Familiarize Electronic Components Part XVIII – Gas Sensors

Gas sensor is the device used for detecting the presence of Gases like Carbon monoxide, LPG, Ethanol, Toluene, Hydrogen etc. These are used as safety measures in work places and Home to avoid hazardous situations. The sensor can be used to generate an alarm or to activate a system to shut down the gas flow. Gas detectors are usually battery operated devices and if the gas level exceeds the preset level, it sends output signal to activate the circuit. Most commonly used gas sensors are LPG sensor, Smoke sensor and Alcohol sensor used in Breath analyzer.

Portable gas detectors are used to monitor rooms and the surroundings of human beings, Breath etc while fixed types are used for detecting one or more types of gases in factories , plants, hospitals etc.

Gas Sensor Module

Fixed Smoke sensors are used in multistoried flats, shops etc. According to the mechanism of operation, Gas sensors may be Semiconductor, Catalytic, Oxidation and Infrared types.

Smoke Sensor

How it works?

The Gas sensor has a sensing element, a base and a cap. The sensing element has a sensing material ( Usually a Semiconductor) and a heater to heat up the sensing material. The different sensing materials used are Tin Oxide, Tungsten oxide etc depending on the type of gas to be sensed. In principle, when the Tin Oxide is heated to high temperature, it adsorbs oxygen on its surface with a negative charge. Then the donor electrons from the Tin oxide crystals passes into the adsorbed oxygen leaving the positive charge in a space charge layer. This creates a surface potential which prevents electron flow. Inside the sensor, the current flows through the boundary of Tin oxide crystals. In the boundary of the crystals, the adsorbed oxygen forms a barrier to prevent the free movement of carriers. This potential barrier gives the electrical resistance to the sensor.

Semiconductor Gas Sensor

So in the standby mode the electrical resistance of the sensor is high. In the presence of a De-Oxidizing gas, the surface density of the adsorbed negatively charged oxygen decreases leading to a reduction in the size of the barrier. This reduced barrier decreases the electrical resistance of the Sensor.

Catalytic Sensors

These are mainly used for Combustible gas detection. These are electro catalytic types having a small sensing element called bead. The bead is made up of platinum wire coil covered with alumina and then with a coating of Palladium or Rhodium.

The sensor element remains heated in the standby mode. When the combustible gas passes over the hot catalytic surface, combustion occurs and the temperature of the bead increases. This alters the resistance of the platinum coil in the bead. This resistance change is directly proportional to the concentration of gas.

Alcohol sensor – Working of Breath Analyzer

It is too sensitive to detect the smell of alcohol in the breath. The sensor element is a heating system made up of Tin oxide and Aluminum oxide. Inside this element, there is a heating coil. There are electrodes between the Tin oxide and Alumina and when the alcohol molecules in the breath pass through the electrodes, Alcohol (Ethanol) burns into Acetic acid. This generates more current in the sensor. The amount of current depends on the number of alcohol molecules in the breath. More current means more alcohol consumed. This level is displayed through the digital display.

3 Pin Gas sensor connection

Test circuit

Pin close to Tab is Pin1 to give  heater voltage in most types. Verify datasheet before using

6 Pin Gas sensors

These are sensitive semiconductor Gas sensors that give an analog voltage at the output when the air contains Combustible gas molecules. In clean air, it gives low voltage and when the gas concentration increases, output voltage also increases accordingly.

The Sensors are provided with 6 pins but 4 pins are sufficient for connection.

  1. H-H pins – These are used for providing 5V DC to the heater element. These pins have no polarity and one H can be connected to 5V DC and the other to the ground
  2. Pins A – A (or B-B). The A and B pins can be used for providing 5 V DC or to get output voltage. If the pins A are used ( by shorting), pins B( by shorting) act as the output or Vice versa.