Connect a TGS2600 Gas Sensor to a Raspberry Pi for Noxious Gas Detection

Detecting volatile sulfur compounds—the primary cause of foul odors—requires a sensor that can sense low‑level concentrations. The Figaro TGS2600 is designed for this purpose. It uses a heated sensing element that changes its electrical resistance when exposed to gases such as hydrogen sulfide or methane. By measuring that resistance change, the sensor outputs an analogue voltage that rises with increasing gas concentration.

Analogue vs Digital Signals

Unlike digital signals, which are strictly binary (0 or 1), analogue signals span a continuous range. A simple analogy is a steering wheel: you can turn it gently or lock it fully, whereas a digital steering system would only allow full left or full right. The TGS2600 produces a full‑range analogue voltage, but our Raspberry Pi’s GPIO pins can only read digital states.

Converting Analogue to Digital on the Pi

The Pi’s GPIO pins are 3.3 V digital inputs. When a voltage below ~1.1 V is applied, the pin reads LOW; above ~1.4 V it reads HIGH. To trigger an alarm when the sensor detects a spike, we can bring the sensor’s output just below the threshold and let any sudden rise cross into the HIGH range. This eliminates the need for an external ADC unless precise concentration values are required.

Setting the Threshold with a Resistor Ladder

The TGS2600’s output can swing from 0 V (clean air) to 3.3 V (severe contamination). A single 47 kΩ resistor pulls the signal toward ground, but background air variations can still push the voltage above the GPIO threshold. By adding a digital‑controlled resistor ladder—four 47 kΩ resistors each tied to a GPIO pin—we can selectively connect resistors to ground and adjust the effective resistance in real time. This “digital‑to‑analogue” conversion lets the Pi maintain a stable detection threshold without manual tuning.

Wiring the Sensor

The sensor has four pins:

1. Heater negative
2. Sensor electrode negative
3. Sensor electrode positive
4. Heater positive

Place the sensor across the breadboard’s central gap. Connect pins 3 and 4 to 3.3 V, and pins 1 and 2 to ground. The sensor’s output (pin 2) will feed the resistor ladder and the trigger GPIO.

Connecting the Trigger Pin

Power down the Pi, then connect the sensor’s output to GPIO 4. Add a 47 kΩ resistor between the output and ground to set the baseline voltage. Additional resistors will be added in the next section.

Building the Resistor Ladder DAC

Each of the remaining four 47 kΩ resistors is connected in parallel to the sensor output and a dedicated GPIO pin. By setting a pin to OUTPUT and driving it LOW, that resistor is connected to ground, pulling the sensor voltage down. Setting the pin to INPUT effectively disconnects the resistor. The combination of active resistors defines the total shunt resistance and allows the Pi to keep the sensor output just below the HIGH threshold under varying background conditions.

How It Works

The total shunt resistance is always less than any individual resistor because they are wired in parallel. As more resistors are activated, the effective resistance decreases, drawing more current from the sensor and lowering its output voltage. This digital control over an analogue signal is essentially a low‑cost DAC, enabling responsive and accurate gas detection.

For more detailed guidance, refer to the full tutorial: Connect a sensor to your Raspberry Pi to warn you when there are noxious gases about!