27904 Parallax Inc, 27904 Datasheet - Page 3

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

Specifications of 27904

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

Pin Definitions and Ratings
* 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.
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.
TP1
TP2
TP3
TP4
Pin
E1
E2
E3
E4
Test Point 1+
Test Point 3+
Test Point 2-
Test Point 4-
Name
HSW
GND
ALR
+5V
Gas Sensor Modules (#27930, #27931, #27932, #27904) v2.2 8/2/2010 Page 3 of 6
Alarm output to host microcontroller
Heat switch input from microcontroller, active low
+5 VDC power
Ground, connects to common ground
Buffered output of sensors (voltage divider)
Ground, connects to common ground
Trip Level voltage set by potentiometer (R4)
Ground, connects to common ground
Function
When their internal heating
0V / Floating*
0V – 5V
0V – 5V
0V / 5V
Level
5V
0V
0V
0V