Datasheet
11.
PRACTICAL
DETECTOR CIRCUIT USING THE
#813
SENSOR
As was mentioned above the TGS
#813
is suitable for use in the detection of
a wide range of
gases
such as naturalgas, L.P.G. and town gas.
Wben
you
design a circuit employing the TGS
#813
you
must consider both the type and
concentration of gas you wish to detect.
Because of its sensitivity characteristics, the
action
of the
Sensor
will
vary according to the type and concentration of gas it is detecting.
Furthermore,
the proper alarm
Point
for the detector should be determined
after considering the following
factors,
a. Where the
Sensor
is to be installed
b. Purpose of detector (gas leak,
automatic
fan
control, air monitoring, etc.)
c. Operation of detector
(Sound,
light, fan control, valve control, etc.)
d. Type of gas being detected or monitored.
When setting the
actual
alarm
Point,
we recommend that you
calibrate
the
detector for 5-10 % of the L.E.L. of the gas being detected.
This figure was decided to meet the requirements for high sensitivity while
at the same time reducing the
Problem
of nuisance alarming.
This is an
extremely
important consideration for
domestic
gas-leak
detectors
because
of the presence of 'noise
gases'
in the home enviroment such as those
result-
ing from
hair-Spray,
alcohol
fumes
or
cooking fumes.
EXAMPLE CIRCUIT
"
Fig. 14 is an example of a simple and economic circuit for a
domestic
gas
leak
detectors.
It
is
designed primarily for the detection of
approxi-
mately
3000 ppm
of methane (natura1 gas).
The
constant
5V output of voltage regulator
1,
is
available for the heater
of the
Sensor
and the detecting circuit.
The detecting circuit consists
of the TGS
11813,
R1,
and
RADJ
in series.
vRl#
output,
which
is the output voltage
across
resistors
R1
and
R~J,
enters the non-inverting input of the
comparator.
Vr,
which
is the reference voltage for the
comparator,
is measured
across
R
4-
This component
is
part of the Temperature Compensating Circuit
which
also
consists-
of
RZ,
R3
AND
Rt
(thermistor).
The Vr value in this circuit
is
designed for
2.5V
at
20°C.
Once you
calibrate
the detector at 3000 ppm by adjusting
R~J
(under
the
conditions 20°C at 65% R.H.) you
tan
get an approximate
2.5V(v~~)
in a
concentration of 3000 ppm methane.
When a combustible gas such as natura1 gas contacts the
Sensor
and the
output of the detection circuit (V,) exceeds the Vr, the output of the
comparator
will go to 'High'.
TR
is then activated and the buzzer is
1
sounded.