8051-Based Mine Air-Quality Monitor with Automatic Exhaust-Fan Control

8051-Based Mine Air-Quality Monitor with Automatic Exhaust-Fan Control is a diploma Mining Engineering hardware project built around a problem our survey kept finding in the stone-aggregate quarries and semi-mechanised workshops around our district: air quality is watched by human noses, not by instruments. Diesel loaders and compressors fill enclosed workshop bays with carbon monoxide, blasting and welding fumes linger longer than they should, and the exhaust fan is switched on only after someone feels uncomfortable.

Our fixed-point monitor continuously reads three parameters — combustible gas (MQ-2), carbon monoxide (MQ-7) and temperature (LM35) — through an ADC0808 analogue-to-digital converter into an AT89S52 microcontroller. When any reading crosses its alarm threshold, the firmware starts the exhaust fan through a relay driver, sounds a buzzer and names the culprit on a 16x2 LCD; the fan runs until every reading recovers below the clear threshold, with hysteresis so the relay never chatters. A stuck-sensor routine flags a channel that reads flat, because an air-quality instrument that silently dies is worse than none.

Every circuit block is simulated in Multisim or TINA before assembly on veroboard, and the gas channels are bump-tested against known sources (incense smoke for CO, a brief unlit LPG puff for combustible gas). The bundle carries the block diagram, control flowchart, Embedded C structure, bill of materials in rupees and a testing table the team fills with their own readings.

In the stone-aggregate quarries, mine workshops and semi-mechanised bays around our district, air quality is monitored by human observation. Diesel equipment produces carbon monoxide, which has no smell; blasting, welding and LPG use accumulates gases in enclosed spaces; and the exhaust fan is switched on only after workers feel discomfort. Handheld gas detectors are expensive and only help if someone carries one, while industrial fixed multi-gas systems cost several lakhs, so small operators go without.

The consequence is a predictable pattern: ventilation starts late, workers are exposed to CO and combustible-gas build-up that no nose can detect, and there is no record of what the air contained. The problem is to design a low-cost, continuously available fixed-point monitor that measures combustible gas, carbon monoxide and temperature, compares them against defined alarm thresholds, starts the exhaust fan automatically when a threshold is crossed, keeps the fan on until readings recover, and flags sensor faults — using the microcontroller, ADC and interfacing skills of the diploma syllabus.

Abstract

Small mines and quarries in India depend on manual observation for air quality, while fixed gas-monitoring systems priced for large mechanised mines remain out of reach. This project designs and builds a low-cost, fixed-point air-quality monitor on the 8051 microcontroller. MQ-2 and MQ-7 sensor channels, an LM35 and an ADC0808 feed the AT89S52, which compares every reading against defined alarm thresholds, starts an exhaust fan automatically on alarm, and displays live values and fault states on a 16x2 LCD. The circuits are simulated in Multisim/TINA and assembled on veroboard; sensor channels are exercised by bump tests with known sources, and the prototype is demonstrated with a 12 V fan standing in for the ventilation circuit.

Introduction

Under the Mines Act 1952, the Directorate General of Mines Safety (DGMS) requires mine air to be kept within safe limits for gases, and ventilation is the primary engineering control. Diesel equipment produces carbon monoxide; blasting produces fumes that need clearance time; and in gassy workings methane is a constant hazard. Large mines deploy approved multi-gas systems; at the small quarries and workshops our team visited, the fan starts when someone notices the smell.

Existing system

• Manual observation: depends on human senses, which adapt quickly and fail dangerously — carbon monoxide has no smell at all.
• Handheld portable detectors: effective but costly per unit, and useless when nobody happens to be carrying one into the room.
• Industrial fixed systems: approved and reliable, but priced in lakhs, which small leaseholders do not spend.
• Limitations: no continuous display, no automatic response, and a long delay between hazard and ventilation.

Proposed system

• Three sensing channels: MQ-2 (combustible gas as a methane/LPG proxy), MQ-7 (carbon monoxide) and LM35 (ambient temperature).
• Digitisation through the ADC0808 with a stable reference, so thresholds are real engineering values.
• Two-level thresholds with hysteresis: a warning level sounds the buzzer, an alarm level starts the exhaust fan; the fan stops only when all channels recover below their clear levels.
• A stuck-sensor check that flags a channel whose reading never changes.
• LCD showing each channel, buzzer alarm and a test/reset switch, with the fan driver demonstrated on a 12 V fan.

Feasibility

• Technical: the 8051, ADC, LCD and relay interfacing are exactly the Microcontroller and Embedded Systems practical syllabus; the gas sensors are standard modules with published datasheets.
• Economic: indicative component cost is about ₹2,900–3,300 including the fan and contactor — a small fraction of any industrial fixed system.
• Operational: the unit mounts on a wall, runs from one mains adapter and needs no operator.
• Schedule: fits the 14-week project semester, with simulation in the first half and hardware and calibration in the second.

Safety boundary

This is an educational prototype. A real mine installation requires DGMS-approved, intrinsically safe equipment and statutory procedures, and the report states this boundary clearly — the project demonstrates the engineering method, not a certified safety product.

In scope

• One fixed monitoring point with three channels: MQ-2, MQ-7 and LM35.
• Two-level thresholds (warning, alarm) plus a clear level, with hysteresis on each channel.
• Automatic exhaust-fan control demonstrated on a 12 V DC fan through a relay, with the 230 V contactor stage explained and tested with a lamp load under supervision.
• LCD display of live values and fault states, buzzer alarm, test/reset switch.
• Bump-test calibration, response/recovery time table and cost estimate.

Out of scope

• Certified, intrinsically safe mine equipment, DGMS approval, or installation in a live mine — this prototype is demonstrated only in the laboratory.
• Methane-specific catalytic or infrared sensing; the MQ-2 is used only as a combustible-gas proxy for demonstration.
• Data logging to memory card, wireless alerts and multi-point networks (future scope).
• Respirable-dust concentration measurement, which is a separate instrument class; the report discusses it as an extension.

We follow a design, simulate, build, test sequence organised as a V-model: every design block has a matching verification planned before it is built.

Threshold table (fixed for the demonstration, justified from datasheets and DGMS literature): CO warning 50 ppm-scale reading, alarm 100 ppm-scale; combustible gas warning 20 per cent of the MQ-2 span, alarm 35 per cent; temperature warning 45 °C. A 10 per cent hysteresis between alarm and clear levels prevents relay chatter. Testing approach: stage-wise voltage checks, logic tests by forcing thresholds with known sources, response-time tests with a stopwatch, and repeatability over five trials per channel; every case is recorded with input, expected, observed and status in the report, and each objective maps to at least one test case.

The monitor is organised in five blocks around the AT89S52.

1. Power supply: 230 V to 12-0 V step-down transformer, bridge rectifier, filter capacitor, 7812 for the relay stage and 7805 for the microcontroller, ADC and sensors. The MQ-7 heater cycle is driven from a transistor switch on a dedicated port pin.
2. Sensing: the MQ-2 and MQ-7 modules output an analogue voltage that falls with gas concentration, and the LM35 gives 10 mV/°C. Each channel passes through an RC filter and an op-amp buffer (LM358) into the ADC0808's multiplexer.
3. Digitisation: the ADC0808's address lines are latched to select a channel; the MCU issues start-of-conversion, waits for end-of-conversion and reads the eight-bit result. Its clock comes from the 8051 ALE pin.
4. Microcontroller: AT89S52 with 11.0592 MHz crystal; Port 1 for ADC data, Port 2 for the LCD in 4-bit mode, Port 3 for buzzer, MQ-7 heater switch, relay driver and switches.
5. Output stage: BC547 driving a 12 V relay with a 1N4007 flyback diode; the relay contact energises the fan circuit (demonstrated on a 12 V fan, with the 230 V contactor stage tested separately under supervision).

flowchart TD
  PS["Power supply (12 V and 5 V)"] --> MCU["AT89S52 microcontroller"]
  MQ2["MQ-2 combustible gas module"] --> BUF["RC filter + LM358 buffer"]
  MQ7["MQ-7 carbon monoxide module"] --> BUF
  LM["LM35 temperature sensor"] --> BUF
  BUF --> ADC["ADC0808 8-channel A/D converter"]
  ADC -->|"8-bit data + EOC handshake"| MCU
  MCU -->|"heater cycle control"| MQ7
  MCU --> LCD["16x2 LCD display"]
  MCU --> BZ["Buzzer alarm"]
  MCU --> RD["Relay driver with flyback diode"]
  RD --> FAN["Exhaust fan: 12 V demonstration / contactor for 230 V"]
  SW["Test and reset switches"] --> MCU

flowchart TD
  S["Read all channels and convert"] --> C{"Any channel above alarm level?"}
  C -->|"Yes"| FANON["Relay ON, buzzer ON, show channel"]
  C -->|"No"| W{"Any channel above warning level?"}
  W -->|"Yes"| BZON["Buzzer ON, fan unchanged"]
  W -->|"No"| CL{"Fan ON and all channels below clear level?"}
  CL -->|"Yes"| FANOFF["Relay OFF, buzzer OFF"]
  CL -->|"No"| ST{"Any channel unchanged for 5 min?"}
  ST -->|"Yes"| FLT["Show SENSOR FAULT on LCD"]
  ST -->|"No"| S
  FANON --> S
  BZON --> S
  FANOFF --> S
  FLT --> S

Firmware structure (Embedded C, Keil C51): a main loop reads each ADC channel three times 20 ms apart and takes the median; calibration factors convert counts to engineering units; the control routine implements the state machine above; Timer 0 provides the MQ-7 heater's 60 s high / 90 s low cycle and the stuck-sensor timer. Alarm and fan decisions require three consecutive readings above threshold, so a passing vehicle's exhaust puff cannot start the fan on its own.

• Data logging to a memory card or serial EEPROM so the office can review trend charts, not just alarms.
• GSM or Wi-Fi alerts to the foreman's phone on alarm or fault.
• Dust (PM10/PM2.5) channel using an optical dust sensor, since respirable dust is the other statutory exposure.
• Multi-point monitoring with several units reporting to one display over RS-485.
• PLC integration, since larger operations already run PLC-based ventilation circuits — the monitor could feed one input to the existing PLC.
• Methane-specific sensing with a catalytic or infrared sensor for gassy seams, moving from proxy to real measurement.