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Can I Modify a Potentiometer?

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Screech

New Member
I nead variable resistance that goes from eg.0- 175K ohms Linear, when a shaft is turned from 0-90degree angle.

The 175 K is just an Example

My Crap Idea!
The image below is a disk that will go on the shaft.
The red thing is a light dependent resistor.
on the other side of the disk is an LED(hidden on the other side.)
as the disk rotates, it starts Unblocking light,and the LDR will vary.
The thickest unblocked cut out will have to be the right size so the LDR will read 0 K, and the thinnest to read 175K
As I said this is a crap design.


Is there a way of using a pot somehow, or modifying it,or some other simple design?
IT would be even better if it was able to be customized, eg: if I decide I nead more resistance @90 degrees.

Does anyone know anything about digital potentiometers?
Thanks
 

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Symon

New Member
You could put two potentiometers in parallel, and linked with a pulley or something. Two 300k pots will give you a 0-150k range. Have a play with different values and you will be able to get what you want.
 

stevez

Active Member
You could use a simple combination of gears. Jameco, among other places, sells them at low cost. There are some low cost vernier drives that provide a reduction though the ratio is fixed.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Screech said:
I nead variable resistance that goes from eg.0- 175K ohms Linear, when a shaft is turned from 0-90degree angle.

The 175 K is just an Example

My Crap Idea!
The image below is a disk that will go on the shaft.
The red thing is a light dependent resistor.
on the other side of the disk is an LED(hidden on the other side.)
as the disk rotates, it starts Unblocking light,and the LDR will vary.
The thickest unblocked cut out will have to be the right size so the LDR will read 0 K, and the thinnest to read 175K
As I said this is a crap design.

Another way of doing it is to use two polarised sheets and rotate one of them, 90 degrees rotation moves from maximum to minimum transparency. I believe this has been used commercially as well.
 

crust

Member
Nigel Goodwin said:
Another way of doing it is to use two polarised sheets and rotate one of them, 90 degrees rotation moves from maximum to minimum transparency. I believe this has been used commercially as well.

I have seen these as well, but they are not linear. The transmission goes with the cos^2(theta) rather than just theta. In some cases this would be linear enough, but if the original poster needs 175k (an irregular value) pi/2 radians ....
 

Roff

Well-Known Member
Screech said:
I nead variable resistance that goes from eg.0- 175K ohms Linear, when a shaft is turned from 0-90degree angle.

The 175 K is just an Example

My Crap Idea!
The image below is a disk that will go on the shaft.
The red thing is a light dependent resistor.
on the other side of the disk is an LED(hidden on the other side.)
as the disk rotates, it starts Unblocking light,and the LDR will vary.
The thickest unblocked cut out will have to be the right size so the LDR will read 0 K, and the thinnest to read 175K
As I said this is a crap design.


Is there a way of using a pot somehow, or modifying it,or some other simple design?
IT would be even better if it was able to be customized, eg: if I decide I nead more resistance @90 degrees.

Does anyone know anything about digital potentiometers?
Thanks
What do you really need to happen as you rotate the shaft? I'm assuming the "pot" controls something. There are generally several ways to solve a problem. By posing your question the way you have, you may be overlooking a much simpler solution.
 

Dean Huster

Well-Known Member
Ordinarily, a commercial application will use a pot with a higher value that will cover just under the range you want in the 90 degree rotation, put a mechanical stop on the pot and then add a couple of trimmers in series and/or parallel to tweak the end values to what you want. I've seen companies "waste" an expensive Bourns or Helipot precision 10-turn pot limiting it to a portion of a turn to get a similar function, just so they could have the linearity. Your biggest problem is finding a linear pot with that high of a value. Linear pots are usually wirewound and wirewound pots are scarce when the values get that high.

There might be a MOSFET electronic solution to the problem, using the MOSFET as the final resistance element with electronics controling the gate.

Dean
 

Screech

New Member
Ron H said:
What do you really need to happen as you rotate the shaft? I'm assuming the "pot" controls something. There are generally several ways to solve a problem. By posing your question the way you have, you may be overlooking a much simpler solution.

Its the variable resistance(resistor) for a monostable timer.
I just relised, could I alter the cap instead, to make any pot suit my timing requirements?

Thanks guys.
 

Roff

Well-Known Member
Screech said:
Ron H said:
What do you really need to happen as you rotate the shaft? I'm assuming the "pot" controls something. There are generally several ways to solve a problem. By posing your question the way you have, you may be overlooking a much simpler solution.

Its the variable resistance(resistor) for a monostable timer.
I just relised, could I alter the cap instead, to make any pot suit my timing requirements?

Thanks guys.
I think you've got it.
 
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