NH3 monitoring made simple with MQ-137 and STM32F302VC
Guardians of purity: Innovating ammonia sensing solution
Published Jul 22, 2025
Click board™
Ammonia Click
Dev. board
CLICKER 4 for STM32F302VCT6
Compiler
NECTO Studio
MCU
STM32F302VC
Our ammonia sensing solution provides accurate and timely information, enabling industries and communities to manage ammonia levels and maintain safe, breathable air
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Hardware Overview
How does it work?
Ammonia Click is based on the MQ-137 gas sensor from Winsen, which uses the SnO2 (tin-oxide) alloy, which decreases its resistance while exposed to the NH3 gas. The greater the NH3 concentration is, the more conductive this material becomes. This can be utilized to obtain the NH3 concentration readings. The sensor contains a small heating element connected to a 5V power supply. The heater element can be controlled via a MOSFET power switch connected to the PWM pin on the mikroBUS™ socket for lower power consumption. It needs to be preheated for a minimum of 24 hours before it can perform as specified. A stainless mesh protects The sensor against particles and mechanical damage; however, exposure to excessive moisture and corrosive gases can damage the inner structure. The measuring circuit consists of the MQ-137 sensor, a power source, and a load resistor (RL) between the output pin and GND. With its internal
resistance, the sensor 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 ambient temperature and humidity affect the sensor's resistance. The sensor can measure relative NH3 concentration change without accurate calibration, 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 labeled ADC SEL. This jumper can redirect the measuring voltage to the ADC for sampling or the AN pin so that it can be used in an external circuitry (external ADC or some other form of measurement signal conditioning). The MCP3551, a 22-bit sigma-delta ADC from Microchip, is used to sample the sensor output 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 digital data, which can be obtained via the SPI interface of the 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. As already mentioned, the ADC uses a 5V power supply. Therefore, this board needs a level conversion circuitry interfacing 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. This Click board™ can operate with either 3.3V or 5V logic voltage levels selected via the VCC SEL jumper. This way, both 3.3V and 5V capable MCUs can use the communication lines properly. Also, this Click board™ comes equipped with a library containing easy-to-use functions and an example code that can be used as a reference for further development.
Features overview
Development board
Clicker 4 for STM32F3 is a compact development board designed as a complete solution, you can use it to quickly build your own gadgets with unique functionalities. Featuring a STM32F302VCT6, four mikroBUS™ sockets for Click boards™ connectivity, power managment, and more, it represents a perfect solution for the rapid development of many different types of applications. At its core, there is a STM32F302VCT6 MCU, a powerful microcontroller by STMicroelectronics, based on the high-
performance Arm® Cortex®-M4 32-bit processor core operating at up to 168 MHz frequency. It provides sufficient processing power for the most demanding tasks, allowing Clicker 4 to adapt to any specific application requirements. Besides two 1x20 pin headers, four improved mikroBUS™ sockets represent the most distinctive connectivity feature, allowing access to a huge base of Click boards™, growing on a daily basis. Each section of Clicker 4 is clearly marked, offering an intuitive and clean interface. This makes working with the development
board much simpler and thus, faster. The usability of Clicker 4 doesn’t end with its ability to accelerate the prototyping and application development stages: it is designed as a complete solution which can be implemented directly into any project, with no additional hardware modifications required. Four mounting holes [4.2mm/0.165”] at all four corners allow simple installation by using mounting screws. For most applications, a nice stylish casing is all that is needed to turn the Clicker 4 development board into a fully functional, custom design.
Microcontroller Overview
MCU Card / MCU
Architecture
ARM Cortex-M4
MCU Memory (KB)
256
Silicon Vendor
STMicroelectronics
Pin count
100
RAM (Bytes)
40960
Used MCU Pins
mikroBUS™ mapper
Take a closer look
Click board™ Schematic
Step by step
Project assembly
Start by selecting your development board and Click board™. Begin with the CLICKER 4 for STM32F302VCT6 as your development board.
Track your results in real time
Application Output
1. Application Output - In Debug mode, the 'Application Output' window enables real-time data monitoring, offering direct insight into execution results. Ensure proper data display by configuring the environment correctly using the provided tutorial.
2. UART Terminal - Use the UART Terminal to monitor data transmission via a USB to UART converter, allowing direct communication between the Click board™ and your development system. Configure the baud rate and other serial settings according to your project's requirements to ensure proper functionality. For step-by-step setup instructions, refer to the provided tutorial.
3. Plot Output - The Plot feature offers a powerful way to visualize real-time sensor data, enabling trend analysis, debugging, and comparison of multiple data points. To set it up correctly, follow the provided tutorial, which includes a step-by-step example of using the Plot feature to display Click board™ readings. To use the Plot feature in your code, use the function: plot(*insert_graph_name*, variable_name);. This is a general format, and it is up to the user to replace 'insert_graph_name' with the actual graph name and 'variable_name' with the parameter to be displayed.
Software Support
Library Description
This library contains API for Ammonia Click driver.
Key functions:
ammonia_heater- Sensor heater functionammonia_data_read- Read data function
Open Source
Code example
The complete application code and a ready-to-use project are available through the NECTO Studio Package Manager for direct installation in the NECTO Studio. The application code can also be found on the MIKROE GitHub account.
/*!
* \file
* \brief Ammonia Click example
*
* # Description
* This example shows the value of ammonia measurement aquired from Ammonia Click board.
*
* The demo application is composed of two sections :
*
* ## Application Init
* Calls functions for driver initializaton used for data conversion and results reading.
*
* ## Application Task
* Reads the level of ammonia in the air every with repetition of 1 second.
* This driver is able to get the level of ammonia gas in the range from 5 to 200 ppm.
* #note#
* Be sure that you correctly set the AD convertor which you want to use.
*
* \author Nemanja Medakovic
*
*/
// ------------------------------------------------------------------- INCLUDES
#include "board.h"
#include "log.h"
#include "ammonia.h"
// ------------------------------------------------------------------ VARIABLES
static ammonia_t ammonia;
static log_t logger;
// ------------------------------------------------------ APPLICATION FUNCTIONS
void application_init ( void )
{
log_cfg_t log_cfg;
/**
* Logger initialization.
* Default baud rate: 115200
* Default log level: LOG_LEVEL_DEBUG
* @note If USB_UART_RX and USB_UART_TX
* are defined as HAL_PIN_NC, you will
* need to define them manually for log to work.
* See @b LOG_MAP_USB_UART macro definition for detailed explanation.
*/
LOG_MAP_USB_UART( log_cfg );
log_init( &logger, &log_cfg );
log_info( &logger, "---- Application Init... ----" );
ammonia_cfg_t ammonia_cfg;
// Click initialization.
ammonia_cfg_setup( &ammonia_cfg );
AMMONIA_MAP_MIKROBUS( ammonia_cfg, MIKROBUS_1 );
if ( ammonia_init( &ammonia, &ammonia_cfg ) == AMMONIA_INIT_ERROR )
{
log_info( &logger, "---- Application Init Error. ----" );
log_info( &logger, "---- Please, run program again... ----" );
for ( ; ; );
}
log_info( &logger, "---- Application Init Done. ----\n" );
}
void application_task ( void )
{
uint16_t nh3_ppm;
if ( ammonia_read_measurement( &ammonia, &nh3_ppm ) == AMMONIA_OK )
{
log_printf( &logger, " NH3 [ppm] : %u\r\n", nh3_ppm );
Delay_ms ( 1000 );
}
}
int main ( void )
{
/* Do not remove this line or clock might not be set correctly. */
#ifdef PREINIT_SUPPORTED
preinit();
#endif
application_init( );
for ( ; ; )
{
application_task( );
}
return 0;
}
// ------------------------------------------------------------------------ END
Additional Support
Resources
Category:Gas
