The gas sensors both have four contacts. Basically, a heater is powered by connecting contact 4 to the required voltage and contact 1 to ground. The second part of the circuit is a variable resistor that is incorporated in each sensor. The resistance between contacts 2 and 3 varies depending on how much of the respective gas is sensed. The higher the gas concentration, the lower the resistance (and the higher the conductivity). Instead of directly measuring that resistance, the voltage level between load resistor and sensor contact 2 gets measured. It can be acquired by using an analog input pin of the Arduino. The sensor and the load resistor together form a voltage divider, so the higher the sensor resistance, the higher the voltage measured. The figure below shows a voltage divider. In our situation R1 corresponds to the sensor resistance, R2 is the load resistance, Vout is the measuring point, and Vin is the circuit voltage powering the sensor circuit.

The sensor resistance – voltage relationship is:

Vout=R2*R1+R2.

It should be considered that the sensors have long break-in periods (2 days CO, 7 days VOC). That means they have to be running for a specific period of time before the readings are stable.

Fig. Circuit wiring of the setup.

The wiring of the sensor pin connections can be found in the datasheets of each sensor. The heater voltage and circuit voltage of the CO sensor are alternating differently, therefore the CO sensor has to be connected to two different digital pins. The heater and circuit voltage of the VOC sensor are both always kept at 5V, which makes it possible to use the same pin as voltage source.