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MODIFICATION OF A SLIDE PROJECTOR FOR REMOTE OPERATION J. H. WOOD and A. D. MELAVEN The University of Tennessee, Knoxville, Tennessee
IN ROUTINE classroom teaching, the lecturer is often faced with the necessity of drawing a complex figure on the blackboard. Drawings of the cyclotron, the blast furnace, the open-hearth furnace, and the nuclear reactor are examples of figures which a lecturer in general chemistry may have to reproduce, usually a t the expense of clarity and class time. In our general chemistry program, we copy photographically most of the figures appearing in the textbook so that these copies may be projected on a screen hanging above the blackboard. Our lecture room is ideally suited for slide projection. There are no windows, and the directional lighting is on a dimmer circuit so that sufficient light can be directed toward the chair-arms for note taking, while the front of the room is fairly dark. All the light controls, including an on-off switch for the slide projector, are located on the lecture table. An automatic slide projector that can be operated efficiently by the lecturer from the front of the room
would be desirable. Alyeal describes an automatic slide changer of the cartridge type in which the slides are pushed into the path of the light beam by a long arm attached to the motor. Alyea's design makes it possible to show a series of 25 slides but does not provide for a reshowing of any given slide. We find that during an average general chemistry lecture, two t o six slides suffice, and frequently we find it desirable t o show certain slides a second or third time in the course of a lecture. Accordingly, we have modified at a cost of about $35 for parts a conventional 3'/4- X 4-inch slide projector (Spencer Model B, 500-watt Delineascope) in such a way as to provide a changer carrying six slides that can be operated by the lecturer from the lecture desk, with the projector at the back of the room. The original slide carrier was replaced by a 16-inch rotating slide carrier made from two aluminum discs2
' ALYEA,H. N.. J. &EM.
EDUC.. 16. 311-2 (19391. sixteen-inch aluminum-base vi"yliie recordings &ere used.
JOURNAL OF CHEMICAL EDUCATION
and a disc of masonite slightly thicker than a mounted glass slide. Before combining these discs, six 31/4 X 4-inch sections symmetrically spaced were cut from the masonite to provide for inserting the slides. Six corresponding sections 3- X 33/&-inchwere cut in each of the aluminum discs. The three discs were theu riveted into a single unit. At the bottom of each slide opening was riveted a spring-brass clip to prevent the slide from falling out when in the inverted position. Rotation of the slide carrier is effected by means of a pair of Selsyn motor^,^ a master Selsyn situated on the lecture table and a "slave" Selsyn situated at the projector. The rotating slide carrier is mounted on the shaft of the slave, and this assembly is secured to the projector by means of a brass strap in such a way that the carrier rotates through the gap provided for the original slide carrier. I n order t o prevent the slide carrier from swaying back and forth because of any unbalance in the carrier, an adjustable spring-backed felt brake is mounted on the projector housing, and the tension of the felt pad against the slide carrier is adjusted so that the carrier can he rotated readily without any tendency to sway when a slide is in projection position. The master Selsyn is mounted in a 5- X 5- X 6-inch plywood box. Rotation of the master Selsyn rotor is done manually by means of a large dial knob. Onesixth of a turn produces a corresponding rotation in the "slave" to bring a new slide into position. Rotation may be clockwise or counterclockwise at will, thus permitting a reshowing of any desired slide. A five-wire cable, permanently installed, leads from a Jones plug outlet a t the projection table to a similar outlet at the lecture table. Each Selsyn motor is connected with a short five-wire cable to a male Jones plug so that each may be plugged into the circuit when slides are to be projected or removed from the circuit when
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W . S. Government Surplus, llOV,60 Cy. A.C., Synchronous. See the following reference for wiring diagrams and a. discussion of synchro motors (Selsyn is a trade name): JAMES,H. M., N. B. NICHOLS, AND R. S. PHILLIPS, "Theory of Servomechanisms," McGraw-Hill Book Co., Inc., New York, 1947, p. 81.
VOLUME 34, NO. 1, JANUARY, 1957
the equipment is stored. In addition to the five wires necessary to operate the Selsyns, a separate 110-volt a.-c. power cable runs to the pover side of the master Selsyn through a toggle switch mounted in the top of the control box. The two Selsyn nlotors are mired in the usual manner. The projector lamp is wired through a two-station on-off switch so that the projector lamp can be turned on or off from either location, or on at one station and off a t the other station (or vice versa). If fewer than six slides are needed for a given lecture, blanks to make up the difference should be inserted to keep the disc balanced. When the machine is being used by lecturers employing many slides, the disc may he removed and replaced by the original slide holder, or it may serve as the slide holder for hand operation in the usual manner. I t will be apparent that several modifications in design are possible. For example, our experience with the 16-inch disc indicates that a much larger disc would work satisfactorily. And of course the openings could be cut to accommodate 2- X 2-inch slides rather than the larger ones. We use the same equipment to project both types; for the miniature slides, 3'/r- X 4-inch masonite holders are inserted in the openings, each holder accommodating two 2- X 2-inch slides. In order to bring the picture size of the miniature slide up to standard, we change from a 20-inch focal length lens to a 12-inch one.
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