monkeyman_stones:
MorganS,
As I thought I'd made pretty clear in previous responses everything except my power supplies is 5 Volts. In response #9 I said as much directly to you.
So what is stopping you from buying a 5V supply?
This is becoming unreasonably and needlessly unpleasant. The question was pretty simple and has been turned into a large load of manure by insignificant questions of no relevance.
Again, the question was and still is: Which (if any) of the boards can be used for 3 sensor (I'm removing the humidity & temp sensor from the mix) and a display? The Uno couldn't even handle 2 as it gave the blue smoke of death upon the power supply being plugged in with the MQ-135 and MQ-2 plugged in, with no display.
If a board can be used as a 3D printer controller, a robotic car with sensors, it is ridiculous that they can't be used for 3 simple sensors and a display.
Which board should be used?
The MQ-135 uses almost nothing if you power it's HEATER from a separate 5volt supply.
The MQ-7 uses almost nothing if you power it's HEATER from a separate 5volt supply.
The MQ-2 uses almost nothing if you power it's HEATER from a separate 5volt supply.
The temp/humidity sensors use almost nothing.
Should be clear now that you need a separate supply for the HEATERS.
Not wise to try to channel that sort of current THROUGH any Arduino.
You have already been told that on post 1 and 2.
Post a schematic or block diagram, so we can check if it will work.
Any 5volt Arduino can be used, but NOT to POWER HEATERS.
Leo..
Wawa,
There is no separate power nor ground for the heater versus any other components of the sensors. That is what has caused and is again causing my confusion. There are 4 pins: 5V+, 5V-(ground), Analog and Digital for 2 of the 3 sensors (3rd has no Analog). As people continue to recommend powering only the heating portion by means of a secondary power supply, I take it I need to cut the + & - pins of that heater away from the board's power and use the separate power supply there, yes? But if I do so how does the board regulate the heating of that component? It would simply be at full power at all times, cooking itself away.
No. Power them all from the same power supply, including the Arduino.
Those sensors have four pins.
Two for the heater, and two for the sensor.
But maybe your sensor boards didn't break out the pins separately.
Did you already post links to the boards?
Leo..
MQ-135
MQ-2 (here is the unit with no Analog pin even though one appears to be there in the pictures)
MQ-7
https://www.amazon.com/gp/aw/d/B00UYSOUL8?psc=1&ref=yo_pop_mb_pd_title
None of these make any claim of a separate heater nor separate current draw while the heaters which there's nearly no sign of is on. They only state 150mA current draw, not 2 different current loads indicating a heater.
Wawa:
Those sensors have four pins.
Two for the heater, and two for the sensor.
But maybe your sensor boards didn't break out the pins separately.
Did you already post links to the boards?
Leo..
How are the sensors able to function without power? If the 5V+ & - are only for the heaters then how are they able to sense gasses and transmit any signal from the units to the Arduino? There's no power to do so...
Dangerous to buy items from stores like ebay and Amazon if you can't reverse-engineer the circuitboards that the sensor are mounted on.
I guess you have to use them as they are designed, and power everything from a 5volt supply (MorganS, post#24).
Leo..
I have a step-down voltage converter of 12V to 5V, so perhaps powering the board with one of my 12V (1A, 2A, 3A & 5A) power supplies and grabbing the 12V positive and negative from the back side of it, through the 5V converter then passing 5V to the power and ground pins of my bread boards to power the sensors, screen and I2C screen device? Then only the current of the output signals from the 3 or 4 boards as well as the I2C display adapter and the display would matter. Sound better?
The one thing I haven't been able to figure code for the display changing to show the readings from each of the multiple sensors for a few seconds, then moving to display of the next sensor and so on. Perhaps I should just use one of my 2004's and have it display them all at the same time rather than the displayed data with the 1602 changing between the various sensors? Certainly sounds faster.
monkeyman_stones:
MQ-135
https://www.amazon.com/gp/aw/d/B00LSG5IZ2?psc=1&ref=yo_pop_mb_pd_titleMQ-2 (here is the unit with no Analog pin even though one appears to be there in the pictures)
https://www.amazon.com/gp/aw/d/B01F2X3VY6?psc=1&ref=yo_pop_mb_pd_titleMQ-7
https://www.amazon.com/gp/aw/d/B00UYSOUL8?psc=1&ref=yo_pop_mb_pd_titleNone of these make any claim of a separate heater nor separate current draw while the heaters which there's nearly no sign of is on. They only state 150mA current draw, not 2 different current loads indicating a heater.
You need to download the datasheets for the elements themselves.
Find the MQ135 data sheet attached.
Then read the data, being aware that they need an initial 24hour burn in period before you can start to get decent readings.
How are you going to measure gas volume?
Can you please post a basic block diagram of how you will layout the sensors and controller?
Tom... 
SNS-MQ135.pdf (145 KB)
TomGeorge,
I used a steel tube to create the greatest vaccum I could with the one of my Unos, each of the sensors and a battery pack for them within. Then I used a gauge upon a gas b-tank for each of the gasses I was concerned with and placed a measured quantity of each of the gasses one at a time, ramping up the volume every 7 minutes. I recorded the responses to each gas type and each volume and looked for the differences between them. I then worked for weeks to develop code to have the report announce which gas was present and caused the alarm.
I used each of the sensors individually to try and create the code for the sensors which are not made to define the gases which are sensed. It's why the code is so unreasonably large as the sensors do not define which gas they sense and I had to find ways to enable as much. Because the sensors are so generalized my code often (32-51% incorrect) provides incorrect responses about which gas is sensed. I'd need many, many months to try and refine the code, but I'm willing to bet that other sensors which automatically do so exist or can be connected for this specific response, but they are near certain to be horrifyingly expensive which is why I attempted to create such code to enable as much.
Selecting between sensor and module
When it comes to measuring or detecting a particular Gas the MQ series Gas sensors are the most inexpensive and commonly used ones. MQ135 is available as a module or as just the sensor alone. If you are trying to only detect (not measuring PPM) the presence of a gas then you can buy it as a module since it comes with an op-amp comparator and a digital output pin. But if you planning to measure the PPM of a gas it is recommend buying the sensor alone without module.
And they explain both operations.
So you call what you don't know manure?
I can power a line of leds with an Uno. How many depends on how much current my limiting resistors let each led have. Any more, I have a transistor per led to switch external power through and the Arduino pins doing the switching with very low power on the transistor gates. The transistors may be inside of a driver chip that has VCC and GND pins that I connect to ... external power.
I'm not deluded into thinking that the only way to use leds is to power them from Arduino pins since a diagram shows leds powered by Arduino pins. It's not one way or the other is the only way. We don't go to the trouble of transistors and external power until we NEED it.
I can power a small number of some devices including sensors with Uno pins. If I want to use more then I connect external power to the VCC pins, make sure that external ground and Uno ground are connected and my sensors run just fine. There are other sensors that I can't power with Uno pins, those run from external power.
The MQ-135 module has pins for power and ground. Do you really think that those MUST connect to Arduino pins? Spend a little time on that if you do because they do not.
You have drawn a number of false conclusions about hardware that you need to drop belief in before you can allow yourself to make progress. Are you able to question your own ideas or will you stick with everyone else must be wrong?
Hi,
I used a steel tube to create the greatest vaccum I could with the one of my Unos, each of the sensors and a battery pack for them within.
So you placed a UNO, sensor and battery complete in a steel vacuum chamber?
How did you get any readings?
What sort of battery is rated for vacuum operation?
If you look at the specs for the sensors you will see that they are designed for operation in 21% O2.
As you are measuring ppm, parts per million, at near vacuum, part per million of what?
These devices are designed for detection (presence) of gases, not empirical measurement of actual broad range of concentrations.
I appreciate your efforts to find any contaminants in your home, but to detect definitive measurable amounts with these sensors is not what they are designed for.
Tom...
There was one project that used color absorbtion to profile different liquids in bottles. That used colored leds as light detectors. CO2 absorbes IR, can't say for the rest just from memory but in a length of tube rather than across the diameter you might get enough to see a difference.
monkeyman_stones:
I have a step-down voltage converter of 12V to 5V ...
Yes. Good idea. How much current can that converter supply when it is given a 12V input?
The Arduino can be powered through its 5V pin. It does not require 12V.
Programming the display is a different problem. Make sure your code is modular so that the display code is separate from the sensor code. If you have a function called readCO2Sensor() then it should not output anything to the screen.
...calibration procedure...
If you are working to that level of detail then you can't get there from here. You need to spend a lot more on the sensors. The best website I know of for gas sensors that fit between "hobby" and "professional" is www.CO2meter.com Check out what they have.
MorganS,
The voltage step down converter claims (is written on it) 3.5 Amperes, I assume that means allowed. If I were running that much through it and with my hand on it (far from the terminals) if it began to become too hot (say, over 80°C) I would immediately pull the power supply plug from the wall outlet.
Is it just me, or does the second Due of mine appear to be a fake? Every Arduino I've seen apart from the Mini's and Micro's are marked for the socket/pins purpose on the plastic. That one unit has black headers and no text at all. I suspect that Amazon sent me a fake for that one (the clear plastic base was originally on the brown header unit and I changed it). A genuine Arduino nearly always includes a plastic base and marked, solid (continuous) headers and a white reset button. That unit came with no base, off the shelf black, unmarked headers and a red reset button instead. Am I alone in my thinking it is not a real (made for/by) Arduino?
That level of detail was simply me trying to save a few thousand dollars (gas sensors with specific recognition are outrageously pricey) and did what I could to try and have a functional unit at the end. But as you saw with the code I kept creating, things continued to get/stay worse. An incorrect recognition level over 3% is unreasonable and is why I'm so very willing to dump that work and use the standard code for as much instead.
GoForSmoke,
#32
I made no statement which earned your disrespectful post. It was my belief that if the Arduino units are meant to enable projects to be simple and simplified, an additional (second) power supply should not be necessary. To accept up to 20 Volts strongly suggests a greater power handling than it actually allows (average automobile electrics run at 6 Volts [pre-1954], 12 Volts [14.4 Volts while running], ignoring 24 Volt systems as they are to high a voltage for direct powering an Arduino).
Was it your thinking that I wear a conductive metal ring and no gloves while changing an automobile battery, forgetfully placing my left (ring hand) on a terminal with the ring in contact, then a steel wrench in the other hand touched to the opposite terminal (instant smoke filled death as the power passes that beating/pumping "thing" between the two hands)?
You will likely find that in life calm wins. Give it a try please.
TomGeorge,
Yes, along with the camera.
"How did you get any readings?" By using a camera designed for the same use as the 12V battery (described below).
The battery type is one contained in a sealed steel case specifically designed for aquatic (self contained underwater breathing apparatus) use. They can handle 5000 feet (1524 meters) of water pressure, they can handle the minimal vacuum one can create in the home. Pressure is pressure regardless of it being negative or positive (so long as the item is built as such - I may drive my vehicle in relatively deep water without ingress, but I wouldn't want to be anywhere near it in space!).
Part per million of near the only gas within the container (some air of course remains because an absolute vacuum is nearly impossible except in space). The vacuum was intended to remove other possible causes of sensor alert. Enough of each gas was added to equal external air pressure, then increased every 7 minutes to measure the response. I work in a lab where access to gas is alway so (one of the benefits of being a former Senior Information Management Analyst who became a Microbiologist).
Rugged Circuits makes 12V capable boards.
" It was my belief that if the Arduino units are meant to enable projects to be simple and simplified, " and when you stayed small it worked.
When you made it bigger, your excuse ran out. You want to do more then learn how, it is not by insisting on dead ends.