I'm building a retracting home theater projector screen and I'd like to make it automatic; press the "up" button once, and it latches until it's up, then shuts off, same in reverse. After tossing around several ideas for "home" and "extended" signals (limit switches, encoder feedback, screw/nut travel distance, single revolution pulses, etc.) I have decided that the simplest method would be to put 2 patches of conductive tape on the back side of the screen roll that will make contact with some probes riding on roll.

So I need some conductive tape that will adhere to fabric. I'm fully capable of googling (and I am googling) but I was hoping for a personal recommendation if anybody has used any of these products and is confident they will perform as I expect. My requirements are that it adheres to fabric and flexes (thousands of times hopefully) without coming loose and is extremely pliable - will not cause a noticeable concave shape from the front of the screen after transitioning from rolled up to extended.

Thank you.

 

3M reflective tape and a small retro-reflective sensor?
As long as the screen itself on the back is not too reflective?
I would have though probes could be too intermittent?
Max.

 

3M reflective tape and a small retro-reflective sensor?
As long as the screen itself on the back is not too reflective?
I would have though probes could be too intermittent?
Max.

I considered the reflector/sensors but I believe they are too expensive. I should probably confirm that. I am used to seeing these sensors sell for 3 digits, but those are the industrial sensors that I deal with on a regular basis. There may be hobbyist-level offerings with a more attractive price. I will look into it.

But I believe that the probes will make good contact; I was thinking something like a length of coat hanger or stiff copper rod that will ride on the roll, with tension applied by a rubber band. Even if intermittent, as long as it makes contact long enough to energize a relay and break the latch circuit, it works good enough for what I need.

 

I considered the reflector/sensors but I believe they are too expensive. .

There is ebay!
But if you can sense close and want to put one together for a couple $$ there is Honeywell HOA1397, HOA0149, and HLC1395 under $10.00 ea from DigiKey.
Cheaper OPB606.
Max.

 

My requirements are that it adheres to fabric and flexes (thousands of times hopefully) without coming loose and is extremely pliable - will not cause a noticeable concave shape from the front of the screen after transitioning from rolled up to extended.

Thank you.

Yeah good luck with that... Not gonna happen IMO.

How about string pots? Or motor current sensing?...

 

aluminum ducting tape ? it sticks on 1 side and is full flexible aluminum on the other,
i gues this whould be your ace tape for your idea ?
http://junyue.en.made-in-china.com/product/WMeJExuladhP/China-Self-Adhesive-Aluminum-Foil-Tape.html

 

aluminum ducting tape ? it sticks on 1 side and is full flexible aluminum on the other,
i guess this would be your ace tape for your idea ?
http://junyue.en.made-in-china.com/product/WMeJExuladhP/China-Self-Adhesive-Aluminum-Foil-Tape.html

Aluminum tape is, "dead soft" when it's new, but it work hardens like copper and always grows an oxide coating in a few minutes. Spray adhesive (like 3M 999 IIRC) is used to help attach it to reticent substrates.

 

Hello,

Could you attach some conductive fabric to the projector screen?
Here is an example for the fabric:
http://www.amazon.com/Conductive-Fa...=1375725878&sr=8-1&keywords=conductive+fabric

Bertus

 

Hello,

Could you attach some conductive fabric to the projector screen?
Here is an example for the fabric:
http://www.amazon.com/Conductive-Fa...=1375725878&sr=8-1&keywords=conductive+fabric

Bertus

Hmmm.... I want to avoid any stitching visible from the front of the screen, but perhaps a couple of drops of superglue in each corner would hold it? Does superglue bond to cloth well? I've never tried it.

 

Maybe you could weave some conductive thread (ie. thin wire) into the existing fabric. You don't need much current carrying, just a detectable drop in resistance.

I was thinking about those rubbery keys on a calculator or a TV remote keypad that are detected when they touch a conducting area on the PCB underneath. I've never looked but maybe you could find conductive rubber sheet.

I guess I'm wondering why you couldn't somehow use a little pressure switch as a limit switch. It just sounds easier to set up to work reliably.

 

I imagine the back of your screen is white? Paint/marker pen some black squares and sense those with reflective opto switches like these?

 

Have you thought of the effect that your coat hanger probes are going to have on the screen after lots of cycles?

A number of interesting and workable ideas have been thrown out. So here's one to add to the mix. Use a proximity sensor such as an ultrasound or IR send/receive to sense if there is anything within an inch or so of the sensor. Then position one at the top such that it is uncovered just as the screen reaches the retracted position and one at the bottom such that the it is covered just as the screen reaches the fully extended position. Of course, I'm thinking in terms of an installation where you can afix the sensors to a wall or a frame or something like that.

Another way would be to connect a string to a bobbin that is tensioned to roll up the string as the screen retracts. Then connect the bobbin to a shaft encoder. This would let you position the screen wherever you wanted. Of course, unless it is an absolute encoder, you have a reference problem. But that could be handled by using a limit switch that is closed when the bar on the bottom of the screen (I'm assuming there is one) reaches the fully retracted position.

 

I don't have or have ever used a screen but things seem to be getting overly complicated here? Don't the screens automatically deploy and retract? Use a simple timed motor? Current sense may work in either direction when it stalls cut the power? There's a hundred ways to do this!

 

? There's a hundred ways to do this!

Hence a hundred suggestions!
Max.

 

I don't have or have ever used a screen but things seem to be getting overly complicated here? Don't the screens automatically deploy and retract? Use a simple timed motor? Current sense may work in either direction when it stalls cut the power? There's a hundred ways to do this!

Yes, there are a hundred or more ways to do it, and I bet I've thought of half of them already. I'll address what's been suggested so far and after, I'll address the rest of my ideas that haven't come up yet. I'm trying to keep it as simple and cheap as possible, ad still be reliable.

timed motor: What if one of the kids grabs it on the way up and stalls it out for a second? then it goes into limbo where it doesn't go up all the way, and then comes back down, past the "extended" position and starts travelling back up winding itself inside out.

Current sense: would work only when returns to home position. on travelling to extended position, there is nothing to stall it out, and it would wrap itself back up inside out as described above.

There is ebay!
But if you can sense close and want to put one together for a couple $$ there is Honeywell HOA1397, HOA0149, and HLC1395 under $10.00 ea from DigiKey.
Cheaper OPB606.
Max.

Those sensors need very close proximity to work. I'm not sure I could pull it off.

How about string pots? Or motor current sensing?...

current sensing, covered,
String pots, eh. I don't have much experience with them. They're spring loaded, right? how much tension do they apply? would it cause my screen to be unevenly loaded and try to spool up unevenly?

Maybe you could weave some conductive thread (ie. thin wire) into the existing fabric. You don't need much current carrying, just a detectable drop in resistance.

The fabric is special projector screen fabric, a composite of cotton and polyester sandwiched with rubber. Weaving something into it after it's already made and delivered would be a chore, and would be visible from the front side.

I was thinking about those rubbery keys on a calculator or a TV remote keypad that are detected when they touch a conducting area on the PCB underneath. I've never looked but maybe you could find conductive rubber sheet.

Perhaps. I wonder how thick it would be. I want whatever it is to be as thin as possible so that everything rolls up nice and uniform, no lumps that would make it try to spool unevenly.

I guess I'm wondering why you couldn't somehow use a little pressure switch as a limit switch. It just sounds easier to set up to work reliably.

I think that the fabric will prove to be thin enough to confound such an approach. too hard to differentiate between one layer spooled or 3 layers spooled. And it will be impossible to differentiate between 3 layers spooled and 3.5 layers spooled; the switch would extend in increments as it passes the beginning of the first layer.

I imagine the back of your screen is white? Paint/marker pen some black squares and sense those with reflective opto switches like these?

That's something I hadn't thought of. But like the sensors MaxHeadRoom suggested, the sensing range is small. 0.2in - my spooled up screen is likely to occupy more space than this.

Have you thought of the effect that your coat hanger probes are going to have on the screen after lots of cycles?

Well I envisioned the bars riding gently on to of the roll, not probing it point-on. I didn't expect there to be any issue, but I suppose after a few thousand cycles it might start to take a toll. But I full expect to have changed the fabric out several times before that becomes an issue, due to little chocolate handprints and gouged holes from falling furniture and such.

A number of interesting and workable ideas have been thrown out. So here's one to add to the mix. Use a proximity sensor such as an ultrasound or IR send/receive to sense if there is anything within an inch or so of the sensor. Then position one at the top such that it is uncovered just as the screen reaches the retracted position and one at the bottom such that the it is covered just as the screen reaches the fully extended position. Of course, I'm thinking in terms of an installation where you can afix the sensors to a wall or a frame or something like that.

The sensors that can see out an inch or so are the type I am used to using. The industrial type. The expensive type. I still have yet to search for a hobbyist-level sensor that can do the job, that is on my to-do list.

Another way would be to connect a string to a bobbin that is tensioned to roll up the string as the screen retracts. Then connect the bobbin to a shaft encoder. This would let you position the screen wherever you wanted. Of course, unless it is an absolute encoder, you have a reference problem. But that could be handled by using a limit switch that is closed when the bar on the bottom of the screen (I'm assuming there is one) reaches the fully retracted position.

Using an encoder is going to be a little more involved than I was planning. It sounds like a fun project, but we don't have a TV right now. The pressure is on to get this thing assembled and working as soon as the fabric arrives (tomorrow).

Now, my other ideas, in case they come up:

• A multi-turn pot affixed to the opposite end, outputting an analog voltage, with comparators to indicate extended and retracted position - probably easier than the encoder, but still a little more involved than I want to get.
• A screw affixed to the opposite end, with a nut on it that is restrained from rotating, and thus travels linearly, activating limit switches in either direction - not sure how much resolution I get out of it, just like the pressure sensor reading how many rolls are remaining. Also mechanically complicated.
• A small pulley affixed to the opposite end which drives a larger pulley; the turns/turn ratio to be matched to number of turns to deploy the screen, so that one full revolution of the large pulley corresponds to a full deployment of the screen, and this pulley could have flags for proximity sensors, or could have a cam for limit switches. - Mechanically involved
• The conductive tape idea - unsure what product will work and not on a time or money budget that supports multiple attempts to get it right
• just having simple momentary controls; you hold the button in until the screen comes all the way down, then release it. simple. - Not very cool. But I may have to live with it.
• Another thought about the optical sensors... These seem like fairly simple sensors; would they be affected by ambient light? While watching a movie, would a scene of an explosion or a disco hall send my screen scrolling upward?
• There are a few more thoughts that escape me now; I'm getting sleepy. But if you have more ideas, please share. And if you have a good recommendation for conductive tape I described, let me know. I'm still considering the conductive fabric and the opto sensors - I don't want anybody to feel like their feedback has fallen through the cracks.

Thanks for all the input!

 

Glue a little ebay neo magnet to the screen, and use a $2 hall switch. It will give really reliable detection (and be dirt proof etc).

I play with those hall switches a bit. With a strong magnet it can be detected from almost an inch, but even with a small weak magnet you should get reliable detection at 1/2 inch or more.

 

As soon as it became apparent that you had access to the mechanism (I was picturing that you had an enclosed motorized screen that you were working with), the first thing that came to mind was to mount a small gear on the shaft and a larger gear next to it. Very simple to do, particularly if you are using a wood endpiece for your mount. Size the gears so that the big gear makes less than one rotation between full-up and full-down. Then expoxy a nail (or something else suitable) so that it sticks out past the rim of the gear (or some other suitable arrangement) and can engage two limit switches positioned appropriately. If your spool is 4" in diamenter, then each rotation is about a foot. So if your screen is six feet tall you would only need a 6:1 gear ratio. Use a 0.5" diameter gear on the shaft and a 3" diameter driven gear. You may be able to epoxy the small gear directly to the end of the shaft.

 

Glue a little ebay neo magnet to the screen, and use a $2 hall switch. It will give really reliable detection (and be dirt proof etc).

I play with those hall switches a bit. With a strong magnet it can be detected from almost an inch, but even with a small weak magnet you should get reliable detection at 1/2 inch or more.

But he's already concerned about tape on the screen being too thick and stiff and distorting the screen, either when it's extended or when it's rolled up.

 

Hello,

You could use some fabric glue to attach the conductive fabric to the projector screen:
http://www.amazon.com/Aleenes-Fabric-Fusion-Permanent-Adhesive/dp/B00178QSE6

Bertus

 

As soon as it became apparent that you had access to the mechanism (I was picturing that you had an enclosed motorized screen that you were working with), the first thing that came to mind was to mount a small gear on the shaft and a larger gear next to it. Very simple to do, particularly if you are using a wood endpiece for your mount. Size the gears so that the big gear makes less than one rotation between full-up and full-down. Then expoxy a nail (or something else suitable) so that it sticks out past the rim of the gear (or some other suitable arrangement) and can engage two limit switches positioned appropriately. If your spool is 4" in diamenter, then each rotation is about a foot. So if your screen is six feet tall you would only need a 6:1 gear ratio. Use a 0.5" diameter gear on the shaft and a 3" diameter driven gear. You may be able to epoxy the small gear directly to the end of the shaft.

Like this

Now, my other ideas, in case they come up:

• A small pulley affixed to the opposite end which drives a larger pulley; the turns/turn ratio to be matched to number of turns to deploy the screen, so that one full revolution of the large pulley corresponds to a full deployment of the screen, and this pulley could have flags for proximity sensors, or could have a cam for limit switches. - Mechanically involved

• 
  
  This probably the most fool-proof method IMO, and I have the resources (lathe) to make the parts needed. I guess it's not actually as mechanically involved as I decided initially. Not very attractive, but I could build a hardwood cover for it at some point in the future; or at least tell my wife that.