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Belgium6774 Posts
Here's some information I already know:
- The sensor uses a heating element that uses 6V. This is kinda gay. My arduino mega (fuck yea mega xtreeeeme!) has an operating voltage of 5V. Boo. Does the 1V really matter? lol
-I need some sort of amplifying component because arduino doesn't have enough accuracy or something? Some sort of op-amp or whatnot, I'm not too sure on what this is and how I should hook it up.
-There's a graphic on the datasheet that explains the relation between ppm and mV. levels of 10k ppm are deadly if I'm not mistaken so there'd have to be a lot of us stinking up the place to get to that level. Maybe the subway. Anyway I'm assuming the only ppm that matter to me are about 0 to 2k ppm. I really wish they'd hire a graphic designer to make these datasheets. *fistwave*
-The graph is exponential making it all the more difficult to derive results from mV readings. Again, *fistwave*
-Even though there's 6 pins, I think I only need to hook up 4! It's like these guys try not to get newbs into electronics by confusing us with such tomfoolery.
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Belgium6774 Posts
Ok that looks pretty sweet, thanks!
A few questions on this diagram:
![[image loading]](http://wiki.medialab-prado.es/images/e/e2/Mg811.jpg)
-Whats the black 6v reg thing with in/gn/out in relation to the arduino board? I mean is it an actual component or what?
-Is a TLC271 easy to find in general electronic component stores?
-What kind of arduino pin do I hook it up to, analog input?
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motbob
United States12546 Posts
Be careful - you'll want to gradually decrease the duty cycle of your pulse-width modulated signal until you hit the critical point. Also make sure there's no D-separation active paths or else your dual vectors may collapse.
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Belgium6774 Posts
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The "6V reg" is just supposed to be some element that generates a 6V current. The arduino itself can't do that because it runs on 5V. Also i don't know if the arduino's powersupply itself can handle serving a heater. So this might even be some external power feed. But basically: Top is either arduino 5V current or any other amount of current in, ground is just ground, and bottom is 6V current out, which is needed to supply the sensor. On the data sheet it says something about 6+-.1V, so you need to be pretty accurate with these 6V i suppose.
Regarding your question for hookup: The TLG271 amplifies the sensor output as far as i understood, the sensor obviously outputs a continuous value, so its an analog signal and needs to be read through one of the arduinos analog inputs. In the second link i posted, you can see a photo of the sensor hooked up to the arduino. I didn't check it but i'm fairly shure they connected the "pin" pin of ARD to the analog input. I guess ARD might be analog readable device or something? too lazy to look up now ^^ But i think you should get the idea now.
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Belgium6774 Posts
Isn't that second link's circuit based on another sensor though? One that operates at 5V as the datasheet seems to indicate. I'm quite convinced of the fact that I'll need one of those amplifiers for the mg811
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yeah its based on another sensor which runs on 5V. Apparently the guy tried it the same way with the sensor you had and ran into some trouble. I guess you do need these additional components.
You still could try it without, but the values would probably be screwed up, at least enough to be of no use.
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Belgium6774 Posts
How much ohm is that R1 resistor?
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How are you powering the Arduino? Over USB or batteries...? You will probably have a 12V or 9V rail to step down to a 6V source. The "6V reg" on that diagram is a simple 6V linear regulator which you will need to supply with something 6V or higher.
Secondly, the output of the sensor is really weak so the TLG271 is probably an op-amp to amplify that signal. I'd imagine that R1 is probably a variable resistor or something to tune the gain (multiplication factor) of the signal. If I had the TLG271 datasheet I could help you here, but really you don't necessarily need to follow this guys diagrams to the law or anything. The chinese datasheet (lol so bad, don't worry, most companies datasheets are hard to follow but not this bad) just shows a regular voltage source to drive the sensor and an amplifier hooked up to the output. This is a very simple circuit, there should be a billion "build a standard non-inverting operational amplifier circuit" things on the internet by now...
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Also you will want it to go to an analog input of course. I hope you have a voltmeter that can detect mV, so you can debug your circuit working...
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On January 07 2011 11:04 Xeofreestyler wrote:
How much ohm is that R1 resistor?
one one photo in the first link you can see they used two identical resistors, so a 10k for both might do i guess? I have absolutely no clue though, how this amplifier thingy works, all i know about electronics is from constructing a home made device like the playstation move controller before it was on the market. Also used an arduino for that, but my data sheets were written in english instead of machine translated japanese hehe, so that went somewhat easier.
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no, that circuit looks like a non-inverting amplifier. The amplification is roughly R1/10K, so you probably want R1 = 100K or higher to get your mV signal up into the range you want to detect.
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Yep NevilleS is right. Any op-amp should do provided its designed for the levels you're working with. A non-inverting amplifier circuit is pretty simple to build. http://en.wikipedia.org/wiki/Operational_amplifier#Non-inverting_amplifier
You can find lm741 op amps at radio shack for pennies. Not sure if they're suitable for this, but they should be if precision and power are not concerns.
So presumably you're going to hook this thing up to a pin and do A/D conversion on it? I'm not familiar with Arduino at all but it looks like its A/D converter is capable of resolving voltages as low as 4.9 mV. Your sensor output is in the range of 30-50mV. How much you choose to amplify that by depends on how much resolution you want, as you are limited to 4.9mV intervals. Keep in mind that your A/D pins can't handle anything over 5V so you do not want to use a resistor big than 1MΩ.
Voltage converters are pretty cheap too. I think Radio Shack should at least have some in a DIP package so you can get at the pins easily. You'll be needing to connect an external source of power to it, so make sure you don't go and order one thats meant to be put on a PCB unless you like soldering under a microscope.
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- TLG271 is just a low power op amp, if you don't have that one in your local electronics shop you can buy any generic one and it will work
- 6 pins are there so you could more easily fit it to your board, if it's complex it really helps (and I don't see how's that confusing  )
- choose R1 based on your voltmeter's range, so that minumum output value of the circuit is barely above the meter's minumum, and circuit's max is close to the meter's max - R1 is only there for that, it doesn't affect the sensor in any way (it would be the best to put a regulated resistor there, for example 1M, and then fit the range)
- 6V is 6V, that 1V may have a crucial meaning, don't use 5V instead
- if you have access to some known CO2 concentration areas, then it would be the best to manually calibrate your circuit. (get a few U=f(CO2) values, then fit it to an exponent)
- don't use arduino, real men print
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Belgium6774 Posts
Hahah well yeah making PCBs would be badass, I agree. But arduino makes it sooo much easier :p
(arduino is powered by usb)
Okay I think a lot of this information will surely help me on the way  Gonna order some parts in the next few days and see what comes of it. Thanks guys!
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Belgium6774 Posts
I'm getting a TLC271 asap, found a very cheap deal on ebay. Does anyone know of a webshop that ships MG811 to belgium and takes paypal? I've only seen it for sale on this site so far: http://www.futurlec.com/Gas_Sensors.shtml , but I dont have a creditcard.
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Reading this thread is like walking into an S&M convention. I have no idea whats going on but find everything incredibly interesting.
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can't you just walk over to a electronics store in the next larger town? We have a shop that sells electronics components over here, i guess they could get that thing or, if not, tell me where i can get it myself.
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EPT
