Parts Query

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MoonGoon

Member
Iv searched and cannot find.

Does anyone know if its possible to get a 8 pole Potentiometer? (for lack of a better description)

i need to vary a number of resistors, all having the same value at the same time. They are not tied together, they are all isolated from each other, as they are each dealing with different voltage at the same time.

Ideally i would want 8 pole Potentiometer.

or better yet:
Does anyone know how i could vary the values of multiple isolated resistors using just one potentiometer?

i hope i have described this well enough. i cant find anything anywhere on it.
 
D

Dippy

Moderator
No, can't help but if you briefly describe your PICAXE project with any schematics then someone might have a solution. A 'second pair of eyes' sometimes helps.
 
hippy

hippy

Ex-Staff (retired)
Stereo "two pole" pots are the most I've ever seen. It is possible to use a single pot and have the voltage used to control eight digital potentiometers but how best to do it depends upon excatly what you're trying to control.
 
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BCJKiwi

Senior Member
In the old days there were massively ganged multiposition switches that could be built up with wafers on long rods.

Something like that could be used, with resistors to create whatever was required - any one still got one of those aluminium chassis with all the spagetti bare wiring and wax caps with glowing tubes on top - bound to have some of these inside!

Or check here;
http://newzealand.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0665837
 
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gengis

New Member
Assuming you really do need a potentiometer(s)

You could use a set of digital potentiometers. Some are designed to work with a "count" input for use with an optical interrupter others for serial control.

These are serial input devices the count input are easy to use with a single knob control:
MCP41XXX/42XXX Microchip parts
DS3501 Dallas

Many others if you search.
 
D

Dippy

Moderator
Can you enlighten us a bit more Moongoon?

Someone may have a good idea to help, otherwise everyone will be polishing their crystal balls so hard that they melt :)

Someone might even suggest some rotary pots and pulley wheels.

Alps do a Quad.

Where is the PICAXE application in this? There maybe another solution.
 
M

MoonGoon

Member
The application is Brightness control of a 7-segment Charlieplexing display.

I first did it via code (and one pot) but i have reached the limit of the Chip and its still too bright and flicker is starting. SO i think doing it in hardware is the solution. I need a way of getting the 8 resistors, (1 per line) currently at 33R, to vary in value from approx 20R to say 500R. All at the same time. Space is critical. and i dont have any spare pins to give to a digital pot.

Manuka's suggestion of Ganged Pots is the closest i think, it would do the job, except they are typically huge so not applicable here i think. Size is crucial.
I am toying with the idea of making my own Ganged Pot, so i can slim it down. I cant have the total length (excluding shaft) exceed 60mm.

Unless somesome knows of a piece of wiring magic to do the job with one pot?
 
hippy

hippy

Ex-Staff (retired)
Andrew : The problem with charliplexing is that there's no constant common andode/cathode.

The obvious solution would seem to be to use PWM brightness control. In this case simply have more off time and less on time. Pure software, no hardware needed at all.
 
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inglewoodpete

Senior Member
Two options come to mind.
1. Use a PICAXE to drive an 8x digital resistor. I'm not sure that they come in lows values or are able to carry the current you need.

2. (This one's a long shot!) I once saw a multiganged pot made of a series of vertical mounting trimpots. A small steel bar was inserted through the screwdriver slots to drive them as one. However, trying to drive 8 trimpots might be too heavy. If you find some light action trimpots, you could try this by mounting them on a piece of strip board to keep them all aligned.

"Think outside the square"

-Peter
 
premelec

premelec

Senior Member
Thinking inside the octagon - 8 LDRs with a central light source so you brighten and dim by using the LDRs resistance to control something...

Note that you can make an isolated current control with 4 diodes and a MOSFET or LM317 as current controlled element using an LDR and LED to control the current...
Isolation is good, temperature stability and linearity are not :) Response slow but ok for visual..

Note that current set inside a diode bridge is bi-directional. The down side is the additional voltage drop to keep the active elements operating - I forgot to mention why four diodes before... it's so you can control current in either current flow direction with the same circuit...
 
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M

MoonGoon

Member
Thats an awsome idea.

Thats just the thinking i like. I tried using software but iv reached my timing limits, i cant get the pauses any lower. im already at 0mS, and still too bright. And considered some other ways of doing it by hardware but no paned out.

LDR's .... that has some serious merit, Thanks for the idea Premelec :)
 
premelec

premelec

Senior Member
It's possible you can find the right LDRs but I was thinking the LDRs biasing a MOSFET or control constant current from LM317 [LM317 + 1 resistor makes an constant current source when you take output from the control pin - resistor goes from output pin to control pin current = 1.2/R]. Anyhow good luck with it!
 
M

MoonGoon

Member
I am liking the idea of using LRD's somehow, but my biggest wall is space, it has to be very small. So far my entire design is 2 chips and a dozen resistors, all will be surface mount.

I am very reluctant to add any more to the board, I can add switches and pots fine, its just what goes on the actual board that im limited too.

I have considered making my own ganged Pot's using surface mount trim pots.
But im really not sure how that will turn out.
 
hippy

hippy

Ex-Staff (retired)
Maybe a silly question, but what happens if you reduce the PICAXE supply voltage ? Does everything dim ?
 
M

MoonGoon

Member
Its actually funny you should mention that Hippy.

A mate of mine at work suggested that same thing to me today. And to be honest, it never occured to me to try it.
When i get home im gonna try it and see how much it affects it.

Thanks
 
M

MoonGoon

Member
You cant imagine how much of a dumb look i have on my face right now.

The idea of changing the voltage NEVER occured to me. My mind was so concentrated on changing the Resistance that i didnt see the obvious.

Quite funny actually.

Hippy, i guess thats why your a Moderator.

So simple, So brilliant. Cheers.
 
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Dippy

Moderator
Now you see why it is (sometimes) a good idea to post the whole problem.
Some people here have been at it (ooer) for many years.
 
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