Need advice on making my lamp! Turn dial into string pulled mechanism?

[bigdoghustler]

TL;DR Summary

Idiot lamp maker seeks smart people for advice.

Excuse my lack of knowledge and please correct me on anything! I'm making a lamp right now and the vision is to have a dimmable lamp in which there are two strings controlling to the potentiometer. To increase the brightness, one string is pulled down and the other will go up. I have a lamp with a rotary dimmer on it right now and I figure the best way to do this is to attach the string to the knob.

The problems I'm attempting to solve are: 1: the range of motion on the knob translates to about 1 inch on the string. Ideally I'd like to pull the string about 5 inches or so to cover the whole range of the dimmer.

and 2: the knob takes a bit of force to turn, and I would like to make it a little bit easier so I don't knock over my lamp when im adjusting the brightness.

Looking for input as to solutions for these problems as well as if anybody has a a completely different idea as to how I should approach this.

Thanks!

~lamp guy

 

[DaveC426913]

The problems I'm attempting to solve are: 1: the range of motion on the knob translates to about 1 inch on the string. Ideally I'd like to pull the string about 5 inches or so to cover the whole range of the dimmer.

and 2: the knob takes a bit of force to turn, and I would like to make it a little bit easier so I don't knock over my lamp when im adjusting the brightness.

Both of these would be solved with a block and tackle system, say, 4:1.

You'll have a challenge making it small enough, but the principle is sound.

The diagram makes it look more complicated than it is. It's really just two axles with a line run twice between them.

 

[berkeman]

I'm making a lamp right now and the vision is to have a dimmable lamp in which there are two strings controlling to the potentiometer.

Welcome to PF.

Can you upload some pictures of the parts you have that you are wanting to use for this? Use the "Attach files" link below the Edit window to do the uploads. It sounds like you already have a potentiometer in mind*, but what are the other pieces you have so far? (support structure, lampshade, bulb(s), etc.)

*Note that you don't control the brightness of an incandescent lamp with a potentiometer. You would traditionally use a rheostat instead (which uses Pulse Width Modulation of the AC Mains voltage to control the brightness setting). Are you wanting to use 120Vac incandescent bulbs for this? Those generally are not available anymore for lamps, so maybe you are going to use LED replacements? Those are controlled in yet a different way...

 

[Filip Larsen]

I assume the movement of the string is translated to rotation of the potentiometer axis via a "wheel" of some diameter around which the string is wound. If so, then you can control the gearing between the linear movement of the string and the rotation simply by using a wheel with suitable diameter. Make the wheel bigger and you would need more string movement for the same rotation of the axis.

 

[Rive]

I have a lamp with a rotary dimmer on it right now

Just to clarify, this means you have an actual, working, dimmable lamp, approved as a product right now and you need help only with the mechanics?

I would say some plastics planetary gear stuff would be ideal for you.

You would need to 3D print a proper housing, of course.

 

[DaveC426913]

I assume the movement of the string is translated to rotation of the potentiometer axis via a "wheel" of some diameter around which the string is wound. If so, then you can control the gearing between the linear movement of the string and the rotation simply by using a wheel with suitable diameter. Make the wheel bigger and you would need more string movement for the same rotation of the axis.

Yeah, that would work too, although I suspect the wheel would be quite large to achieve the 5:1 advantage he's looking for.


What we need is to know the size - the physical range - of the device he's using (be it a potentiometer or a rheostat). @bigdoghustler ?

 

[Baluncore]

I have a lamp with a rotary dimmer on it right now and I figure the best way to do this is to attach the string to the knob.

Using a wheel that is five times the radius of the existing knob will give the required ratio. The problem will be keeping the string on the rim of the wheel.

The use of a low-cost stepped belt and sprocket from eBay might do that, and the belt could stay as a loop, but that would not hang neatly. Stepped pulleys and belts are available in many diameters and pitches.

The next conceptual refinement would be to replace the string or belt with a ball chain, or a more flexible bead chain. A ball chain will run on a nylon pulley, with a groove and round holes drilled, indexed around the rim. There is a blue one shown here that can be 3D printed: https://www.thingiverse.com/thing:70179

Those pulleys and ball chains are used in some roller blinds. The chain is very widely available, while the pulleys are much more difficult to find. I would make my own pulley and chain guide, but you may be able to use the spare parts for a common design. Alternatively, salvage the sprocket and chain from a blind that is being replaced. Search eBay for:
"Blind Parts 38 43mm Geared Side winder Clutch Chain Drive Roller Blinds"

 

[DaveC426913]

Using a wheel that is five times the radius of the existing knob will give the required ratio.

Yeah but if the range of his rheo/pot spans a mere inch, that makes a wheel five inches in diameter.

I assume that's too big for a lamp.

Even if we only use a segment of the wheel, it still swings across a five inch arc.

My suggestion for a 4:1 block and tackle is limited in size only by the blocks, which can be arbitrarily small.

 

[Baluncore]

My suggestion for a 4:1 block and tackle is limited in size only by the blocks, which can be arbitrarily small.

... Blind Parts 38 43mm Geared Side winder ...

It appears that the blind actuator may contain a reduction gearbox.