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Topic: Digital Potmeter: shifting resistor interval (Read 2936 times)previous topic - next topic

jopiek

Jan 19, 2014, 06:20 pm
I have an (educational) project that has a small resistor that I need to influence. The original resistance is about 10?.
I want to influence it using something like a digital potmeter. But they seem to be mostly available in versions with much higher  resistance.

Any good ideas on how we should approach this? We already tried several components (including a normal potmeter), but we want to have a good resolution so we hope to approach the 10? level (probably 100? would be very fine as well).

robtillaart

#1
Jan 19, 2014, 07:22 pm
maybe tell a bit more about what you want to demonstrate.
possibly it can be done with other values as well
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

jopiek

#2
Jan 19, 2014, 08:01 pm
well, my current circuit is like this, it is an experimental project to see how things work, (I help some students that are doing the project):
a1                  a0
5V ---[ 100k ] -|- [ x ohm ] -|- [ 100k] --- GND

at a0 and a1 we read the voltage. We want to vary the resistor of x ohm, that normally is 10 ? fixed. But, the problem is that if we connect a 10k potmeter between a1 and a0 it is just way too course (because it has 256 steps).

MAS3

#3
Jan 19, 2014, 08:28 pm
So your X value is a 10 Ohms resistor, which has some kind of variable resistor in parallel ?
If so, the resistance X will always be lower than 10 Ohms, is that your goal ?

I'm wondering what you want to show.
You are encapsulating your 10 Ohms in 2 resistors that are a factor 10.000 larger.
Any changes to that 10 Ohm resistor will result in a very, very small change that is no match to the tolerance of any 100K resistor you'll come up with.
If a0 and a1 are analog inputs of your Arduino, do you think 100 K and 100 K influenced by 10 minus a small deviation will be measured by the 1023 steps you have between GND and 5 volt ?
Have a look at "blink without delay".
Did you connect the grounds ?
Je kunt hier ook in het Nederlands terecht: http://arduino.cc/forum/index.php/board,77.0.html

jopiek

#4
Jan 19, 2014, 08:31 pm
Well we tested it with a variable resistor (actually 1K normal potmeter, 100k resistors were 10k in out first setup) in parallel but also replaced it with a variable resistor. Both options worked. But... we want to control it, and that is why we need a small controllable variable potmeter, but they are hard to find. Doesn't matter if we get a more complex circuit from that.

Grumpy_Mike

#5
Jan 19, 2014, 08:51 pm
If you did then your response in no way addressed the concerns expressed in it.

Quote
(actually 1K normal potmeter, 100k resistors were 10k in out first setup) in parallel but also replaced it with a variable resistor.

That is totally not what you described at first.

jopiek

#6
Jan 19, 2014, 08:54 pm
Well it is, but we did several setups, so the latter is complimentary. About the previous answer, I am aware of that, but that is as it is. We only want to influence the in-between resistor.

Grumpy_Mike

#7
Jan 19, 2014, 08:57 pm

Well it is, but we did several setups, so the latter is complimentary. About the previous answer, I am aware of that, but that is as it is. We only want to influence the in-between resistor.

Sorry with a response like that there is no point in trying to help is there?
Good luck.

jopiek

#8
Jan 19, 2014, 09:00 pm
well, I'm just doing my best trying to explain it... if I was more affluent at it I didn't need to ask did I? I always try myself first. The application works, we only want to improve it.

MAS3

#9
Jan 19, 2014, 09:29 pm
At least tell us you want to do this with an Arduino's analog inputs or not.
You mentioning a0 and a1 makes it feasible to think so.
But as we are in General Electronics here, we can't know unless you tell us.

Also, 10K and 1K is a huge difference to 100K and 0.01K, but you just told us about your awareness regarding this.
Have a look at "blink without delay".
Did you connect the grounds ?
Je kunt hier ook in het Nederlands terecht: http://arduino.cc/forum/index.php/board,77.0.html

jopiek

#10
Jan 19, 2014, 10:31 pm
I agree. I think the only solution is to make one myself, using e.g. an 8-output digital switch and a resistor array. Will give a reasonable result I think. As I understand, that is actually how digital potmeters work inside.

MAS3

#11
Jan 19, 2014, 11:37 pm
Lots of success with that.
I hope you'll be able to get the message across to your students and they will actually be taught some lesson.
You certainly don't get any message across over here.
Have a look at "blink without delay".
Did you connect the grounds ?
Je kunt hier ook in het Nederlands terecht: http://arduino.cc/forum/index.php/board,77.0.html

Jiggy-Ninja

#12
Jan 20, 2014, 12:26 am
What is this small resistance being used for? Why must it be so small? Giving us a bigger picture view of what you are trying to accomplish might help us determine a good solution.