These are standard MQ-2 gas sensors which are sensitive to smoke among other gases. They can be directly connected to ADCs for ease-of-use.
Ability to detect : LPG, i-butane, propane, methane, alcohol, Hydrogen, smoke.
MQ-2 sensors are quite sensitive and give a quick response. The resistance of these sensors changes proportionally to the type of gas and concentration of the gas detected. Just connect power, add a load resistance and connect it to an input of an ADC to measure the output of the sensor.