Saturday, February 4, 2023

Designing Wireless Gas Detection and Monitoring Systems

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MQ-3

This sensor is suitable for detecting alcohol concentration in the breath and is commonly used in hand-held breathalyser devices to test the alcohol level of drivers. The sensor requires a heater voltage and provides an output voltage proportional to the sensed alcohol concentration.

MICS-2700

This sensor is a 4-pin semiconductor device used to detect the concentrations of the NO2 gas in the range 0.05 to 5ppm. The measurement is done with a heater power of 43mW which raises the temperature of the sensing resistor to about 220ºC.

Designing a wireless gas detection & monitoring system

Gas detection systems are life-saving devices as they detect the presence of toxic gasses and alert the people to leave the area. Such devices are recommended to be used in every home and in public places such as in schools, hospitals, train station, airports, and so on.

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In this section the design of a microcontroller based wireless LPG gas detection and monitoring system is given. The system described here is multi-node and is therefore suitable for use in places like caravan sites, holiday homes, students’ halls of residence, old peoples’ homes, university campus, and other places where groups of people live and large number of detectors may be required. The block diagram of the designed system is shown in below.

The system consists of two parts: gas detection stations, and gas monitoring station. A gas detection station consists of a semiconductor gas sensor, a microcontroller that receives signals from the gas sensor and activates a suitable transmitter module to send a message to the monitoring station together with the unique ID of the station to alert that gas leakage has been detected at the specified station. Appropriate action is then taken at the monitoring station such as sounding an alarm and attending the requesting station for help. In this design up to 16 stations can be used in the system with their unique IDs selected with switches and ranging from 0 to 15.

Block diagram of the designed system
Block diagram of the designed system

Both the detection stations and the monitoring station use low-cost 8-bit medium performance PIC18F family of microcontrollers. The hardware details of each station are given below.

Detection Stations

The MQ-6 semiconductor LPG gas sensor chip is used in the design since it is highly sensitive, low-cost and is easy to use. LPG concentrations above 1000ppm are accepted to be toxic and should be avoided. The system has been designed so that it is activated if the LPG concentration is sensed to go above 1000ppm.

MQ-6 LPG gas sensor chip
MQ-6 LPG gas sensor chip

The MQ-6 pins are organised in groups of three pins on each side of the sensor where pins 2 and 5 are the heater pins, and 1,3 and 4,6 are the sensor circuit pins.

A load resistor RL is connected in series with sensor resistance Rs to convert the resistance Rs to a voltage. Shown below is the sensor output voltage when a 4.7K load resistor is used.

Output voltage of the sensor
Output voltage of the sensor

The circuit diagram of a detection station is shown below. A PIC18F2550 type microcontroller is used in the design with an external 8MHz crystal. The PLL clock multiplier is used to increase the operating clock frequency from 8MHz to 48MHz. The sensor output is connected to a 10K variable resistor which feeds into analog input channel RA1 of the microcontroller. The sensitivity of the device can easily be adjusted via this potentiometer. Whenever a gas leakage is detected, a message is sent to the monitoring station together with the unique station ID.

Circuit diagram of a gas detection station
Circuit diagram of a gas detection station

Every station is assigned a unique ID by using the switches connected to PORTB. In this design 4 switches are used, making it possible to have up to 16 detection stations (0 to 15).

Depending upon the applications the distance between the detection stations and the monitoring station could be hundreds of meters or even more. For example, in a large caravan site this distance could even be a few kilometres. For this reason, it was decided to use RF transmitter and receiver modules in the design.

The Radiometrix TX2H transmitter module was found to be suitable for use in the design. This is a 433.92MHz UHF FM transmitter module with a power output of 25mW, having a typical range of 500 metres, consuming only 23mA of current. Radiometrix is a leading designer and manufacturer in UK of professional grade VHF and UHF low power radio modules according to the international FCC/ETSI standards. A large selection of transmitter, receiver, and transceiver modules and associated products are available from transmit powers of 5mW to 500mW.

TX2H UHF transmitter module
TX2H UHF transmitter module

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