Naked Science Forum
Non Life Sciences => Technology => Topic started by: Karsten on 13/12/2009 21:49:24
-
My daughter and I went to the movies the other day. Since we were the last to leave the theater afterward, we took a peek into the projector room and received a little tour from the owner. We also received a roll of 2:30 minute movie trailer as a gift. I thought it would be pretty cool to build a wooden machine that with the help of a flashlight and hand-crank could project this trailer onto a 5cm by 5cm screen (2"x2"). I guess I have to flash the flashlight (or whatever light source) just at the right moments and in sync with the film forward movement to create the illusion of movement. How do I do this simply? I am not looking for great quality, just something that illustrates the principle. Something mechanical rather than electronic.
I remember seeing human powered apparatuses (sp?) in the "Deutsches Museum" in Munich, Germany that were VERY simple and showed a little clip. I forget how those worked though.
Anyone?
-
I was going to suggest buying a second hand projector/ viewer on ebay, but if the film is 35mm they cost hundreds of dollars.
http://cgi.ebay.ca/MOVIOLA-D-20-Table-Top-Motorized-35mm-Film-Viewer_W0QQitemZ350273067744QQihZ022QQcategoryZ4791QQcmdZViewItem
If you just want to see a magnified image of the frames of film you can buy a magnifier called a lupe for ~$10 ...
http://cgi.ebay.co.uk/Magnifier-Lupe-Jessops--8x-Magnification-With-packaging_W0QQitemZ330380778333QQcmdZViewItemQQimsxq20091126?IMSfp=TL091126203007r7977
http://www.comparestoreprices.co.uk/photography-accessories/kaiser-k2334-8x-magnification-loupe.asp
-
My daughter and I went to the movies the other day. Since we were the last to leave the theater afterward, we took a peek into the projector room and received a little tour from the owner. We also received a roll of 2:30 minute movie trailer as a gift. I thought it would be pretty cool to build a wooden machine that with the help of a flashlight and hand-crank could project this trailer onto a 5cm by 5cm screen (2"x2"). I guess I have to flash the flashlight (or whatever light source) just at the right moments and in sync with the film forward movement to create the illusion of movement. How do I do this simply? I am not looking for great quality, just something that illustrates the principle. Something mechanical rather than electronic.
I remember seeing human powered apparatuses (sp?) in the "Deutsches Museum" in Munich, Germany that were VERY simple and showed a little clip. I forget how those worked though.
Anyone?
I think there was a Victorian device that was essentially a rotating drum with a series of different pictures attached to it. The drum rotated around a vertical axis to produce a moving image. What I don't understand is how it managed to freeze each frame for the viewer. Maybe it didn't actually freeze each frame (as a projector does) but merely provided a brief glimpse of each image, relying instead on the persistence of the human eye. That would have the same effect, although the image might be a bit dim.
OK, this might work. Rig up a mechanism that lets you pull the film through some sort of guide. You can get fancy with winders etc, but you could also simply pull it by hand.
You need a light. I think a LED would be ideal if you can get one that's bright enough. You need to rig it so that the LED turns on and back lights the image on each frame for a fairly short interval, otherwise the image will appear to move. This might work with a LED because they turn off and on virtually instantaneously. (It will not work with an incandescent light bulb because they are too slow.) BTW, as this "projector" relies on the persistence of the human retina, it will only work in a darkened room, if it works at all!
The "tricky bit" is the timing wheel. It needs to engage with the sprocket holes in the film and turn on the LED briefly as each frame is centered over the LED. If you can make or obtain a metal sprocket wheel, you might simply run a sprung contact against it to make the circuit to the LED. To break the circuit, stick a piece of vinyl insulating tape on the sprocket wheel. Of course, you could use a microswitch driven by a cam or lots of other techniques, but the vinyl tape method makes it really easy to adjust the timing and "dwell angle" of the LED.
You might replace the LED with an incandescent bulb if you can arrange for the sprocket to drive, or even be, a shutter that blocks the light. Yes. That might work too. Build a rather large sprocket wheel so that you can put a light source in the middle of it. You'll need a concentric shutter wheel that has a series of smallish holes centered behind each frame. As a frame enters the viewing area, the hole lines up with a stationary hole to illuminate the frame from the light source. This is probably closer to the Victorian contraption.
Here we go, but it's not much like the thing I "remembered".
http://en.wikipedia.org/wiki/Zoetrope
-
The "tricky bit" is the timing wheel. It needs to engage with the sprocket holes in the film and turn on the LED briefly as each frame is centered over the LED. If you can make or obtain a metal sprocket wheel, you might simply run a sprung contact against it to make the circuit to the LED. To break the circuit, stick a piece of vinyl insulating tape on the sprocket wheel. Of course, you could use a microswitch driven by a cam or lots of other techniques, but the vinyl tape method makes it really easy to adjust the timing and "dwell angle" of the LED.
An optical switch (http://en.wikipedia.org/wiki/Slotted_optical_switch) could be used to detect the black edge of a preceding frame, (or to count the sprocket holes), to synchronise the flash, (no sprocket wheels required). The flash would have to be very brief ~1/100th of a second, even then the image may be blurred by the motion of the film.
In a real cine projector the film is momentarily held still when it is projected (http://en.wikipedia.org/wiki/Intermittent_mechanism), i.e. the film does not move through the projector at a constant rate, it is jerked past the gate.
-
The "tricky bit" is the timing wheel. It needs to engage with the sprocket holes in the film and turn on the LED briefly as each frame is centered over the LED. If you can make or obtain a metal sprocket wheel, you might simply run a sprung contact against it to make the circuit to the LED. To break the circuit, stick a piece of vinyl insulating tape on the sprocket wheel. Of course, you could use a microswitch driven by a cam or lots of other techniques, but the vinyl tape method makes it really easy to adjust the timing and "dwell angle" of the LED.
An optical switch (http://en.wikipedia.org/wiki/Slotted_optical_switch) could be used to detect the black edge of a preceding frame, (or to count the sprocket holes), to synchronise the flash, (no sprocket wheels required). The flash would have to be very brief ~1/100th of a second, even then the image may be blurred by the motion of the film.
In a real cine projector the film is momentarily held still when it is projected (http://en.wikipedia.org/wiki/Intermittent_mechanism), i.e. the film does not move through the projector at a constant rate, it is jerked past the gate.
Quite correct RD. I think I mentioned that a projector holds the frame still for an interval. Making a mechanism to do that is rather complicated though, so I was trying to avoid it by relying on the image storage capability of the human retina. Do you think that would be insufficient?
Your idea of counting sprocket holes would certainly work. I suppose it depends on whether Karsten prefers an electronic or a more mechanical device. He could scan every frame of the film into a computer and replay the sequence entirely electronically, but I'm not sure that was quite what he had in mind.
A rather nasty way of holding the frame stationary for an interval is to use a torque limiting sprocket wheel. If the wheel has spring loaded detents, it will stop until the film exerts sufficient torque on the wheel to overcome the detent. It will then rapidly advance to the next detent, etc., etc. One problem with this approach is that the sprocket holes in the film will take a bit of a beating.
-
The flash would have to be very brief ~1/100th of a second, even then the image may be blurred by the motion of the film.
At 25 frames per second better make that 1/1000th of a second.
Even at 1/1000th the film will move 1/40th of a frame during its illumination.
It's gonna be blurred in the direction of motion of the film...
[ Invalid Attachment ]
and very flickery.
-
Oops! I just updated my previous post.
-
Just a couple of essential things (some of which have already been mentioned by others) you'll need to incorporate:
1. Each frame needs to be held stationary while it's being illuminated.
2. You'll need to be able to advance the film one frame at a time.
3. You'll need to be be able to draw the film off the spool, and draw the film onto the take-up spool, smoothly and continuously so as not to stress the film.
4. To achieve 3. you need to incorporate buffer loops either side of the gate (the place where the frame is held stationary and illuminated) so that as the film is advanced by each frame it is taken from the buffer loop and not directly from the spool. Similarly, as each frame is feed out of the gate, it runs into the take-up buffer loop.
5. You'll need a shutter that is syncronised with the film movement through the gate; the shutter closes when the film moves and opens once the frame is positioned. A spinning disk with cut-outs is probably the easiest solution here as it just needs to spin at the correct constant speed.
6. If you're using strong and hot illumination you'll need a blower to cool the film while it's stationary in the gate, other wise it might melt.
7. If you're running a long length of film then the film and take-up spools will need to change their angular speed to insure constant linear speed. The film can be drawn off the film spool by a constant speed toothed sprocket but the take-up spool will need some sort of slipping clutch.
In practice, you'll need two sets of toothed drive sprockets arranged as an inner pair, to step the film through the gate, and an outer pair to smoothly fill the feed buffer loop and empty the take-up buffer loop.
-
Just a couple of essential things (some of which have already been mentioned by others) you'll need to incorporate:
1. Each frame needs to be held stationary while it's being illuminated.
2. You'll need to be able to advance the film one frame at a time.
3. You'll need to be be able to draw the film off the spool, and draw the film onto the take-up spool, smoothly and continuously so as not to stress the film.
4. To achieve 3. you need to incorporate buffer loops either side of the gate (the place where the frame is held stationary and illuminated) so that as the film is advanced by each frame it is taken from the buffer loop and not directly from the spool. Similarly, as each frame is feed out of the gate, it runs into the take-up buffer loop.
5. You'll need a shutter that is syncronised with the film movement through the gate; the shutter closes when the film moves and opens once the frame is positioned. A spinning disk with cut-outs is probably the easiest solution here as it just needs to spin at the correct constant speed.
6. If you're using strong and hot illumination you'll need a blower to cool the film while it's stationary in the gate, other wise it might melt.
7. If you're running a long length of film then the film and take-up spools will need to change their angular speed to insure constant linear speed. The film can be drawn off the film spool by a constant speed toothed sprocket but the take-up spool will need some sort of slipping clutch.
In practice, you'll need two sets of toothed drive sprockets arranged as an inner pair, to step the film through the gate, and an outer pair to smoothly fill the feed buffer loop and empty the take-up buffer loop.
Blimey! I hope he's got a lot of time and some decent machine tools [:D] Mind you, if he does make a projector that good he'll be able to set up in business, although the market may be rather limited these days.
-
Some good ideas here. Some I will consider overkill for this project. I will not bother with holding the film still for every frame. That is beyond what I want to achieve. If it can't be focused enough without, oh well, the project is dead.
I think the idea of the "brief glimpse" and the persistence of the eye is what worked in the simple device I saw in this museum. It was, and Geezer found it too, the zoetrope.
Picture here: http://library.thinkquest.org/C0118600/index.phtml?menu=en%3B4%3Bci4008.html
Don't know if the detail in a modern 35mm film is not too much to create the illusion of movement. It might just be a blur. I will have to line up a mechanism to allow the brief glimpse at the right time. Maybe a slotted drum can be pressed against the permanently illuminated film and .... no, that won't work. Somehow the film needs to be run by the light source while not giving the viewer visual access to the film at all times. Hmmm....
-
You might even take a chunk of the film and mount it around the circumference of a bicycle wheel (offset of course). Beg, borrow or midnight requisition a variable frequency strobe light. Spin the wheel and manually adjust the frequency of the strobe to sync it with the frames on the film. If you can't get your hands on a strobe, you could make one from a LED flashlight and a simple electronic circuit.
Of course, you'll not see too much of the movie with this method, but it might demonstrate the principle.
-
Viewing/projecting the film via a rotating mirror could cancel out the film's motion if the mirror's rotation was perfectly synchronised with the film motion, (difficult).
[ Invalid Attachment ] [ Invalid Attachment ]
single rotating mirror octagonal rotating mirror
http://web.media.mit.edu/~stefanm/TinyProjector/TinyProjector_labnotebook_prototypes.html
-
I seem to remember that manual film editors had a prism instead of a gate. My father had one and I think I took it apart to clean it once, but I could be confused.
-
Viewing/projecting the film via a rotating mirror could cancel out the film's motion if the mirror's rotation was perfectly synchronised with the film motion, (difficult).
[ Invalid Attachment ] [ Invalid Attachment ]
single rotating mirror octagonal rotating mirror
http://web.media.mit.edu/~stefanm/TinyProjector/TinyProjector_labnotebook_prototypes.html
Synchronizing this mechanically instead would be easier since the film has those handy perforations. Another good idea!
-
I knew I'd seen the rotating mirror principle used somewhere before ...
[ Invalid Attachment ]
http://www.kidestore.co.uk/kidzlabs-animation-praxinoscope-p-273.html
http://en.wikipedia.org/wiki/Praxinoscope
-
You could copy each frame onto a bit of paper and make a "flick book". Not high tech, but practical.
http://en.wikipedia.org/wiki/Flick_book
-
This project sounds like a job for Heath Robinson.
-
I would Suggest you use a Geneva Drive for your Film projector. A Geneva Drive splits the spinning into steps. It is used in most 35mm projectors.