The op-amp LM358 is designed in a comparator mode which compares the signal received from the sensor to a threshold voltage(reference voltage). Potentiometer of 10kΩ is connected at pin no. 2.It is set at 2V(reference voltage). The voltage 2V is equal to output voltage of gas sensor in normal air condition i.e. in absence of gas. LM358 compares two input i.e. preset voltage i/p at inverting end with gas sensor o/p voltage at non-inverting end. In absence of gas the output voltage of gas sensor is 2V thus overall output voltage of op-amp will be 0V. In presence combustible gases input voltage reduces and output voltage of gas sensor increases thus creating difference between two input voltage of op-amp. Thus output voltage of LM358 lead to triggering of buzzer and thus alarm sound generates and LED glows showing the presence of combustible gas.