Pololu MQ Gas Sensor Carrier board for MQ3, MQ2, MQ4 + many other gas sensors
logic voltage: Dependent on the gas sensor used.
Load Resistor Recomendations: The resistive load should be calibrated for your particular application using the equations in the datasheet, but a good starting value for the resistor is 20 kΩ.
Dimensions: 0.7″ x 0.95″
(Usage information and diagrams provided by Pololu)
Above shows where a load resistor should be placed, remember to check the datasheet for your particular gas sensor to help determine what load resistance you should use.