LPG Gas detector Circuit design and prototype (Lab Experiment)

1. Design Lab Report
On
Design of Electronic circuit for Gas sensor
array
Supervised By
Prof. M.C. Bhattnagar
Submitted by
Ajay Singh
at
E n g i n eeri n g Ph ys i c s D e p artmen t
I n d i an I n s ti tu te o f T ec h n o l o g y D el h i
J u l y 2 0 1 3 – No vemb er 2 0 1 3
Introduction:

2. A gas detector is a device which detects the presence of various gases within an area, usually as
part of a safety system. This type of equipment is used to detect a gas leak and interface with a
control system so a process can be automatically shut down. A gas detector can also sound an
alarm to operators in the area where the leak is occurring, giving them the opportunity to leave
the area. This type of device is important because there are many gases that can be harmful to
organic life, such as humans or animals.
Gas detectors can be used to detect combustible, flammable and toxic gases, and oxygen
depletion. This type of device is used widely in industry and can be found in a variety of
locations such as on oil rigs, to monitor manufacture processes and emerging technologies such
as photovoltaic. They may also be used in fire fighting.
As detectors measure a gas concentration, the sensor responds to a calibration gas, which serves
as the reference point or scale. As a sensor’s detection exceeds a present alarm level, the alarm or
signal will be activated. As units, gas detectors are produced as portable or stationary devices.
Originally, detectors were produced to detect a single gas, but modern units may detect several
toxic or combustible gases, or even a combination of both types.
2. Working Principles and Structure of Measuring Head
2.1 Operating principle of semiconductor type gas sensors
This operating principle applies to all TGS8xx,
TGS2xxx and TGS3xxx series sensors.
Fig. 1 shows basic gas sensor structure of
TGS26xx series sensors as an example.
The gas sensor is composed of sensing element,
sensor base and sensor cap. The sensing element
contains sensing material and heater to heat
upsensing element (eg. 400℃). Depending on the
target gas, the sensing element will utilize
different materials such as
Tin dioxide (SnO2), Tungsten oxide (WO3), etc.
When a metal oxide crystal such as SnO2 is heated
at a certain high temperature in air, oxygen is
adsorbed on the crystal surface with a negative
charge. Then donor electrons in the crystal surface
are transferred to the adsorbed oxygen, resulting in
leaving positive charges in a space charge layer.
Thus, surface potential is formed to serve as a
potential barrier against electron flow (Figure 2).
Inside the sensor, electric current flows through
the conjunction parts (grain boundary) of
SnO2 micro crystals. At grain boundaries,
adsorbed oxygen forms a potential barrier which
prevents carriers from moving freely. The
electrical resistance of the sensoris attributed to
http://aesensors.nl/wp-
content/uploads/2015/01/OPERATING-PRICIPLE.pdf

3. this potential barrier.
In the presence of a deoxidizing gas, the surface
density of the negatively charged oxyge decreases,
so the barrier height in the grain boundary is
reduced (Figures 3 and 4). The reduced barrier
height decreases sensor resistance.
The relationship between sensor resistance and the
concentration of deoxidizing gas can be expressed
by the following equation over a certain range
of gas concentration:
Rs = A[C] –α
where: Rs = electrical resistance of the sensor
A = constant
[C] = gas concentration
α = slope of Rs curve
Due to the logarithmic relationship between sensor
resistance and gas concentration, semiconductor
type sensors have an advantage of high sensitivity
to gas even at low gas concentration.
The excellent stability and performance of these
semiconductor type sensors provides maintenance-
free, long lived, and low cost gas detection.
Figaro provides various sensors which have
different sensitivity characteristics by selecting the
most suitable combinations of sensing material,
temperature and activity of sensor materials.
http://aesensors.nl/wp-content/uploads/2015/01/OPERATING-
PRICIPLE.pdf
2.2 TGS2610 gas sensor:
For this design we have used TGS2610 gas sensor. TGS2610 is a semiconductor type gas
sensor which combines very high sensitivity to LP gas with low power consumption and long
life. Due to miniaturization of its sensing chip, TGS2610 requires a heater current of only
56mA and the device is housed in a standard TO-5 package.

4. Figure5. Different type of TGS2610 gas sensor.
The TGS2610 is available in two different models which have different external housings but
identical sensitivity to LP gas. Both models are able to satisfy the requirements of
performance standards such as UL1484 and EN50194.
1. TGS2610-C00 possesses small size and quick gas response, making it suitable for gas
leakage checkers.
2. TGS2610-D00 uses filter material in its housing which eliminates the influence of
interference gases such as alcohol, resulting in highly selective response to LP gas.
This feature makes the sensor ideal for residential gas leakage detectors which require
durability and resistance against interference gas.
The figure below represents typical sensitivity characteristics; all data having been gathered
at standard test conditions. The Y-axis is indicated as sensor resistance ratio (Rs/Ro) which is
defined as follows:
Rs = Sensor resistance in displayed gases at various concentrations
Ro = Sensor resistance in 1800ppm of iso-butane
Figure6. Plot shows the sensitivity characteristics of TGS2610-C00 and TGS2610-D00
(http://micros.com.pl/en/catalog/129035/czujniki-gaz%C3%B3w-figaro)

5. 2.2.1. Basic Measuring Circuit:
The sensor requires two voltage inputs: heater voltage (VH) and circuit voltage (VC). The
heater voltage (VH) is applied to the integrated heater in order to maintain the sensing
element at a specific temperature which is optimal for sensing. Circuit voltage (VC) is
applied to allow measurement of voltage (VRL) across a load resistor (RL) which is
connected in series with the sensor. A common power supply circuit can be used for both
VC and VH to fulfill the sensor's electrical requirements. The value of the load resistor
(RL) should be chosen to optimize the alarm threshold value, keeping power dissipation
(PS) of the semiconductor below a limit of 15mW. Power dissipation (PS) will be highest
when the value of RS is equal to RL on exposure to gas.
2.2.2. Specifications:
http://micros.com.pl/en/catalog/129035/czujniki-gaz%C3%B3w-figaro

6. 2.2.3. Structure and Dimensions:
In TGS 2610, there are four different pin connections. Pin connection:
1: Heater
2: Sensor electrode (-)
3: Sensor electrode (+)
4: Heater
The value of power dissipation (PS) can be calculated by utilizing the following formula:
Sensor resistance (Rs) is calculated with a measured value of VRL by using the following
formula:
3.Experiment procedure
3.1 Circuit components
I. Transformer (15-015/220v, 1A)
II. Bridge rectifier (four-IN4007 diodes)
III. Filter circuit (capacitors –two 2200µf,two 100nf ,two 220 µf ,
ICs-7812&7912 )
IV. Gas sensor (TGS2610)
V. Potential divider (Resisters 10kΩ, 4.7 kΩ)
VI. LEDs (Green,Red,Yellow)
VII. IC LM324
VIII. PCB
3.2 Circuit Diagram:

7. Fig. Circuit diagram [..]
Figure : Image of the designed circuit for a single sensor.
When there is LPG in surrounding , The gas sensor’s resistance changes drastically ,which
reflects there as a change in voltage across the potential divider. Correspondingly voltage
across LED increases and LED glows.

8. Discussion:
The designed circuit is basic circuit which contains gas sensor (TGS2610) to detect LPG
gas . Similarly other sensors can be used to detect another gases using the same circuit. In the
circuit we can also amplify the output which can be used to supply for electric bell to alarm.
The circuit can also be extended to supply a trigger circuit or electric relay for atomisation of
a system. The same circuit can also be used for multiple gas sensing using an array of
sensors. The circuit is based on the change in resistance , so using multiple colour LEDs can
also be used which will indicate the concentration like if only red glows it means lower
concentration of the leaked gas, if red and yellow LEDs glows it means higher concentration,
and if red,yellow and green LEDs glow means much higher leakage.
The circuit can also be interfaced with some data acquisition software’s to present the signal
on screen, then the signal data can also be recorded for further analysis.
References :
http://en.wikipedia.org/wiki/Gas_detector
http://www.figarosensor.com/products/2610pdf.pdf
http://www.tsdpl.com/gas_sensors/operating_principle.html
http://www.electronics-lab.com/projects/power/011/
http://www.raesystems.com/industry/industrial-safety
http://www.gasdetection.com/
http://www.thomasnet.com/articles/instruments-controls/How-Gas-Detectors-Work