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CH4 (Methane) Gas Sensor Module (#27930)
CO (Carbon Monoxide) Gas Sensor Module (#27931)
LPG (Propane) Gas Sensor Module (#27932)
C2H5OH (Alcohol) Gas Sensor Module (#27904)
These Gas Sensor Modules are designed to allow a microcontroller to determine when a preset gas level
has been reached or exceeded. The type of gas detected depends on which module you are using.
27930 detects Methane (CH4), 27931 detects Carbon Monoxide (CO), 27932 detects Propane (LPG) and
27904 detects Alcohol (C2H5OH). Interfacing with these sensors is done through a 4-pin SIP header and
requires two I/O pins from the host microcontroller. The sensor modules are intended to provide a
means of comparing gas sources and being able to set an alarm limit when the source becomes
excessive.
Parallax does not provide gas calibration data on these modules and such data as
well as the alarm settings are the responsibility of the user to define. For information on
calibration please see page 3.
Features
Easy SIP header interface
Compatible with most microcontrollers
Low-power standby mode
Application Ideas
Gas level over-limit alarm
Stand-alone/background sensing device
Environmental monitoring equipment
Key Specifications
Power requirements: 5 VDC @ ~165 mA
(heater on) / ~60 mA (heater off)
Interface: 1 TTL compatible input
(HSW), 1 TTL compatible output (ALR)
Operating temperature:
14 to 122 °F (-10 to 50°C)
Dimensions: 1.50 x 1.0 x 1.0 in
(38.1 x 25.4 x 25.4 mm)
Packing List
Gas Sensor
Gas Sensor PCB Assembly
Potentiometer adjustment tool
Copyright © Parallax Inc.
Gas Sensor Modules (#27930, #27931, #27932, #27904)
v2.2 8/2/2010 Page 1 of 6
Precautions
Be aware that the gasses detected by these gas sensors can be deadly in high
concentrations. Automotive exhaust and charcoal cooking grill exhausts emit carbon
monoxide (CO). Propane Gas (LPG) can leak from propane grills and furnaces and Methane
(CH4) can be found in animal farming areas. Always be careful to perform gas tests in well-
ventilated areas.
THESE GAS SENSOR MODULES ARE NOT DESIGNED FOR OR APPROVED FOR ANY
APPLICATION INVOLVING HEALTH OR HUMAN SAFETY. THESE GAS SENSOR MODULES ARE
FOR EXPERIMENTAL PURPOSES ONLY. PARALLAX, INC. ABSOLVES ITSELF OF ALL LIABILITY
AND RESPONSIBILITY ASSOCIATED WITH THE CUSTOMER'S USE OF THESE GAS SENSOR
MODULES AND IS NOT RESPONSIBLE FOR ANY BODILY INJURY, DEATH OR PROPERTY
DAMAGE AS A RESULT OF USING THESE GAS SENSOR MODULES.
Connecting and Testing
The gas sensor canister plugs into the socket on the front of the module. The gas sensors are essentially
resistive devices and are not polarized, so there is no need to be concerned about plugging it in
“backwards.” It will work in either orientation.
The 4-pin SIP header on the Gas Sensor Module makes it easy to connect to a breadboard or SIP socket.
The four connections are defined in the table below. Connection to a 5V microcontroller, such as the
BASIC Stamp
®
module, would be pretty straight forward and require two I/O pins; one input for detecting
the alarm signal and the other an output for controlling the internal heater.
For a 3.3V microcontroller such as the Propeller
TM
chip, a 3.9 kΩ
(10 kΩ could be used) resistor would be
required from the ALR output to the Propeller chip input pin. For the Propeller to control the heater
switch input (HSW) you would need an NPN switching transistor, such as a 2N3904 and a 1 kΩ
resistor.
The schematic for this connection is shown below.
Switching circuit for the Propeller
chip. Note: HIGH signal is
required to turn on heater due to
inversion of signal by the
transistor.
Copyright © Parallax Inc.
Gas Sensor Modules (#27930, #27931, #27932, #27904)
v2.2 8/2/2010 Page 2 of 6
Pin Definitions and Ratings
Pin
E1
E2
Name
ALR
HSW
Function
Alarm output to host microcontroller
Heat switch input from microcontroller, active low
Level
0V / 5V
0V / Floating*
5V
0V
0V – 5V
0V
0V – 5V
0V
E3
+5V
+5 VDC power
E4
GND
Ground, connects to common ground
TP1
Test Point 1+ Buffered output of sensors (voltage divider)
TP2
Test Point 2- Ground, connects to common ground
TP3
Test Point 3+ Trip Level voltage set by potentiometer (R4)
TP4
Test Point 4- Ground, connects to common ground
* The HSW line is internally pulled up to 5 V via a 10 kΩ
resistor.
NOTE: Sensor may rattle if shaken – this is normal.
Theory of Operation
These gas sensor modules use gas sensors from Hanwei Electronics. When their internal heating
elements are activated, these gas sensors respond to their specific gas by reducing their resistance in
proportion to the amount of that gas present in the air exposed to the internal element. On the gas
sensor modules this is part of a voltage divider formed by the internal element of each gas sensor and
potentiometer R3 (Set Point). The output of this voltage divider is fed into the non-inverting inputs of
the two op-amps on the LT1013 dual op-amp IC. Op-amp A is configured as a buffer with unity gain and
is used to provide a non-loaded test point for the signal voltage at TP1 (+) and TP2 (-). The signal
voltage is also being fed into op-amp B which is configured as a comparator that gets its reference
voltage at the inverting input from potentiometer R4 (Trip Level) and is also available at TP3 (+) and TP4
(-).
The output of op-amp B goes out to the ALR pin through a 1 kΩ
resistor providing a TTL-compatible
signal to a microcontroller. This output also connects to a red LED on the gas sensor modules. The zero
gas span adjustment is set via potentiometer R3. As mentioned above R3 allows you to change the
span/range of the voltage divider formed by the gas sensor and R3 which is the bottom leg of the divider,
electrically speaking. Adjusting R3 to lower values will make the gas sensor less sensitive but more
stable. Avoid setting R3 below 200 ohms as at this point you will be close to shorting the output to
ground. Setting R3 to higher values will make the gas sensor more responsive, but without a minimum
load it will become unstable after a certain point. The trip level adjustment is set via potentiometer R4.
This is just a simple voltage divider that lets you set the voltage from 0V to 5V. This voltage is compared
to the voltage coming from the gas sensor/R3 divider. When the voltage from the gas sensor is higher
than the voltage set by potentiometer R4 the red LED will light and the ALR output will be high (5 V).
The section below describes how to configure these gas sensor modules to detect gas with minimal
calibration.
Copyright © Parallax Inc.
Gas Sensor Modules (#27930, #27931, #27932, #27904)
v2.2 8/2/2010 Page 3 of 6
Calibration
The procedure for setting these potentiometers is explained below. Please note that turning the
potentiometer clockwise decreases voltage, while turning the potentiometer counter-clockwise increases
it. This can be compared to a water valve.
For the CO sensor, please read the CO Sensor Specifics on page 5 before attempting calibration.
Place the Gas Sensor Module in a clean air environment and supply power to the module. The
heater should be active during this time. Allow at least 10 minutes before making adjustments.
Adjust potentiometer R4 until the voltage across TP3 (+) and TP4 (-) reads approximately
0.80 V.
Adjust potentiometer R3 until the voltage across TP1 (+) and TP2 (-) reads approximately
0.80 V.
At this point adjusting R3 up/down should make the LED toggle on/off. Adjust R3 so the LED
just goes off.
Apply your gas source to the gas sensor. The LED should light up.
Remove the gas source and allow the sensor to settle. The LED should go back out.
If the LED does not go out within 60 seconds, adjust R3 until the LED goes out and repeat the
two previous steps.
The gas sensor module’s ALR pin should only be checked when the heater is on and the readings have
stabilized.
Note: Temperature and humidity are factors that could affect the sensor, making calibration difficult. If
this should happen try adjusting the voltages used in the calibration up or down as necessary to find a
more stable range.
Copyright © Parallax Inc.
Gas Sensor Modules (#27930, #27931, #27932, #27904)
v2.2 8/2/2010 Page 4 of 6
CO Sensor Specifics
The CO gas sensor must cycle through alternating voltages on its heater when active. This is unlike the
CH4, LPG, and C2H5OH gas sensors where the heater is on or off. To sense CO gas using the CO gas
sensor the heater must be driven at 5 V (full power) for the purge phase and 1.4 V (low power) for the
sense phase. The full-power (purge) phase runs for 60 seconds and the low-power (sense) phase runs
for 90 seconds. These phases cycle continuously as long as the sensor is active. The ALR output is
checked during the sense phase only. Checking it during the purge phase or when the heater is not
active could result in false positives.
In order to power the CO gas sensor from 5V and still obtain 1.4 V for the sense phase it is necessary to
pulse-width modulate (PWM) the HSW pin. Example source code is provided for the BASIC Stamp 2
module (page 6) and Propeller chip (see the Propeller Object Exchange, http://obex.parallax.com).
Sources of Gas for Calibration / Testing
The use of a standard butane hair curler is an easy, safe source to use for a very low carbon monoxide
(CO) gas concentration (less than 500 ppm) and can be used during calibration and testing of the CO Gas
Sensor Module.
An unlit propane torch can be used as a source for calibration and testing of the LPG Gas Sensor Module.
These are readily available from any hardware store and most department stores. Be careful not to leave
the valve open very long. Simply cracking the valve just a little can trigger the gas sensor.
We have not found an easy reliable source of methane for calibration and testing of the CH4 Gas Sensor
Module, however the CH4 gas sensor is responsive to propane as well so that could be used for testing.
Breathing gently through a small plastic tube (a drinking straw will work) can be used for calibrating the
C2H5OH Gas Sensor Module. The tube will help get the gas to the sensor and should condense most of
the water vapor in your breath that may interfere with the sensor.
Resources and Downloads
You may download the module schematic as well as the manufacturer datasheet for the sensor from
each gas sensor module’s product page. Go to
http://www.parallax.com,
type the specific product
number into our search box and click “GO.”
For gas calibration information you can also visit the following PDF on this website:
http://www.gotgas.com/pdf/GasSnsrCalibratn.pdf
Copyright © Parallax Inc.
Gas Sensor Modules (#27930, #27931, #27932, #27904)
v2.2 8/2/2010 Page 5 of 6
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