is powered by 5V from the mikroBUS™ power rail. The reference voltage is used to calculate the value of the input voltage based on data received from the SPI bus, by using the following formula:

V_SENS = DATA x V_REF / FS

V_REF is the same as the power supply for this circuit, which means V_REF = 5V, and the highest number written with 22 bits (FS) is 4,194,303. DATA is the 22-bit conversion value.

As already mentioned, the ADC uses a 5V power supply. Therefore, this board needs a level conversion circuitry in order to be interfaced with 3.3V MCUs. This Click board™ uses the TXB0106 IC, a 6-bit bidirectional level shifting IC from Texas Instruments, which is used to shift communication logic voltage levels from 5V to 3.3V. The voltage shift depends on the reference voltage on the VCCA pin, which can be selected with the SMD jumper, labelled as the VCCIO SEL.

Specifications

Pinout diagram

This table shows how the pinout of the Ozone 2 Click Board™ corresponds to the pinout on the mikroBUS™ socket (the latter shown in the two middle columns).

Notes	Pin					Pin	Notes
Analog pin	AN	1	AN	PWM	16	NC
NC	2	RST	INT	15	NC
SPI Chip Select	CS	3	CS	TX	14	NC
SPI clock	SCK	4	SCK	RX	13	NC
SPI Data OUT	SDO	5	MISO	SCL	12	NC
NC	6	MOSI	SDA	11	NC
Power supply	+3.3V	7	3.3V	5V	10	+5V	Power supply
Ground	GND	8	GND	GND	9	GND	Ground

Onboard Jumpers and Settings