CHAPTER-1

Introduction:

Alerts the user via audio and visual inSdications. The heart of this The simple engineering

project Here is a LPG gas leakage sensor circuit that detects the outflow of LPG gas and

circuit is a gas sensor module SEN 1327. QM 6 gas sensor is used in the SEN 1327 module.

The output signal from SEN 1327 gas sensor module is used to drive a 555 timer based

astable multivibrator circuit. Here 555 timer works as a tone generator, the frequency of tone

can be altered by varying the preset VR1.

1

Chapter- 2

Concept of Project:

The gas leakage alarm circuit operates off a 9V PP3 battery. Zener diode ZD1 is used to

convert 9V into 5V DC to drive the gas sensor module. 

A preset in the module is used to set the threshold. . Interfacing with the sensor module is

done through a 4-pin SIP header.  

Whenever there is LPG concentration of 1000 ppm in the area, the OUT pin of the sensor

module goes high. This signal drives timer IC 555, which is wired as an astable multivibrator.

The multivibrator basically works as a tone generator. 

Output pin 3 of IC 555 is connected to LED1 and speaker-driver transistor SL100 through

current-limiting resistors R5 and R4, respectively. LED1 glows and the alarm sounds to alert

the user of gas leakage. The pitch of the tone can be changed by varying preset VR1. Use a

suitable heat-sink for transistor SL100. 

2

Chapter 3

Project Details:

3.1 Circuit Diagram:

Fig.:3.1 Gas Leakage Alarm

Fig.:3.2 MQ-6 Sensor

3

3.2 Working:

The gas leakage alarm circuit operates off a 9V PP3 battery. Zener diode ZD1 is used to

convert 9V into 5V DC to drive the gas sensor module. 

A preset in the module is used to set the threshold. . Interfacing with the sensor module is

done through a 4-pin SIP header.  

Whenever there is LPG concentration of 1000 ppm in the area, the OUT pin of the sensor

module goes high. This signal drives timer IC 555, which is wired as an astable multivibrator.

The multivibrator basically works as a tone generator. 

Output pin 3 of IC 555 is connected to LED1 and speaker-driver transistor SL100 through

current-limiting resistors R5 and R4, respectively. LED1 glows and the alarm sounds to alert

the user of gas leakage. The pitch of the tone can be changed by varying preset VR1. Use a

suitable heat-sink for transistor SL100. 

Here is a LPG gas leakage sensor circuit that detects the outflow of LPG gas and alerts the

user via audio and visual indications. The heart of this The simple engineering project circuit

is a gas sensor module SEN 1327. QM 6 gas sensor is used in the SEN 1327 module. The

output signal from SEN 1327 gas sensor module is used to drive a 555 timer based astable

multivibrator circuit. Here 555 timer works as a tone generator, the frequency of tone can be

altered by varying the preset VR1.

4

3.3 Pcb Layout:

Fig.: 3.3 Layout

5

Chapter- 4

Components Required:

1. Resistors (22Ω, 4.7k Ω, 10k Ω, 1k Ω, 560 Ω) 

2. Preset (VR1=10k Ω) 

3. Zener diode (5V) 

4. Capacitors (100nF x 2) 

5. Gas sensor module SEN 1327 

6. 555 timer IC 

7. Transistor (SL 100) 

8. Loudspeaker (8 Ω, 0.5W) 

9. LED

6

Chapter- 5

5.1 555 timer IC

Fig.: 5.1 NE555 dual-in-line package

Fig.:5.2 Internal block diagram

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation,

and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and

as a flip-flop element. Derivatives provide up to four timing circuits in one package.

Introduced in 1972 by Signetics, the 555 is still in widespread use, thanks to its ease of use,

low price, and good stability. It is now made by many companies in the original bipolar and

also in low-power CMOS types. As of 2003, it was estimated that 1 billion units are

manufactured every year.

7

Design

The IC was designed in 1971 by Hans R. Camenzind under contract to Signetics, which was

later acquired by Philips.

Depending on the manufacturer, the standard 555 package includes 25 transistors, 2 diodes

and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8).[2]

Variants available include the 556 (a 14-pin DIP combining two 555s on one chip), and the

two 558 & 559s (both a 16-pin DIP combining four slightly modified 555s with DIS & THR

connected internally, and TR is falling edge sensitive instead of level sensitive). There is no

557.

The NE555 parts were commercial temperature range, 0 °C to +70 °C, and the SE555 part

number designated the military temperature range, −55 °C to +125 °C. These were available

in both high-reliability metal can (T package) and inexpensive epoxy plastic (V package)

packages. Thus the full part numbers were NE555V, NE555T, SE555V, and SE555T. It has

been hypothesized that the 555 got its name from the three 5 kΩ resistors used within, but

Hans Camenzind has stated that the number was arbitrary.

Low-power versions of the 555 are also available, such as the 7555 and CMOS TLC555. The

7555 is designed to cause less supply noise than the classic 555 and the manufacturer claims

that it usually does not require a "control" capacitor and in many cases does not require a

decoupling capacitor on the power supply. Such a practice should nevertheless be avoided,

because noise produced by the timer or variation in power supply voltage might interfere with

other parts of a circuit or influence its threshold voltage

8

The connection of the pins for a DIP package is as follows:

Pin Name Purpose1 GND Ground, low level (0 V)

2 TRIG OUT rises, and interval starts, when this input falls below 1/2 of CTRL voltage.

3 OUT This output is driven to approximately 1.7V below +VCC or GND.

4 RESET

A timing interval may be reset by driving this input to GND, but the timing does

not begin again until RESET rises above approximately 0.7 volts. Overrides

TRIG which overrides THR.

5 CTRL "Control" access to the internal voltage divider (by default, 2/3 VCC).

6 THR The interval ends when the voltage at THR is greater than at CTRL.

7 DISOpen collector output; may discharge a capacitor between intervals. In phase

with output.

8 VCC Positive supply voltage is usually between 3 and 15 V.

Note-PIN 5 is also called control voltage pin! By applying a voltage to the CONTROL

VOLTAGE input, pin 5, you can alter the timing characteristics of the device. In most

applications, the CONTROL VOLTAGE input is not used. It is usual to connect a 10 nF

capacitor between pin 5 and 0 V to prevent interference. The CONTROL VOLTAGE input

can be used to build an astable with a frequency modulated output.

Modes

The 555 has three operating modes:

Monostable mode: in this mode, the 555 functions as a "one-shot" pulse generator.

Applications include timers, missing pulse detection, bouncefree switches, touch switches,

frequency divider, capacitance measurement, pulse-width modulation (PWM) and so on.

Astable: free running mode: the 555 can operate as an oscillator. Uses include LED and lamp

flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position

modulation and so on. The 555 can be used as a simple ADC, converting an analog value to a

pulse length. E.g. selecting a thermistor as timing resistor allows the use of the 555 in a

temperature sensor: the period of the output pulse is determined by the temperature. The use

9

of a microprocessor based circuit can then convert the pulse period to temperature, linearize it

and even provide calibration means.

Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not

connected and no capacitor is used. Uses include bounce-free latched switches.

Monostable

Fig.:5.3 Schematic of a 555 in monostable mode

Fig.: 5.4 The relationships of the trigger signal, the voltage on C and the pulse width in

monostable mode

In the monostable mode, the 555 timer acts as a "one-shot" pulse generator. The pulse begins

when the 555 timer receives a signal at the trigger input that falls below a third of the voltage

supply. The width of the output pulse is determined by the time constant of an RC network,

which consists of a capacitor (C) and a resistor (R). The output pulse ends when the voltage

10

on the capacitor equals 2/3 of the supply voltage. The output pulse width can be lengthened

or shortened to the need of the specific application by adjusting the values of R and C.

The output pulse width of time t, which is the time it takes to charge C to 2/3 of the supply

voltage, is given by

where t is in seconds, R is in ohms and C is in farads.

While using the timer IC in monostable mode, the main disadvantage is that the time span

between any two triggering pulses must be greater than the RC time constant.

Bistable

Fig.: 5.5 Schematic of a 555 in bistable mode

In bistable mode, the 555 timer acts as a basic flip-flop. The trigger and reset inputs (pins 2

and 4 respectively on a 555) are held high via Pull-up resistors while the threshold input (pin

6) is simply grounded. Thus configured, pulling the trigger momentarily to ground acts as a

'set' and transitions the output pin (pin 3) to Vcc (high state). Pulling the reset input to ground

acts as a 'reset' and transitions the output pin to ground (low state). No capacitors are required

in a bistable configuration. Pin 5 (control) is connected to ground via a small-value capacitor

(usually 0.01 to 0.1 uF); pin 7 (discharge) is left floating.

11

Astable

Fig.: 5.6 Standard 555 astable circuit

In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a

specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and

another resistor (R2) is connected between the discharge pin (pin 7), and the trigger (pin 2)

and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through

R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during

output low intervals of the cycle, therefore discharging the capacitor.

In the astable mode, the frequency of the pulse stream depends on the values of R1, R2 and C:

The high time from each pulse is given by:

and the low time from each pulse is given by:

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in

farads.

12

The power capability of R1 must be greater than .

Particularly with bipolar 555s, low values of R1 must be avoided so that the output stays

saturated near zero volts during discharge, as assumed by the above equation. Otherwise the

output low time will be greater than calculated above. It should be noted that the first cycle

will take appreciably longer than the calculated time, as the capacitor must charge from 0V to

2/3 of VCC from power-up, but only from 1/3 of VCC to 2/3 of VCC on subsequent cycles.

To achieve a duty cycle of less than 50% a diode (that is fast enough for the application) can

be added in parallel with R2 towards the capacitor. This bypasses R2 during the high part of

the cycle so that the high interval depends approximately only on R1 and C. The presence of

the diode is a voltage drop that slows charging on the capacitor so that the high time is longer

than the often-cited ln(2)*R1C = 0.69 R1C. The low time is the same as without the diode as

shown above. With a diode, the high time is

where Vdiode is determined when the diode has a current of 1/2 of Vcc/R1. As a extreme

example, when Vcc= 5 and Vdiode= 0.7, high time = 1.00 R1C which is 45% longer than the

"expected" 0.693 R1C. At the other extreme, when Vcc= 15 and Vdiode= 0.3, high time = 0.725

R1C, 4.6% longer. The equation reduces to 0.693 R1C if Vdiode= 0.

The operation of RESET in this mode is not well defined, some manufacturers' parts will

hold the output state to what it was when RESET is taken low, others will send the output

either high or low

13

5.2 MQ-3 Gas Sensor

This is an alcohol sensor from futurlec, named MQ-3, which detects ethanol in the air. It is

one of the straightforward gas sensors so it works almost the same way with other gas

sensors. It costs $6.90.Typically, it is used as part of the breathalyzers or breath testers for the

detection of ethanol in the human breath.

Fig.:5.7 MQ-3 Gas Sensor

14

Datasheet

Here is a datasheet, only 2 pages. It shows features, applications, specifications and

configurations etc. It is a pretty simple datasheet. Since this datasheet was not prepared in

English,the translation is not very accurate.

How It Looks

Basically, it has 6pins, the cover and the body. Even though it has 6 pins, you can use only 4

of them. Two of them are for the heating system, which I call H and the other 2 are for

connecting power and ground, which I called A and B.

If you look at the inside of the sensor, you will find the little tube. Basically, this tube is a

heating system that is made of aluminum oxide and tin dioxide and inside of it there are

heater coils, which practically produce the heat. And you can also find 6 pins. 2 pins that I

called Pin H are connected to the heater coils and the other ones are connected to the tube.

15

How It Works

How does it work? The core system is the cube. As you can see in this cross-sectional view,

basically, it is an Alumina tube cover by SnO2, which is tin dioxide. And between them there

is an Aurum electrode, the black one. And also you can see how the wires are connected. So,

why do we need them? Basically, the alumina tube and the coils are the heating system, the

yellow, brown parts and the coils in the picture.

Fig.:5.8 Cross Sectional View

16

SnO2 ceramics will become the semi - conductor, so there are more movable electrons, which

means that it is ready to make more current flow.

Then, when the alcohol molecules in the air meet the electrode that is between alumina and

tin dioxide, ethanol burns into acetic acid then more current is produced. So the more alcohol

molecules there are, the more current we will get. Because of this current

17

5.3 Resistor:

Fig.:5.9 Resistor

A resistor is a two-terminal electronic component that produces a voltage across its terminals

that is proportional to the electric current through it in accordance with Ohm's law:

[V = IR]

Resistors are elements of electrical networks and electronic circuits and are ubiquitous in

most electronic equipment. Practical resistors can be made of various compounds and films,

as well as resistance wire (wire made of a high-resistivity alloy, such as nickel-chrome).

The primary characteristics of a resistor are the resistance, the tolerance, the maximum

working voltage and the power rating. Other characteristics include temperature coefficient,

noise, and inductance. Less well-known is critical resistance the value below which power

dissipation limits the maximum permitted current, and above which the limit is applied

voltage. Resistors can be integrated into hybrid and printed circuits, as well as integrated

circuits. Size, and position of leads (or terminals), are relevant to equipment designers;

resistors must be physically large enough not to overheat when dissipating their power.

18

5.4 Variable Resistor:

Fig.:5.10 Variable Resistor

A resistor may have one or more fixed tapping points so that the resistance can be changed by

moving the connecting wires to different terminals. Some wire wound power resistors have a

tapping point that can slide along the resistance element, allowing a larger or smaller part of

the resistance to be used. Where continuous adjustment of the resistance value during

operation of equipment is required, the sliding resistance tap can be connected to a knob

accessible to an operator. Such a device is called a rheostat and has two terminals.

19

5.5 Zener Diode:

Fig.:5.11 Zener Diode

The zener diode's operation depends on the heavy doping of its p-n junction. The depletion

region formed in the diode is very thin (<0.000001 m) and the electric field is consequently

very high (about 500000 V/m) even for a small reverse bias voltage of about 5 V,

allowing electrons to tunnel from the valence band of the p-type material to the conduction

band of the n-type material.

In the atomic scale, this tunneling corresponds to the transport of valence band electrons into

the empty conduction band states; as a result of the reduced barrier between these bands and

high electric fields that are induced due to the relatively high levels of dopings on both

sides. The breakdown voltage can be controlled quite accurately in the doping process. While

tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%.

Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to

200 volts.

20

5.6 TRANSISTOR:

INTRODUCTION:

A transistor is a semiconductor device used to amplify and switch electronic signals and

power. It is composed of a semiconductor material with at least three terminals for

connection to an external circuit. A voltage or current applied to one pair of the transistor's

terminals changes the current flowing through another pair of terminals. Because the

controlled (output) power can be higher than the controlling (input) power, a transistor can

amplify a signal. Today, some transistors are packaged individually, but many more are

found embedded in integrated circuits.

The transistor is the fundamental building block of modern electronic devices, and is

ubiquitous in modern electronic systems. Following its development in the early 1950s the

transistor revolutionized the field of electronics, and paved the way for smaller and cheaper

radios, calculators, and computers, among other things.

Fig.:5.12 Transistor

5.6.1 TYPES:

Transistors are categorized bySemiconductor material (date first used): the metalloids

germanium (1947) and silicon (1954)— in amorphous, polycrystalline and monocrystalline

21

form; the compounds gallium arsenide (1966) and silicon carbide (1997), the alloy silicon-

germanium (1989), the allotrope of carbon graphene (research ongoing since 2004), etc.—see

Semiconductor material

Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types"

Electrical polarity (positive and negative) : NPN, PNP (BJTs); N-channel, P-channel

(FETs)Maximum power rating: low, medium, high Maximum operating frequency: low,

medium, high, radio frequency (RF), microwave (The maximum effective frequency of a

transistor is denoted by the term , an abbreviation for transition frequency—the frequency of

transition is the frequency at which the transistor yields unity gain)

Application: switch, general purpose, audio, high voltage, super-beta, matched pair Physical

packaging: through-hole metal, through-hole plastic, surface mount, ball grid array, power

modules—see Packaging

Amplification factor hfe or βF (transistor beta) Thus, a particular transistor may be described

as silicon, surface mount, BJT, NPN, low power, high frequency switch.

5.7 Capacitor:

22

Fig.:5.13 Capacitors

A capacitor (formerly known as condenser) is a device for storing electric charge. The forms

of practical capacitors vary widely, but all contain at least two conductors separated by a non-

conductor. Capacitors used as parts of electrical systems, for example, consist of metal foils

separated by a layer of insulating film.

A capacitor is a passive electronic component consisting of a pair of conductors separated by

a dielectric (insulator). When there is a potential difference across the conductors, a static

electric field develops across the dielectric, causing positive charge to collect on one plate

and negative charge on the other plate. Energy is stored in the electrostatic field. An ideal

capacitor is characterized by a single constant value, capacitance, measured in farads. This is

the ratio of the electric charge on each conductor to the potential difference between them.

Capacitors are widely used in electronic circuits for blocking direct current while allowing

alternating current to pass, in filter networks, for smoothing the output of power supplies, in

the resonant circuits that tune radios to particular frequencies and for many other purposes.

The capacitance is greatest when there is a narrow separation between large areas of

conductor, hence capacitor conductors are often called "plates", referring to an early means of

construction. In practice the dielectric between the plates passes a small amount of leakage

current and also has an electric field strength limit, resulting in a breakdown voltage, while

the conductors and leads introduce an undesired inductance and resistance.

5.8 Switches:

In electronics, a switch is an electrical component that can break an electrical circuit,

interrupting the current or diverting it from one conductor to another.

23

Fig.:5.14 Switches

The most familiar form of switch is a manually operated electromechanical device with one

or more sets of electrical contacts, which are connected to external circuits. Each set of

contacts can be in one of two states: either "closed" meaning the contacts are touching and

electricity can flow between them, or "open", meaning the contacts are separated and the

switch is non conducting. The mechanism actuating the transition between these two states

(open or closed) can be either a "toggle" (flip switch for continuous "on" or "off") or

"momentary" (push-for "on" or push-for "off") type.

A switch may be directly manipulated by a human as a control signal to a system, such as a

computer keyboard button, or to control power flow in a circuit, such as a light switch.

5.9 Battery:

An electrical battery is one or more electrochemical cells that convert stored chemical energy

into electrical energy. Since the invention of the first battery (or "voltaic pile") in 1800 by

Alessandro Volta, batteries have become a common power source for many household and

24

industrial applications. According to a 2005 estimate, the worldwide battery industry

generates US$48 billion in sales each year, with 6% annual growth.

Fig.:5.15 Battery

There are two types of batteries: primary batteries (disposable batteries), which are designed

to be used once and discarded, and secondary batteries (rechargeable batteries), which are

designed to be recharged and used multiple times. Batteries come in many sizes, from

miniature cells used to power hearing aids and wristwatches to battery banks the size of

rooms that provide standby power for telephone exchanges and computer data centers.

Chapter-6

Advantages & Disadvantages:

Recommended for checking gas leaks in pipeline joints/flanges

25

Quick response to combustible gas Leak

Miniature size, single handed operation design

Long and flexible goose-neck tube

Adjustable sensitivity

Automatic quick warm-up, quick response time

Audio level following gas concentration

10 Level light for gas leak concentration

Hi-HM battery, continuous 8 hours working

Low battery alarming indication

Chapter-7

Applications:

The ability to determine the location of the leak.

26

Tthe detection speed

The ability to estimate the size of the leak.

High flow rates while at low flow rates a mass balance

It can also detect gases like isopropyl alcohol, (methanol, ethanol, butane) ether, ketone

(butane, proton), hydrogen, and toluen.

Conclusion

LPG gas is supplied in pressurised steel cylinders. As this gas is heavier than air, when it

leaks from a cylinder it flows along floor and tends to settle in low spots such  as a basement.

This can cause fire or suffocation if not dealt with. 

27

Here is a circuit that detects the leakage of LPG gas and alerts the user through audio-visual

indications. 

Zener diode ZD1 is used to convert 9V into 5V DC to drive the gas sensor module. 

The SEN-1327 gas sensor module from RhydoLABZ is used in this circuit. Its output goes

high when the gas level reaches or exceeds certain point. A preset in the module is used to set

the threshold. Interfacing with the sensor module is done through a 4-pin SIP header.  

Whenever there is LPG concentration of 1000 ppm (parts per million) in the area, the OUT

pin of the sensor module goes high. This signal drives timer IC 555, which is wired as an

astable multivibrator. The multivibrator basically works as a tone generator. 

It was experience in making this project as this project consist of so many components hence

it give a chance to know about various electronics components. We have successfully

completed this project “Gas Leakage Alarm”. We sincerely thanks Mr. Naresh Soni and Mr.

Nitin Jain for their invaluable guidance and all the lab assistants for their constant support

throughout the making of the project.

References

1. http://www.electronicsforu.com/electronicsforu/circuitarchives/view_article.asp?

sno=600&title=gas sensor

2. http://en.wikipedia.org/wiki/Electronic_component

3. http://octopart.com/tda2005m-stmicroelectronics-407800

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4. en.wikipedia.org/wiki/555Timer

29