# Gas Detector circuit design

#### asimraufawan

Joined Jul 4, 2008
29
Hi everyone,

Recently i have made a Gas Detector circuit which detects the leakage of
methane Gas at 10% LEL (see figure 1) & it works fine.
The sensor i m using is TGS2611.
The sesnsor has a variable resistance Rs which decreases the resistance with the concentration of GAS & when VRL exceeds VREF the alarm signal generates.
please note that the value or RL is matched with the sessor ID #18
at 10% LEL (RL=3.65KOhm)

Now i m trying to built the same circuit using two conditions
1. At 10% LEL it generates an alarm sinnal.
2. At 20% LEL the system should shut down.
(see Figure 2)
The Value for RL at 20 % LEL should be 2.61KOhm.

Please see the attachments, the figure 2 represents two level gas detector.
Kindly suggest me the resistance value that best suits the application in Figure 2.

THANKS.

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#### eblc1388

Joined Nov 28, 2008
1,543
You cannot change RL values on the fly so you can use only a single value for RL. I have selected RL to be 3.65K and based all subsequent calculations on this value.

I am assuming @20% LEL RL=2.61K you have provided is the correct value as I have no way to tell or check.

When LEL increases, voltage across RL also increases. Therefore your 20% alarm point should operate with a higher voltage, the reverse of that shown in your circuit. The connection of the comparator is correct but the top one will then be the "shutdown" output while the lower on be the "alarm output".

The resistor values R7, R8 and R9 is what you'll need to get that trip point.

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#### asimraufawan

Joined Jul 4, 2008
29
Thanks for the quick reply may i know what software you have used
for the simulation purpose or you have tried it with formulas.

#### eblc1388

Joined Nov 28, 2008
1,543
Your problem can be worked out using just ohm's law without a simulation but I got lazy so I do it the easy way.

The program is LTSPICE from Linear Technology. It is free and one can get it from the link below:

LTspice IV high performance Spice III simulator

#### VASUDEV2009

Joined Mar 20, 2009
1
Hello
Dear please can you give me the PCB design of working version.
I am trying to do this circuit but failed.May be not able to fig out exactly.
thanks