# Connect the NO2 gas sensor MiCS-2714 (Environmental project)

Hi there, thank you for reading.

I’m trying to to a quality air sensor connected to the internet. For this reason I bought this sensor: MiCS-2714.
Where I got it I read that it doesn’t need 5V feed, instead only 1.8. I thought this means that I can connect it to the 3V3 power that the Spark Core has integrated. Could it be done? In that case which kind of resistors do I have to place?

Here is the data sheet info. http://www.cdiweb.com/datasheets/e2v/mics-2714.pdf
Thank you for helping!

There is a power circuit example in the data sheet. Try that

Ok. I didn’t wanted to burn anything connecting it to 3v3 instead to a 5V.
i’ll try. thanks

Also, in the datasheet it states 1.8V as maximum, so whatever you do, if you go past 1.8V you could damage the chip

Edit : actually it is 1.7V max (it says typical in the data sheet but it also states that if you exceed the typical you damage it)

Ok, thank you for your help.
there’s something I don’t know how to calculate.

If 131 Ω is the R for 5V.
Which one I have to place if I connect it to the built in 3V3 Spark Core supply?
thanks!

EDIT.
I did: 131 X 3,3 / 5 = 86,46.

So I guess that I have to put a 86Ω
Is it Correct?

Sadly no, that is incorrect I think.

The total resistance of the circuit is not 131 ohm but 131 ohm + 66 (typical resistance of the heater) ohm. That is 197 total. At 5v that equals a current of 0.02538071A and thus 0,1269W.

So at 3.3V we need 0,03845562A. That equals a total resistance of 85,81 ohm. The heater is 66ohm so the extra resistor should be 20 ohms.

However, the datasheet is a bit inconsistent, for instance look at the typical heating power and the maximum the datasheet states, the typical is way over the maximum so I’m a bit confused. Perhaps somebody else could verify my calculations since I doubt myself at the moment.

Wow!
Thank you for the replay!

Ok. If anybody else corrects it I will try with it… and see what happen… thank you.

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Hi,

0.03845 amp passing through 66 ohms gives a voltage drop across the resistor of 2.5 volts way above your requirement. I believe a good point would be 62 ohms (edit for standard value) plus the 66 ohm of the sensor. This gives a current of approximately 0.026 A this current through a 66 ohm resistor gives a voltage drop of 1.70 volts.
The 5 volt example is a total of 197 ohms at 5 volts giving 0.025 amp, the voltage drop across the 66 ohm resistor is approximately 1.68 volts.

Best regards,
Clive

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I was doubting between keeping the power or the current the same. It all makes sense now

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Thank you Casm for your clarification. Also @TheHawk1337 for your kind support.

@Casm is correct, you want to keep the current the same across the heater resistor

@TheHawk1337 A Note to your power comment. To keep the power the same at the heater resistor you need to keep the current the same. Your power will actually be the voltage drop across that resistor * the current. The external resistor is just used to adjust the total current - keeping the power the same.

@Fosforo2 There is a p-channel mosfet on the circuit in the datasheet. This is used to pre-heat the MiCS NO2 sensor. I think you need to pre-heat the sensor for 30 seconds and then let it run at the lower power level. Also please keep in mind that the sensor needs to run for about 24 hours the first time just to burn off any manufacturing impurities. Then you can read the ‘sense’ voltage using an analog input on the Spark. Going from a sense voltage to the NO2 ppm is a different story - these sensors are non-linear.

Hi @mtnscott,
Sorry can you explain me in what you said in the end?

Going from a sense voltage to the NO2 ppm is a different story - these sensors are non-linear.

Means that the NO2 ppm info is not trustable?

thank you

@Fosforo2 What I was trying to say is that the calculation to get the NO2 value from the analog voltage is not linear. The analog voltage present at pin ‘B’ across the sense resistor (Rs) will vary based on the level of NO2 detected. If you look at the datasheet page 2 figure 4 you can see the NO2 concentration vs Rs / R0 curve is non-linear. This means you will need to use polynomial equation to calculate the NO2 ppm from the sense voltage.

Thanks @mtnscott
All clear now.

Hi

I found a datasheet on using the sensor and some information on the preheat, hope this helps, the sensor is from sgxsensortech.com, if you send me a private message with your email i can send the document to you. it seems i cant attach it here.

Pre-heating

The MICS 2714 and the NO2 part of MICS 4514 both need preheating

Preheating is a manner to accelerate the stabilization time of the NO2 sensor. Because this sensor is operated at a lower operating temperature in order to increase its ability to detect the oxidizing gases, it takes more time to reach equilibrium with the air sample.

In automotive application usual pre-heating period is 30 seconds.

However, if initial response time is not an issue, preheating can be avoided.

It is recommended to use preheating each time it is used if non operation period is longer than 1 hour.

Pre-heating is requested for NO2 sensor only in order to help reduce the time of convergence towards a stable value at power on. 30 seconds at 2.4V followed by steady state operation at 1.6 to 1.7V is what we advise.

This procedure takes place at each power on.

best regards,
Clive

@casm
I have found the sgxsensortech datasheets to be confusing. Most of the information is based on 5v supply power. They have recommended circuits for heating and sensing. They are all based on 5v. Since they don’t specify the effective resistance of the heating element it’s difficult to determine what series resistance is needed to heat them when operating at 3.3v. Providing a pre-heat and steady state voltages is not enough unless you know the current at those voltages.

I just went to the web site and pulled off the datasheet (I actually have an application sheet covering usage in more detail). I can see the resistance data for the heater am I missing something here?

Best regards,
Clive

@casm Thanks, you are correct, where I’m struggling is with the sense resistor when running w/ 3.3v. It’s simple math to convert from 5v to 3.3v on the heater circuit, but what about the sense circuit? Do we also reduce the sense resistor when operating under 3.3v?

I think the clue is in the datasheet and I agree it is not easy to decipher, I also hope I am correct here! They say maximum dissipation for the sense resistor should be 8mW, sensor resistance range 0.8 to 20,000 ohms. Working back from this we can get to resistor values. There are a few things they use multiple ranges when using the sensor and this may be needed for precision at different levels. Just working a simple example:
3.3 volt supply and 3K resistor gives a voltage of 2.61 for the lowest resistance and 0.43 volts for the highest resistance a total range of 2.18 volts. Dissipation in the sense resistor is a maximum of around 2mW
3.3 volts and a 10 bit A2D converter measuring a range of 2.18 volts is 676 counts from top to bottom of sensor range.
These numbers work but as the sensor is massively non-linear it probably needs a much closer look.
I hope this helps.

Best regards,
Clive

@clive Yes, thanks. I have a few breakout PCB’s I designed on their way that will let me qualify the two sensors on the MiCS4514. (or at least connect them up and start measurements) Being non-linear and sensitive to multiple gasses I hope to better understand their behavior.