Last night saw significant progress on the telecine project. I made a new back for the projector that replaces the original motor & lamp housing, then successfully tested the new DC motor.

I began with what was once an ECU cover (or more accurately, a DME cover from a BMW). I made a cardboard template of the rear of the projector earlier, than transferred the outline to the cover. From there, I used a dremel to cut the outline out and drilled and tapped four 3mm x .50 holes for machine screws.

With the plate taken care of, I turned  my attention towards the drive gear. The cassette player I sourced the gears from had two identical gears with both inner and outer teeth as pictured. My plans use one of the gears to replace the original drive pulley with the outer teeth being driven. The motor uses the inner teeth of the remaining gear to engage the outer teeth of the now replaced drive pulley, but the gear must be ground down to only the center for it to clear. I used a dremel and grinder to first cut up the gear, then grind the remaining metal down until only the center remained.

The newly ground gear was then mounted to the DC motor. A hole slightly large than the gear was drilled into the new back plate, then the motor was mounted to the back plate. A spacer had to be used for proper clearance, and the spacer also made it easier to mount the DC motor using very shallow screws. With everything mounted, I applied +12v  DC to the motor and watched the projector run!

I spent a significant portion of my weekend working on the telecine project. The test films I ordered from ebay arrived Friday and I immediately popped one on the projector. Unfortunately, the take up reel/spool kept turning too quickly and pulling the film against the second (lower) sprocket. Having never used a projector before and knowing this projector was formerly in working condition, I assumed the problem was a user error. After triple checking everything, I decided the take up reel must be the wrong size… but then I checked ebay listings and the take up reels looked identical. I then pulled the service panel off the projector since I had it off earlier – perhaps I misplaced a gear? Nope! But I did find a spring loaded mechanism at the take up reel and after looking closely, I determined that the take up reel is supposed to slip except it wasn’t. I held the gear in place and forced the take up reel until it finally gave way. After that, it slipped much more easily and the projector began to operate correctly!

After a successful test of the projector, I then started disassembly. The film needs to be advanced frame by frame for the telecine so the motor either needs to be controlled directly or replaced entirely. Pictured above is the view of the projector with the motor and lamp housing removed entirely. The Bell and Howell Regent Design 122 Model L appears to be a great telecine platform because the assembly above rotates exactly once for every frame. Rather than control the original motor, I have decided to build a new motor assembly that will be controlled by a microcontroller. For the lamp, I have decided to reverse the projection and mount the camera where the old lamp used to be with an LED light source mounted where the current projection lens is in front.

The new motor assembly is based on parts ripped from an old cassette deck. The dc motor is from the deck along with the gears needed to make everything work. The gears need machining to fit the projector, but otherwise look like a good fit. I have already done the necessary machining to mount one of the gears.

The microcontroller is not able to directly control a powerful DC motor, so a control circuit is needed. I first modeled the circuit in multisim, then breadboarded it for testing. The circuit above is not identical to the breadboarded circuit – I omitted the capacitor and the 2N2222A is directly driven from the 5v rail on a PC power supply; I threw the 7905 in because it’s already mounted to the spare perf board I have from a previous project.

The motor will need to carefully advance the film and cannot run too quickly. I drafted not particularly elegant PWM code on the propeller and ran a few tests. This will allow the motor to run at lower power levels so as not to over advance the film.

I think this was at 66% duty cycle, but I don’t recall exactly. Each square is 1 ms.

Next to come is the mounting of the DC motor and closed circuit control.  Once completed, all that remains is to mount the light source and camera, then scan some film!

I got to work on the Telecine project last night. No permanent modifications have been made yet as I’d like to experience the projector in its original 8mm glory first. Even then, I’d like to avoid modifying the projector as much as possible so it can be returned to its original condition. At first, I thought this would be an unattainable goal, but after disassembling the projector and studying its construction, I feel this is actually realistic.

The projector appears to have four main parts (using my own terms): the base, the lamp housing & motor assembly, the gate assembly, and the film advance. The base does exactly what the name implies – it supports for the projector and houses some simple controls (light switch & motor switch). I will probably leave this as-is.

The next part, the lamp housing and motor assembly, is important. The entire assembly is self contained and separated from the gate, sprockets, and reel drive system.  The motor drives a rubber (?) disc that is loosely coupled to a drive wheel on the gate. For every one revolution of the gate, the film advances exactly one frame. The entire lamp housing and original motor can be removed from the projector while leaving all the equipment needed to advance and display the film intact, making this projector an excellent platform for a telecine.

With the lamp housing and original motor removed, a new motor must be fitted to advance the film. As this is intended to be a telecine, we want to advance each frame slowly and accurately. You should be thinking to yourself that a stepper motor would fit the bill, and indeed it does. However, I failed to find an appropriate stepper motor in my junk pile so I will use a drive motor from an old cassette player instead. This drive motor will hopefully provide ample torque is operated from a 12 volt DC source and it worked fine during a dry run (held by hand with no film loaded in the projector). A few challenges remained – the motor itself, like most small DC motors, does not provide sufficient torque at a 1:1 coupling and it runs at far too great of a speed; in fact, the motor indicates it is rated to run at 2400 RPM with no load. To reduce the speed and increase torque, the motor was originally coupled via a rubber drive belt and pulleys. However, the projector presents a larger load than the cassette deck and the original drive belt was very long. Rather than searching for a new drive belt and fitting it to the projector, I have elected to build a gear box.

As this project relies largely on parts I have available in my junk pile, no real calculations were done. The cassette deck kindly donated two metal gears in addition to the drive motor, and they fit nicely on both the projector’s gate & and the drive motor. They should provide slightly greater torque than the hand held test and I am confident the drive system will work. A metal plate will be fabricated that matches the original lamp housing & motor to which the new DC motor will be mounted. As the motor is simply a “dumb” DC motor, external sensors will be used to detect when a frame is advanced. My current idea is to detect a specific spot on the gate wheel using either a microswitch or optical trigger. This will allow a microcontroller (likely either my existing Parallax Propeller or an Arduino) to know when to stop and start the motor. Additionally, to protect against over-advancing and to ensure the gate is not blocking view of the film, a sensor will detect the gate position.

A further benefit of the lamp housing being removed is that the film can be “reverse projected” with the light source on the on the outside and the camera mounted where the light bulb used to be. This greatly opens up camera choices as size becomes a non-issue (within reason). My camera choice is still up in the air with the SL202 still being an option, but webcams, such as this Microsoft LifeCam HD-3000, becoming more attractive.

Pictures to come!