how to read resistance of variable resistor with ADC?

Junior in EE at UIUC. Most of our coursework thus far has been more theoretical than practical - for example I know all about how a mosfet works at the atomic level, but I have been taught about using them in only the simplest of circuits. I believe the senior year will be more about practical circuits, but still I personally don't agree with this way of teaching.

-Mike

How about doing what joysticks do then. Convert the resistance to a pulse width (555 circuit left for the interested student).

Theer's literally no connection ?

Shame. Means it'll have to be calibrated for the track resistance. Actually, are you sure the joystick lever even moves it the whole way ? I suspect it doesn't. Accuracy issues will be the last of your problems.

Graham

are

Your 100k variable R is likely anywhere betwen 90k and 110 k track resitance. Use say a 100uA current and you'll get an FSD of anywhere between 9V and 11V.

From what I suspect, the max resistance you'll get may be less than that anyway. I'd suggest you measue the resistance span on the D connector. Didn't realise you were using a stock joystick, though you might be making your own improved one.

Graham

Unfortunately. Also the impedance is a bit high to go right into many ADCs. Using a current source or sink means that the maximum source impedance could be as high as 100K.

Best regards, Spehro Pefhany

Sounds daft to me to but typically *academic*. They seem to think it's al labout thero when most engineers I know who are any good want to get down with the nitty gritty. I do believe the practical first method teaches a hell of a lot more, faster.

Graham

spoon

Given that he's designing his own electronics, I reckoned a voltage follower wasn't too tricky to add. However it isn't an actual pot it seems so that's out anyway. The pot would have been 25K max at centre position of course.

Graham

OK, fair enough - I've seen the nightmare IBM joystick interface. Maybe you could just hang another 100K resistor from it to ground, and read a voltage from 2.5 to 5, and scale it accordingly.

But, for something like this, it seems to me it'd be kind of a daunting project to make something that's guaranteed to work with every possible input configuration; but the "standard" joystick circuit does use the pot in a current source, in which case reading the voltage across a series resistor should still work, depending on if the drive is pulsed, and that sort of thing.

Good Luck! RIch

No, but there is a variable resistor, which, when combined with another resistor, does, in fact, create a voltage divider.

M Noone will have to redo the software, of course, to read the ADC value instead of just timing the ramp.

Cheers! Rich

Right - but then linearity is lost, which is what I'm doing my best to avoid. I was thinking there'd just be some standard circuit for this. The current source seems like the best method to me, though.

OF COURSE IT IS!!!!!

Actually this reminds me of the year I spent at University College London on an Electronic Engineering degree course. It was *rubbish* . I learnt vastly more ( about electronics at least ) from my self-study. I distinctly remember 2 of my fellow students on the course commenting after they'd passed their exams that they still didn't know how a transistor worked ( as in a practical circuit ). It put me right off. It is seriously no way to teach engineering. Few ppl even need to know how a mosfet works - leave it til later for heavens sake. Engineers just need to know how to use the data.

Graham

Have you found a circuit for acurrent source yet ? 2 pnp transistors and a resistor will do what you need.

Actually, linearity may be screwed up a bit by the mechanical linkage used.

Graham

It's far from linear sadly.

Graahm

I've seen that type of circuit before - but it is very much non linear, which is why I'm avoiding it. I'd probabaly have to use a lookup table or something if I were to use that kind of circuit, which is not what I want to do...

-Mike

-Mike

", but still I personally don't agree with this way of teaching. "

That is because you think you are in school to learn, this is a tragic misconception. School's primary purpose is to create jobs for teachers and create a market for criminally overpriced textbooks. The reason you are not seeing any practical circuits is that it would cost the school too much. It's far easier to stick a bunch of kids in a plain room with some overpriced books and rake the money in. Did your professor also write the textbook?

Why not?

Okay, here's about the cheapest way to get a linear ADC reading (it's also ratiometric to supply voltage, so it assumes that the ADC - and + references are Vss and Vdd.

------------------+ | | .-. | VCC | | | + 8K | | ----+ | | '-' _V_ | | | +--|___|-| | .-. | | === 24K | | | 100K | GND | | | | '-' | | | | |\\| | . ' -|-\\ | | | >- '------------> to ADC input 200mV +-----------|+/ | |/| 0.2 to 2.7V (1.27V at middle) .-. 1K | | | | 1/2 LM358 '-' | | === GND

It doesn't lend itself very well to filtering of junk on the pot wires, so the circuit would be better built into the joystick than elsewhere.

Best regards, Spehro Pefhany

Is that entirely linear ? I'm thinking of the +1 term in the gain equation.

Not sure the op-amp will like a lot of capacitance from the cable on the - input either.

Graham

Yes, it's entirely linear (assuming an ideal op-amp), just with a constant +200mV offset. The rheostat sees a constant current of

LM358s are pretty hard to get to oscillate at the worst of times, unless you start loading the output heavily, but if any problems came up, a 10n from output to inverting input would solve it. And 51R on the output wouldn't hurt if it's a long cable to the ADC.

Best regards, Spehro Pefhany