MQ-7 Semiconductor Sensor for Carbon Monoxide
- 1 Introduction
- 2 Character
- 3 Application
- 4 Configuration
- 5 Technical Data
- 6 Basic test loop
- 7 Sensitivity Characteristics
- 8 Influence of Temperature/Humidity
- 9 Structure and configuration
- 10 Notification
- 11 Document
Sensitive material of MQ-7 gas sensor is SnO2, which with lower conductivity in clean air. It make detection by method of cycle high and low temperature, and detect CO when low temperature (heated by 1.5V). The sensor’s conductivity is more higher along with the gas concentration rising. When high temperature (heated by 5.0V), it cleans the other gases adsorbed under low temperature. Please use simple electrocircuit, Convert change of conductivity to correspond output signal of gas concentration. MQ-7 gas sensor has high sensitity to Carbon Monoxide. The sensor could be used to detect different gases contains CO, it is with low cost and suitable for different application.
- Good sensitivity to Combustible gas in wide range
- High sensitivity to Natural gas
- Long life and low cost
- Simple drive circuit
- Domestic gas leakage detector
- Industrial CO detector
- Portable gas detector
Basic test loop
The above is basic test circuit of the sensor. The sensor need to be put 2 voltage, heater voltage（VH）and test voltage（VC）. VH used to supply certified working temperature to the sensor, while VC used to detect voltage (VRL) on load resistance （RL）whom is in series with sensor. The sensor has light polarity, Vc need DC power. VC and VH could use same power circuit with precondition to assure performance of sensor. In order to make the sensor with better performance, suitable RL value is needed:
Power of Sensitivity body(Ps):
Resistance of sensor(Rs): Rs=(Vc/VRL-1)×RL
Fig.1 shows the typical sensitivity characteristics of the MQ-7, ordinate means resistance ratio of the sensor (Rs/Ro), abscissa is concentration of gases. Rs means resistance in different gases, Ro means resistance of sensor in 1000ppm Hydrogen. All test are under standard test conditions.
Influence of Temperature/Humidity
Fig.2 shows the typical temperature and humidity characteristics. Ordinate means resistance ratio of the sensor (Rs/Ro), Rs means resistance of sensor in 100ppm CO under different tem. and humidity. Ro means resistance of the sensor in environment of 100ppm CO, 20℃/65%RH
Structure and configuration
Structure and configuration of MQ-7 gas sensor is shown as Fig. 3, sensor composed by micro AL2O3 ceramic tube, Tin Dioxide (SnO2) sensitive layer, measuring electrode and heater are fixed into a crust made by plastic and stainless steel net. The heater provides necessary work conditions for work of sensitive components. The enveloped MQ-7 have 6 pin, 4 of them are used to fetch signals, and other 2 are used for providing heating current.
1 Following conditions must be prohibited
1.1 Exposed to organic silicon steam
Organic silicon steam cause sensors invalid, sensors must be avoid exposing to silicon bond, fixature, silicon latex, putty or plastic contain silicon environment
1.2 High Corrosive gas
If the sensors exposed to high concentration corrosive gas (such as H2Sz, SOX，Cl2，HCl etc), it will not only result in corrosion of sensors structure, also it cause sincere sensitivity attenuation.
1.3 Alkali, Alkali metals salt, halogen pollution
The sensors performance will be changed badly if sensors be sprayed polluted by alkali metals salt especially brine, or be exposed to halogen such as fluorin.
1.4 Touch water
Sensitivity of the sensors will be reduced when spattered or dipped in water.
Do avoid icing on sensor’surface, otherwise sensor would lose sensitivity.
1.6 Applied voltage higher
Applied voltage on sensor should not be higher than stipulated value, otherwise it cause down-line or heater damaged, and bring on sensors’ sensitivity characteristic changed badly.
1.7 Voltage on wrong pins For 6 pins sensor, if apply voltage on 1、3 pins or 4、6 pins, it will make lead broken, and without signal when apply on 2、4 pins
2 Following conditions must be avoided
2.1 Water Condensation
Indoor conditions, slight water condensation will effect sensors performance lightly. However, if water condensation on sensors surface and keep a certain period, sensor’ sensitivity will be decreased.
2.2 Used in high gas concentration
No matter the sensor is electrified or not, if long time placed in high gas concentration, if will affect sensors characteristic.
2.3 Long time storage
The sensors resistance produce reversible drift if it’s stored for long time without electrify, this drift is related with storage conditions. Sensors should be stored in airproof without silicon gel bag with clean air. For the sensors with long time storage but no electrify, they need long aging time for stbility before using.
2.4 Long time exposed to adverse environment
No matter the sensors electrified or not, if exposed to adverse environment for long time, such as high humidity, high temperature, or high pollution etc, it will effect the sensors performance badly.
Continual vibration will result in sensors down-lead response then repture. In transportation or assembling line, pneumatic screwdriver/ultrasonic welding machine can lead this vibration.
If sensors meet strong concussion, it may lead its lead wire disconnected.
For sensor, handmade welding is optimal way. If use wave crest welding should meet the following conditions: 2.7.1Soldering flux: Rosin soldering flux contains least chlorine
2.7.2Speed: 1-2 Meter/ Minute
2.7.51 time pass wave crest welding machine
If disobey the above using terms, sensors sensitivity will be reduced.