interfacing 8051 smoke sensor tgs2600 with adc0809

hey guys,

i want to interface smoke sensor to my at89s52. the concept is the sensor reading will determine when the fan (actuator) should go low speed,higher speed,or full.

but i want to make this doubt clear first.
the sensor im using is figaro tgs 2600. (datasheet attached). the adc+lcd is taken from this site (ref:**broken link removed**)

the code is the same. the difference is i interfaced my sensor as the datasheet drew (attached).

the doubt that i want to ask are:
1. the adc range displayed (0-255) , keep changing in a room with no contaminant gases. ex: 1xxx-2xxx-1xx-00x-0xx in a random way.is it normal? or should it stand still in certain condition?
2. the sensor need RL(variative). the sensor said RS should be 10k-90k in air. i go for RL=10k, i got Vo=0.177 ,hence in calculation my RS goes around 270K Ohm. not in a range that datasheet tells me. if i increase RL then RS increased too. even when i used 470 Ohm still not meet the range.DS suggest the RL is >450Ohm.
is there something wrong with the circuit or sensor?

the formula that i used is in the datasheet.
View attachment figaro tgs 2600.pdf

i measure Vo through this point

Did you verify that the heater voltage is in the specified 5.0 +/- 0.2V range? Otherwise Rs may be different.

FvM said:

Did you verify that the heater voltage is in the specified 5.0 +/- 0.2V range? Otherwise Rs may be different.

Click to expand...

i give it the same voltage with the + voltage (5volt dc). though i measured it and its around 4.7x-4.8x

*bump* anybody had experience this before?

I'm not sure how reliable the "clean air" sensor resistance value specification is. A higher resistance must not necessarily indicate a faulty (or wrongly operated) sensor. Sensitivity to air contaminations should be the primary criterion.

FvM said:

I'm not sure how reliable the "clean air" sensor resistance value specification is. A higher resistance must not necessarily indicate a faulty (or wrongly operated) sensor. Sensitivity to air contaminations should be the primary criterion.

Click to expand...

and how do i know if the sensor is sensitive or not? what is the indicator?

Exposing the sensor to a small amount of test substance...

FvM said:

Exposing the sensor to a small amount of test substance...

Click to expand...

the output is stable after i apply pull up resistor to P0 as input port from adc data0-data7.

the sensor have graph showing 0-100ppm. my lcd still display it in binary 0-255. does it mean 1 binary=0.39ppm(100/255)?