your question
It's almost impossible to guess at your professor's motivations for remaining somewhat opaque in conversations with you. If I had to throw mud at the wall as a guess, I'd say that you are being evaluated to see if you can rise up to the occasion (or not.) What exactly that occasion is, I don't know.
But I'll write a few things and see if any of it helps add context you can use. Of course, only you will be able to tell.
I'll stick with the MQ-7 sensor. I've no interest in repeating myself for all three of them.
my professor then asked me to pick a sensor and explain/draw on theboard how the sensor's resistance affects the voltage
what does he expect of me to draw/show.
the exact wordings he used is along the line of: 'how does this changein resistance now translate to voltage' next thing he gave a marker. Idon't even what I was supposed to draw atp
It's possible you are being asked to show a basic understanding of a simple voltage divider equation. Equation 1 is just one of its many reformulations. Or perhaps an idea is to also see if you've even thought about what kinds of plots may be interesting and/or what such plots might show if you'd bothered to try.
I admit that seems a little odd (perhaps too easy) for a 500-level student/class. But I don't know where else to start. What you've written down for us doesn't provide much to go on.
Your whole question seems to boil down to being more about asking us what's going in the professor's mind than a specific technical question. And if so, that's really not an appropriate question for this site. But if we could read minds, then I suppose there would then be a technical dimension that makes the question more appropriate. I'll just assume for now that the words can be taken in a more literal way and follow through with some thoughts. It's all I can offer.
draw/show a plot how the sensor's resistance affects the voltage
This provides some kind of focus, at least, as it excludes plotting how ppm CO affects the voltage, for example. This is something to hang a hat on. Let's take it literally.
Several ideas come from the sensor datasheet, given a specific sensor unit:
- \$R_{\small{0}}\$: The measured resistance of that sensor when exposed in a calibration step to 100 ppm CO.
- \$R_{\small{S}}\$: The measured resistance of that sensor when exposed to a situation of interest.
- \$\frac{R_{\small{S}}}{R_{\small{0}}}\$: A unitless ratio when that sensor is exposed to a situation of interest.
A specific circuit implementation will also include this:
- \$R_{\small{L}}\$: The load resistance used in a specific circuit implementation that includes that sensor.
From this table entry in the datasheet:
Any two sensors picked out of a box may exhibit values over an entire decade range, when exposed to the same 100 ppm CO situation. The geometric mean would be \$\sqrt{2\:\text{k}\Omega\,\cdot\,20\:\text{k}\Omega}\approx 6.32\:\text{k}\Omega\$. and that any particular sensor drawn from a box of them may be anywhere from 31.6% to 316% of that value. Quite a range.
But for purposes of creating a plot, let's take the central value of \$R_{\small{0}}=6.32\:\text{k}\Omega\$.
Another useful idea to remember now is the maximum power transfer theorem. It suggests (not a proof, yet) that the peak sensitivity of a circuit including the sensor may be found when \$R_{\small{L}}=R_{\small{0}}\$(assuming that 100 ppm CO is in the center of the desired measurement range.) So let's take \$R_{\small{L}}=R_{\small{0}}=6.32\:\text{k}\Omega\$ for plotting purposes.
You should also already know (being an engineer) that most everything important happens within an order of magnitude in either direction. So set the x-axis to show a resistance that varies one order of magnitude around the nominal value, together with the expected y-axis values and a curve:
This isn't a straight line. (I suspect that's the first thing to be checked in your drawing.)
The above curve comes out of the basic voltage divider equation: \$V_{\small{RL}}=V_{\small{C}}\cdot\frac{R_{\small{L}}}{R_{\small{L}}+R_{\small{S}}}=V_{\small{C}}\cdot\frac{1}{1+\frac{R_{\small{S}}}{R_{\small{L}}}}\$.
(You should be able to quickly develop equation 1 on the datasheet from the above equation.)
You should be able to work out that when \$R_{\small{S}}=632\:\Omega\$ then the factor (multiplier) will be about \$\frac{1}{1.1}\approx 91\%\$ and when \$R_{\small{S}}=63.2\:\text{k}\Omega\$ then the factor (multiplier) will be about \$\frac{1}{11}\approx 9.1\%\$. (Obviously when \$R_{\small{S}}=6.32\:\text{k}\Omega\$ then the factor will be \$\frac{1}{2}= 50\%\$.)
Just put those points down immediately, before drawing the rest of the curve (as shown already above.) A few more simple calculations can get you a few more points to set down and, from there, you can just hand-draw in the rest.
Perhaps that was the reason for the question. No idea, though.
notes
If you use \$R_{\small{L}}=6.32\:\text{k}\Omega\$ in every unit built (no calibration step where \$R_{\small{L}}\$ is set to \$R_{\small{0}}\$) then it turns out that the peak sensitivity may be degraded and only 75% of optimal, unit to unit. This may be okay. Or maybe not.
There's more, of course. Assuming each unit isn't calibrated but instead every unit uses \$R_{\small{L}}=6.32\:\text{k}\Omega\$ and given a matched sensor where \$R_{\small{0}}=6.32\:\text{k}\Omega\$, then the measurement range over the above graphed range spanning a decade to either side of center is about 3 ppm CO to about 3400 ppm CO.
(You should be able to work this out from Figure 3 in the datasheet.)
But what happens if a sensor isn't well-matched. In fact, suppose it is one of the more extreme devices with \$R_{\small{0}}=2\:\text{k}\Omega\$ or \$R_{\small{0}}=20\:\text{k}\Omega\$? What is the measurement range of those units, assuming that units are shipped to customers without any calibration step and adjustment of the load resistance in order to calibrate them?
For a \$R_{\small{0}}=2\:\text{k}\Omega\$ unit this would be from 0.5 ppm CO to about 580 ppm CO and that it would read 17 ppm CO when the actual case was 100 ppm CO. That may not be a good thing.
For a \$R_{\small{0}}=20\:\text{k}\Omega\$ unit this would be from 17 ppm CO to about 20,000 ppm CO (which isn't even charted on the datasheet.) It would read 580 ppm CO when the actual case was 100 ppm CO. That also may not be a good thing.
This suggests strongly that a calibration step is needed with the load resistor set so that \$R_{\small{L}}\approx R_{\small{0}}\$.
I've not disclosed how exactly I developed the mapping of ppm CO, just discussed. But you should be able to develop all this entirely on your own, without any hand-holding from others. If that's not the case, you still have work ahead.
The datasheet also recommends "that you calibrate the detector for 200 ppm CO in air" and use a somewhat higher value for \$R_{\small{L}}\$ than I suggested earlier, above. Can you come up with a reason for that suggestion in the datasheet?
My guess is that your professor would like to see just how much you are able to handle on your own and if you are able to be turned loose on a project with some reasonable expectations that you can identify issues, explore solutions, and can identify areas where a specification may be missing and needs definition.