An Overview of MQ138 VOC Gas Sensor
MQ138 gas sensor has a high sensitivity to ethylene glycol, benzene formaldehyde, tetrachloroethylene, toluene, methylene chloride, and other VOC gases. The sensor could recognize different organic steam. The sensor provides various applications at a lower cost.
Internal Structure
They form this MQ sensor with a micro AL2O3 ceramic tube, a sensitive layer of tin dioxide (SnO2), measuring electrode, and heater, which are fixed into a crust of stainless steel net and plastic. The heater provides certain work conditions for sensitive components. The MQ-138 sensor has six pins, four of them are used to fetch signals, while it used the other two for providing heating current.
Working Principle
The MQ-138 sensor contains tin dioxide (SnO2), which has a smaller conductivity in clean air. When there are VOCs like, ethylene glycol, benzene formaldehyde, tetrachloroethylene, toluene, methylene chloride, the conductivity of the sensor becomes higher.
Features and Specifications of MQ138 Gas
Features
- Dual signal output (Analog output and TTL Level output)
- TTL output effective signal is low. When the output is low, the signal light is on. Can be connected to single-chip Microcomputer IO Mouth）
- Analog output increases with concentration. The higher the concentration, the higher the voltage.
- It has high sensitivity for formaldehyde – suitable for the detection of aldehydes, alcohols, ketones, and aromatic compounds. Test concentration range of gas sensors:
- Benzene 1 to 100ppm
- Toluene 10 to 100ppm
- Methanol 5 to 100ppm
- Alcohol 30 to 300ppm
- Acetone 10 to 300ppm
- Formaldehyde 1 to 10ppm
- It has long-term service life and reliable stability.；
- Fast response recovery features
- With mounting hole. Easy to fix and install；
- Probe plug-in design. Convenient Test。
Specifications
- Size：32mm X22mm X27mm L x W x G
- Main chip：LM393.
- Operating voltage: DC 5V
- Heating voltage: 5 ±0.2V (AC·DC)
- Working current: 180mA.
- Circuit voltage: DC5V (Max DC 24V)
- Load resistance: 10K (adjustable)
- Test concentration range :1-100ppm.
- Clean air voltage: < 1.5V.
- Sensitivity: > 3%
- Response time: < 1S (3-5 minute warm-up, theory preheating time 48 hours)
Pinouts of MQ138 Sensor
| Pin Number | Pin Name | Description |
| 1 | Vcc | The [in is there to power the module, requires 5V |
| 2 | Ground | To connect the module to the system’s common ground |
| 3 | Digital Out | Digital output pin, the Threshold value can be set by using the potentiometer |
| 4 | Analog Out | Analog output pin. Analog voltage based on the concentration of the gas |
Applications of MQ138
Domestic VOC gas alarm
Domestic VOC gas alarms use sensors to monitor the VOC pollutants which are toxic to human health. And, for this purpose, the device uses a microcontroller with an MQ-138 sensor.
Industrial VOC Gas Leakage alarm
In this Gas Alarm, If the device recognizes the level of gas in the air that outstrips the safety level of the environment. it will immediately activate the
alarm to warn the people working in the industry of the abnormal condition so that they will take necessary actions.
Portable VOC gas Detector
We use the term Portable gas detection for gas detectors that are carried by an individual being. Generally, battery-operated requires sensors. We use portable monitors for toxic or combustible gas detection, and hence -138 is a brilliant use.
