Here in this project we show that how we detect a alcohols or any type of  gases and if we detect a gases then alarm is on and the same time lcd display shows the gas detection display. If the gas is detected then relay is off and break the supply of gas applied to the vehicle or we can control any type of electrical connection on or off by this relay operated logic.


Main part of this project is gas sensor.

Here we use TGS gas sensor. This sensor is a 6 pin sensor . Top and  bottom of the sensor is covered with 100 mesh stainless stell 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.


Logic circuit.

Gas sensor interface.

Schimtt trigger circuit.

Microcontroller interface.

Lcd interface.

Alarm  control

Relay on/off  control




IN the  gas sensor interface we use gas  sensor as a input logic. We attach one ic 555 as  with this sensor . Here ic 555 work as a monostable timer.  Sensor output is connected to the  ic 555 pin no 2 via npn transistor. Pin no 2 is a negative trigger ic. When sensor activate then positive output is available and  this positive output  is converted into  negative with the help of the NPN transistor. Output of the NPN transistor is now connected to the  pin no 2 of the ic 555. Pinno 4 and 8 of the ic 555 is connected to the positive supply. Pin no 1 is connected to the ground pin. Pin no 7 is the output pin. Pin no 6 and 7 is the time constant pin. Pin no 6 and 7 provide a time constant option. When ic 555 works then  output is available on the pin no 3 . Now this output is switching by the further NPN transistor.One led is also connected to the  transistor circuit. Output of the npn transistor is available on the collector point. This output  from the collector of NPN is  now connected to the microcontroller pin no 1 which is port p1.0. Pin no 40 of the  controller is connected to the positive supply. We supply a 5 volt regulated power supply on this pin. For regulated power supply we use one step down transformer  . Step down transformer step down the voltage from 220 volt ac to 9 volt ac. This ac is converted into  dc with the help of the rectifier circuit. Here we use two diode as a  full wave rectifier. Output of the rectifier is now converted into 5 volt regulated supply with the help of 7805 regulator. IC 7805 regulator  provide a 5 volt constant  supply to pin no 40 of the ic. Pin no 20 is connected to the ground pin.  LCD is connected to the port 0. Here we use 2 by 16 lcd for display. 2 by 16 means  2 line and 16 character.


Two output is available from the controller from port P2. One output is buzzer and second output    is relay output. Buzzer is on when  circuit sense a gas and  relay  switch off the supply chain.


To control a relay we use two transistor circuit. Output from the controller is  drive the pnp transistor through  1 k ohm resistor. Collector of the pnp transistor is connected to the npn transistor through 1 k ohm resistor. Collector of the NPN transistor is connected to the  relay coil  directly. . Here relay is for the switching purpose only . As the  transistor is on  relay is on and relay break the contact of supply  chain.





DIODE    IN 4007(2)

CAPACITOR  ,1000 MFD (1), 33 MFD (1), 27 PF (2).



RESISTOR  100 K (2), 1 K (4), 470 OHM (1), 10 K (3)



L.E.D (3)

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