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Sync-Strobe 35mm Film Looper
works of  Carl C Pisaturo
The Film Looper is a simple device which allows a short loop of 35 mm movie film to "come alive".

This is achieved by moving the film over a special light table which flashes in sync with the position of the individual frames.

The light table portion is  4 feet tall, which accommodate 62 frames of movie film - each a miniature animation.  The loop repeats about every 5 seconds.



Design and Manufacture: Carl Pisaturo, 2014.

Size:  66" tall x 4" x 4"; installed by attachment to room column.

Materials: machined aluminum, white-tint acrylic, Delrin, polycarbonate, stainless steel, ball bearings, linear slider, DC motor, LEDs, custom electronics, found film.
ELECTROMECHANICAL
At the bottom of the Film Looper are the electromechanical systems dealing with:

Film Transport

Film Tensioning

Frame Position Sensing
STROBING LIGHT TABLE
FILM TRANSPORT

A length of movie film is closed into a loop with special tape (with punched holes matching those on the film), and installed onto the Film Looper machine.  There, it assumes an oval form, supported on 2 capstans. The bottom capstan provides the motion, being driven by an electric motor, and the top capstan free-wheels.  A DC motor is connected to the driving capstan by a urethane round belt which also reduces the motors speed by a factor of 4.  

The motor has a stainless steel fly wheel on the shaft to help the film move smoothly, and the motor is "shock mounted" on a rubber o-ring to reduce acoustic noise. 

The capstans (bottom one pictured at right) are lathed out of 2 1/8" aluminum rod and ride on ball bearings.  The film is gently held by the edges only, since the center of the capstan is cut lower.

For this system to work, the film must be under some tension or the capstan would slip.  Note: the black toothed sprocket is not driving the film, it is part of the sensor system.
FILM TENSIONING

A surplus linear slider (rectangular object at left bottom) was used to build a gravity based film tensioning system.  

Compared to spring arms or other methods of tensioning, this strategy is straightforward, allows slight loop length variation,  and gives a clean geometry to the film loop (oval).  It also makes installing/uninstalling film loops easy - just lift the motor assembly to slack the film.

The substantial weight of the capstan, motor and base are transmitted entirely to the film, giving an appropriate and even amount of tension to the film loop.

Tensioning the film is important for the function of the transport, keeping the film even on the light table, and the sensor working
reliably.
FILM POSITION SENSING

In order to flash the light table in sync with the position of film frames, the longitudnal position of the film must be accurately sensed.  This is simplified by the fact that there are 4 perforations in the side of the film per frame.  So the task of our sensing system is to command a flash to occur every 4th hole-pass.

I first tried to directly sense the holes with a slot sensor, but this turned out to be difficult due to the fact that the holes allow only slightly more light to pass than the film itself.  Also, film position was critical, and interference from ambient light was problematic.

The implemented strategy utilizes an off-the-shelf plastic 16 tooth sprocket (black in photo at right) which is joined coaxially with a custom aluminum 4 slot disk.  One movie frame causes the toothed sprocket to make 1/4 revolution and the aluminum disk to make one slot-pass.  The slots are sensed by an ordinary slot sensor.  This system is reliable and  accurate. 
The strobing light table carried over some techniques from my Transmutascope project: the same high current / short duration LED pulsing PCB was used, with roughly the same type of LEDs  (white 1 watt) wired in parallel.

In this case, 48 such LEDs were soldered in parallel to copper bus wire (above) spaced about 1" apart.  Note the green marks to prevent reverse soldering. This flimsy strip was then epoxied down to a strip of black 1/4" thick polycarbonate to form the base of the unit. 

The main design challenge here was achieving a smooth brightness over a large area (no brightness "hot spots") without using hundreds of LEDs or very thick energy-wasting diffusers.  The solution I came up with through experiments is 2 thin layers of diffuser material with about an inch of air-gap between them. 

Constructing the structure (right) of 10 components, each a strip 48" long, required epoxy, lots of clamps, and patience.  After this glued-up structure was finished, the bottom lamp layer was screwed in from the sides.
End-on veiws (left and below) of strobing light table

The top and middle layers are 1/8" white tinted acrylic. Bottom layer is 1/4" black polycarbonate.

 Below the top layer are a pair of 1/4" square aluminum bars to dim the area of film outboard of the images.

Sides are 1/8" aluminum with 1/16" aluminum strips epoxied on to act as spacing aids.
ONE SHOT V3 Circuit Board (below)

This versatile board allows safe high current LED strobing by automatically restricting pulse durations if frequency gets too high.  It is used in the Film Looper, Transmutascopes, and general purpose LED strobes.  In a nutshell, when triggered, the big capacitor dumps current into a bank of parallel wired LEDs for a controlled amount of time.
35mm MOVIE FILM
An amazing and increasingly rare analog medium...

A single frame of widescreen format 35mm movie film.  24 of these would be projected per second.

Interestingly, high quality stereo sound is represented optically in the 2 white strips to the left of the image.