27904 Parallax Inc, 27904 Datasheet - Page 5

• Manufacturer Part Number: 27904
• Description: ALCOHOL BENZINE GAS SENSOR
• Manufacturer: Parallax Inc

Specifications of 27904

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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