Hi,i want to use MQ137( NH3 sensor) in aviculture. i want to connect it to PIC microcontrllers. i want a circuit to alarm when gas(NH3) concentration reaches or wants to go higher than
25ppm.
first see the data sheets and application circuit of this sensor:
MQ137 datasheet
MQ137 New datasheet
MQ series circuit
Gas sensor calibration
Application circuit

but i don't think i need that complicated circuit, i think i can do it more simple with microcontroller,lets see the basic circuit in data sheet:


Basic circuit

first look at basic test circuit of MQ137(JPG file). We have this formula in the data sheet:
Rs=((Vc/VRL)-1) × RL

Rs= Resistance of the sensor
Vc= +5 volt
VH= +5 volt
my application is alarming when NH3 concentration reaches or wants to go higher than 25ppm.
So i can measure the VRL value(voltage) and put this value in the formula to find the Rs value.
1- What value of RL are you suggest for my application?
2-What's the value of the Rs in 25ppm NH3?
3- The Rs value and the gas concentration in ppm have the inverse ratio or direct ratio? ( i think it's inverse ratio)

If i have RL value and measuring the VRL, i will put these values in the formula and find the Rs value.
If i have Rs value in 25ppm NH3(standard value of Rs for MQ137 in 25ppm that you will tell me), i compare the Rs value from the formula ( result of the formula) with the Rs value in 25ppm NH3 (the standard value that you will tell) and when they are equal or when the measured value lower than the standard value(that you will tell) i will turn on the alarm because when they are equal or the measured value is lower than the standard value( that you will tell me) the gas concentration is 25ppm or more than 25 ppm.(because i think they have inverse ratio, if they have direct ratio, when the two value Rs reaches to each other or the measured value wants to go higher than the standard value i will turn on the alarm)
I think it's the best way to alarm when gas concentration reaches or wants to go higher than 25 ppm.
please answer those three questions.
Am i thinking right? if not,please help me to do this project, i really need help
Thank you very much

I think most of your answers are in the datasheet.

1- What value of RL are you suggest for my application?

might as well start with the 47K they suggest in paragraph "sensitivity adjustent" of the datasheet.


2-What's the value of the Rs in 25ppm NH3?

Fig 3 of datasheet shows it's less than resistance in air by a ratio of about 2.5/1.1, or baout 44% of resistance in air. Look at the relative hight of blue and green lines.

3. 3- The Rs value and the gas concentration in ppm have the inverse ratio or direct ratio? ( i think it's inverse ratio)

Try a formula of this sort:
Rs = Ro X (1 - alpha X C) where C is concentration and alpha is as given in "Table C. Senstivity Characteristic".

A simple regulator and bridge circuit would do - i think . Surely you dont need a microcomputer unless yuo're just good with them and have one handy. Personally I am scared to death of them. Too old to learn them i guess.
Look into applications on LM324 datasheet. I'd include an analog meter...

I'm curious - as strong as ammonia smells why would one need an alarm?

good luck, keep us posted...

a.

but datasheet said use 47K for calibrating sensor in 10ppm, what about 25ppm?
could you explain more about your answers? i haven't worked with these sensors yet,could you tell me the exact values?
is my theory right or not(in first post)?
i want to use the PIC microcontroller because i wan to make a aviculture monitoring system that measure temperature, humidity and ammonia concentration and controlling fans,heathers and etc.

""......
but datasheet said use 47K for calibrating sensor in 10ppm, what about 25ppm?
...............""

Use the same value. Exact value is not important just so it is small compared to Rsensor.
Note their recommended 47K is twenty times smaller than lowest Rsensor (in air), probably twelve times lower than Rsensor in 200ppm ammonia.
Measure carefully.

""......
could you explain more about your answers? i haven't worked with these sensors yet,..
.......""

Think of the sensor as a self-adjusting resistor Rs.
It adjusts itself according to ammonia concentration.

Place it in series with 5 volts and a resistor RL.

Measure VL,the voltage across RL.
By ohm's law, I= VL/RL
By Kirchoff's law, voltage across sensor is (5-VL)
and again by ohm's law Rs = (5-VL)/I, which is (5-VL)* RL/VL

So,,, Rsensor = (5-VL) * RL/VL
Now, you know RL and after you measure VL you'll know Rsensor.
You need to do that twice--
once in air and once in (some concentration of) ammonia.
The value you get in air is R0.
The value you get in ammonia is R(insert percent ammonia).


Now you have calibrated your sensor and resistor as a pair.
You know by how much the resistance changed in response to ammonia.

""....
could you tell me the exact values?.
....""

No I cannot. Neither can the people who build them.

The ratio R(percent)/R0 is fairly predictable.
The value of R0 is random within a range of limits.
As stated in the datasheet that range is between 900kohms and 4900kohms.
That's a pretty wide range.
You don't know where in the range you'll start, only what direction you'll move.

That's why you have to 'calibrate' each and every one of the sensors.
Whether you use 47K or 10K or 100K for RL is unimportant.
Just measure carefully.

Their datasheet figure 3, sensitivity curve(upper graph) must have some mistake in it.
The AIR line should be on the 1.0 line not 3.5, because in air RS IS R0 and R0/R0 = 1..

your theory is right.
What is wrong is to expect an exact value without taking careful measurements.


Here's a link on sensors - i've never used these particular ones either.
http://www.gasalarmsystems.co.uk/NewsDetail.aspx?referrer=newsPage&id=15

For your application it might be advantageous to put the sensor in a bridge circuit and measure the unbalance with your PIC..
That way you could "null" the bridge when sensor is installed rather than storing R0 in software... Note that's what they did in the pdf drawing - sensor and R1 are one leg of bridge, R4, R5, and RP are the other.

by the way - that link to a jpg file locked up my computer solid and i had to turn power off. Have you had any trouble with that site?

a.

for calibrating sensor i should connect the 47K resistor as RL, then i use that application circuit and use potentiometer to adjust it in 25 ppm? so i need an environment that has 25ppm NH3 concentration and i change the potentiometer value until the alarm turning on.is it right? is there any way that don't need an environment with 25ppm NH3 concentration? i don't have an experimental environment.how can i adjust it?
in datasheet said that calibrate sensor for 10ppm in clean air, so there is 10 ppm NH3 in clean air? and i should calibrate it to alarm in 10ppm? am i thinking right or i haven't get it?


thank you

"".......
for calibrating sensor i should connect the 47K resistor as RL, then i use that application circuit and use potentiometer to adjust it in 25 ppm? so i need an environment that has 25ppm NH3 concentration and i change the potentiometer value until the alarm turning on.is it right? ........
...........""

You're pretty close.

in this, one of your links above:
http://amindzo.250free.com/co%20sensor.pdf

they are saying to "calibrate" it with a mix of NH2 and air.
See this paragraph in that link:
"Testing through Calibration gas
Preheating the gas leak alarm for 10 minutes, Making the calibrating gas concentration
according to volume proportion, put the gas leak alarm in the calibrated gas. after 10
seconds ,the gas sensor sensing the gas adequately , then adjust the RP slightly to make
buzzer sound exactly and RED LED light, the calibration will be completed after repeated it
several times."

"".............
in datasheet said that calibrate sensor for 10ppm in clean air, so there is 10 ppm NH3 in clean air?
'''''''''''
...............""

no, they're saying make yourself a mixture of clean air and ammonia , 10ppm . You should "calibrate" with the approximate concentration you want to be your alarm point.

And there's not that much ammonia in normal air.

"Natural ammonia levels in the atmosphere are in the low ppb range. Ammonia is toxic to humans at low ppm levels and needs an effective detection solution in most industrial environments."
http://www.ir-microsystems.com/web/applications/ammonia.php

i'd use a simpler circuit than that alarm. Let the microprocessor do the thinking.

Are you familiar with the Wheatstone Bridge? That'll make a DC voltage you can measure with microcontroller's A/D converter and alarm from calculated result.
Google 'wheatstone bridge'..

a.

thank you very much,i will try to find an experimental environment for calibrating sensor. datasheet said preheating time is 48 hours, i want to know that i should turn on the heater of the sensor 48 hour before calibrating in clean air or is should put the sensor in experimental environment(with NH3) and then turn on the heater for 48 hours?

I noticed that statement also.

Honestly I don't have firsthand experience with that sensor.
I'm guessing it's this type of sensor:
"Chemosorption (MOS) sensors
Chemosorption sensors consist of a metal oxide semiconductor, such as tin dioxide (SnO2) on a sintered alumina ceramic bead contained within a flame arrestor. In clean air the electrical conductivity is low. Oxidation of the measured gas on the sensing element increases conductivity. An electrical circuit is used to convert the change in conductivity to an output signal which corresponds to the gas concentration. Sensitivity to a particular gas is alterable by changing the temperature of the sensing element....
.........................""""
(from that link i posted, http://www.gasalarmsystems.co.uk/NewsDetail.aspx?referrer=newsPage&id=15 )

The 24 hours is probably to cook all the moisture out of the moisture out of the alumina.

Air should work fine, the dryer the better.

This is interesting subject. Was that sensor expensive? I'm looking for a Freon detector and this looks like a possibility. Where'd you buy it?

thanks, a.

you should see the web site:
http://www.hwsensor.com

Thanks!

that one comes up in what looks like Chinese - here's their english link if anybody else is interested..
http://www.hwsensor.com/English/