Here we use MQ3 gas sensor. This sensor is a 6 pin sensor . Top and bottom of the sensor is covered with 100 mesh stainless steel wire cloth. The heart of the sensor is the cylindrical form in the middle of the unit. The cylinder is a ceramic material with the SnO2 material deposited on its surface. The heater coil is located inside the ceramic cylinder. The heater has a resistance of 38 ohms.
Output of the gas sensor is connected the ic 555 as a monostable trigger ic. As the sensor is detect a gas then ic 555 activate automatically and then output of the ic 555 is connected to the microcontroller circuit.
In the microcontroller circuit we use ic 89s51 controller. Use of this is is to control the one or many relay logic and at the same time show the message on the lcd display. If no gas is detected then display shows a everything is ok . If the gas is detected then show a warning message on the screen and at the same time relay off the supply unit.
Two output is available from the controller from port P2. One output is buzzer and second output is relay output.
STEP DOWN TRANSFORMER 220 TO 9 VOLT AC
DIODE IN 4007(2)
CAPACITOR ,1000 MFD (1), 33 MFD (1), 27 PF (2).
NPN TRANSISTOR BC 548(4)
PNP TRANSISTOR BC 558(2)
RESISTOR 100 K (2), 1 K (4), 470 OHM (1), 10 K (3)
PULL UP RESISITOR ARRAY. 10 K (1)
VARIABLE RESISTOR 4.7 K (1)
RELAY 12 VOLT
Buzzer is on when circuit sense a gas and relay switch off the supply chain.