Instructions: The above fig is the basic test circuit of MQ-4.The sensor requires two voltage inputs: heater
voltage (VH) and circuit voltage (VC). VH is used to supply standard working temperature to the sensor and
it can adopt DC or AC power, while VRL is the voltage of load resistance RL which is in series with sensor. Vc
supplies the detect voltage to load resistance RL and it should adopts DC power.