Resistive sheet material for homemade potentiometer

[Karen O]

Hi,

My latest Narrow Band Television activities require an unusual kind of potentiometer. I need a continuous circular track with four terminals such that the wiper can do a four way fade. It is for an experimental disk motor controller.

Were such potentiometers ever manufactured?

More importantly for me, does anyone know a source of resistive sheet material with which I can attempt to make my own?

Many Thanks,
KO

[kalee20]

Dunno. Maybe,but I have not come across any.

As for sheet material, firms who make potentiometers may use the stuff - or they may cut insulating material to size and then coat it with resistive stuff afterwards. I don't know that either! But somebody at a company like Omeg, or Blore Edwards, may be able to help. Or you could try using Teledeltos paper, but doubt if it will give you a long life with a wiper running across it.

[Radio Wrangler]

Penney & Giles?

David

Going a bit digital, how about http://uk.farnell.com/bourns/eaw0j-b...tt=ace+encoder

Short life but OK for testing, antistatic bag materal (the black stuff).

Very thin steel (shim stock) make it big enough to give a reasonable resistance.


Magnets and hall sensor http://uk.farnell.com/honeywell-s-c/...oic/dp/1811312 these give a sin/cos output.

[Diabolical Artificer]

A few ideas off the top of my head.

You can draw a resistor using a graphite pencil. I'd try grinding the pencil lead into powder and mixing with glue,stick it on a washer. This coat would be resistive.

You could use a wire-wound track.

What about glueing two pot tracks together on a former?

Also I,ve just tried one of those grahite type tubes that are used in power cables. They have a very high resistance though, 7 meg +. Maybe you could cut them into slivers with an angle grinder/Dremmel.

Lastly,I did have an address for a pot/slider manufacturer who were happy to do bespoke items (Canadian company?). I've lost it though. The page address is on this forum somewhere in one of the threads. Under the heading Slider wanted or Mixer something. Just looked but unable to find it.

I have two big old Welwyn pots your welcome to. They are wire wound but the body is thick and would be easy-ish to modify. They're easy to dismantle.

Andy

[woodchips]

Not certain I understand how the tracks are arranged, is it just one continuous track? Are the terminals to the track at 90 degrees? Just one wiper? Four wipers at 90 degrees?

Why not just use four pots geared together? Not 360 degree rotation but is the effect the same?

[Karen O]

Hi All,

Thanks for all the suggestions.

The first thing I tried was an anti-static storage box. When I poked my meter probes about 1cm apart, it showed a readable resistance - about 200k. But then the reading started to drop rapidly, eventually reaching 18k (and still creeping downward even then). I wonder what that's all about

I did think of using a crank arrangement with two pots at 90 degrees. This could be made fairly easily by drilling large plastic knobs and inserting self tapping screws into the tops for pivots. Two pots would work, although the used travel would perhaps only be 100 degrees. I could overcome this by using four pots, all operating near the ends of their travel.

Another thought was linear pots in an arrangement not unlike the pistons on a steam train.

The plan I favour at the moment, and mentioned by one of the responders, is to make a continuous circular track by cutting up the tracks from two pots. Conductive paint would allow connection to the four tap points, two of which will be on the joins.

Woodchips: Yes that is the requirement - continuous track, four tap points, one slider.

LDRs and a rotating bulb?

[Karen O]

Yes, LDRs would be a good solution but perhaps a little tricky to calibrate.

I forgot to mention that the effect I'm looking for can actually be achieved using a joystick - so long as the lever is kept to the outside of the conical guide. The two pots will then implement sin and cos.

I want my solution to be repeatable and so I might settle for a solution based on two linear potentiometers and a peculiar double crank mechanism that operates push rods 90 degrees out of phase.

The crank assembly is then the main challenge. But given that this would replace a potentially more difficult mechanism (an adjustable optical pick-up for the disk) I think it can be justified.

Apologies if I have strayed into matters NBTV. Many thanks for all your answers - I have a lot to go away and think about. Back to my 625-to-405 converter

[Bazz4CQJ]

I wonder if you could achieve anything by taking fine graphite powder (as available for lubing locks) and sprinkling that on to some sticky-backed plastic. The problem with all things like this is that even if you find a company that makes it, their minimum quantities often make them unattractive propositions for hobbyists.

[dseymo1]

I have an item of scrap testgear containing a rather nice large four-gang pot.
It's assembled in such a way that it *might* be possible to change the orentation of the individual elements if required.
No idea of (resistance) value, but can check if of interest.
Yours for the cost of postage if it's any use.
PM me for details if you like.

Google 'sin cos pot' no prices just 'apply for quote'.

[Malcolm G6ANZ]

Karen, I have got an ex equipment continous rotation pot with two wipers, I believe the two wipers are 90' apart. 20k track resistance @ 1% lin. Its Bourns 6657S-ZUP-203 however I can't find data on it in the Bourns site! It is 'slightly' specialist. Your welcome to try it for the cost of postage.
As suggested by Merlinmaxwell it is a sin/cos pot
Malcolm

[charambo]

A teacher of mine made a two wiper sin/cos pot by winding resistive wire on a square piece of plastic. He then put two wipers 90 degrees apart sliding over the wire.
When wipers travel perpendicular to the wire, there is a fast change in the output voltage, which will be around 0V, if the ends of the wire are supplied with equal voltages of opposite sign. As the wiper travels near the + or - terminals (the ends of the wire), it goes in the same direction of the wires, and the output for a given angle rotation will change little. Changing the shape of the base plate allows for generating different functions.
If you are interested in this, I might try to make a sketch of how he made it.

[brianc]

Companies which supply parts and materials to piano technicians usually carry a product used for lubricating wood against wood bearings called Winslip. This is colloid of powdered graphite and a very volatile liquid. The Winslip is painted onto the bearing surfaces (or Tufnel!) and the graphite is deposited as a thin layer which can be thickened by several applications. The surface is hardened by burnishing after each application. I have used this stuff to effect a repair to a pot track so maybe it's worth a try.
It can be found at the bottom of this page:
http://www.fletcher-newman.co.uk/frameset.html

[Karen O]

Once again I am overwhelmed by the generosity of the members on this forum (on re-reading that sounded like it might be sarcastic but it definitely is not I can assure you).

It is comfort enough to know that continuous turn potentiometers were once made and that I didn't dream that these things once existed!

However, I'm not sure that a sin/cos pot will meet my needs in this case. I will have to go away and think about it for a while and get back to those who have made such kind offers. I am also a little mindful of those in the NBTVA who might want to repeat what I have done, in which event I have to be biased towards solutions based on parts and materials available today. The LDR suggestion is perhaps the front runner in this respect.

Yes Charambo, uniform excitation of a sheet of material would yield a true sin/cos output from a wiper. What I had in mind originally was a linear fade between the compass points so to speak. That would describe a square rather than a circle in the phase plane, which is still useable I'm sure but would result in slightly unsmooth adjustment.

Once again, thank you to all those who have made kind offers.

[Anthony Thomas]

Hi Karen O

I just chanced across this thread and wondered if I might come with a couple more suggestions to add more confusion or food for thought?

1) Radio Wrangler suggested Penny and Giles and I took that to be a joystick control for wheelchairs, I might still have some offcast parts from joystick controls that you could experiment with.

2) The motor trade used to have repair "resistive paint or laquer" for repairing heated windscreens and suchlike, expensive but readily obtainable localy perhaps.

Drop me a pm if you would like me to dig out those joystick components, I seem to think that one had two potentiometers arranged in a crosswise fashion that could be adapted not to spring bak into the middle position or the "off" position.

I have no real idea what I am talking about but have a fertile imagination

Tony

[Karen O]

Hi Tony,

I've only just noticed your post - sorry.

That is a very kind offer but I've settled for building something myself based on two slider potentiometers and a quadrature crank on the control shaft. I'll try to make a video if I get it to work!

Thanks again.

[Anthony Thomas]

Hi Karen,

I was trawling through my potentiometers a few days ago and found two of these and if they are of any use to you I can send them in the post.

http://components.arrow.com/part/det...paign=octopart

Please let me know if they are of interest to you, It seems like you have possibly found a solution. These two controls were removed from a reading screen for those with impaired vision.

Best wishes
Tony