If you are looking for a new hobby, here is a project which is exciting and yet so easy to construct. Imagine inviting your guests to watch the next foot ball world championship on your home theater from a small TV.
The thing which can make this possible is a lens called frensel lens.
Construction of the projector unit
The projector can be made from almost any material, cardboard being the easiest to work with. It is advisable to l make the first construction with cardboard and, after gain- ing confidence, the final case may be made of wood or decorative boards. The projector is built as shown in Fig. 1. The inside measurements A and B correspond to width and height respectively of the TV screen, the depth, is 48.25 cm and is the same for all sizes of TV sets. The rear of the case remains open. The front has a square opening where the-objective unit will slide in. The case can be made more stable by using good adhesive and masking tape around the corners and angles. Stability of the case is very important. The square opening D x D should be 30.5 cm x 30.5 cm for all TV sets above and up to 33 cm screen size.
Objective unit
This is made of same material as the projector unit. Here the front is open while the rear carries the lens. The wall thickness of the projector unit should be such that one should be able to slide this objective unit easily and horizontally, yet not so loose that it gets tilted in the square opening of the projector unit. The length E of the objective unit is 22.9 cm. The lens carrying sheet has a 14.6cm diameter hole in the center ( Fig. 1). Tape the special fresnel lens over this hole. Make sure you tape the edge of the lens only and not on the grooves as this will affect the clarity of the picture. The grooves on the lens, should always face towards the objective unit. When pushed into the projector, the grooved side should face the TV screen.
Mirror
Though the lens will magnify the TV picture, it will be inverted. lf the picture is vertically inverted, placing the TV set upside down can be avoided in the following manner:
I. Turn the deflection coil at the neck of the picture tube by 180 degree.
2. Alternatively, invert the terminals of the vertical and horizontal deflection coils through a slide switch as per the connection plan in Fig. 2; This will allow you to see the picture projected on giant screen or, by removing the projector unit, normally on TV by simply sliding the switch either way·as desired. If you have a 33cm transistorised portable set, ideal for projection sharpness, you can turn the set upside down as this will not damage the set anyway. lf the picture is inverted side wise it can be remedied by inverting the vertical deflection coils. As an alternate method a mirror can be used. The mirror of 30.5 cm x 30.5 cm in size should be fixed on the projector mounting plate with the help of a mirror holder. The mirror holder is easily made by using two wooden planks of l .27 cm x 5 cm x 30.5 cm each and fixing them at the mirror edge by contact adhesive. Now the mirror can stand vertically. The mirror should stand vertically making 450 angle to the lens. The light path through the TV projector is shown in Fig. 3. Final fixing of the mirror holder and the projector unit on a board should be done only after the final adjustments fora good. picture have been made.
Final construction
After adjusting the TV set to obtain an upright picture, bring the projector unit exactly in front of the TV screen. Use a supporting board of proper size and height for resting} the projector unit. Slide in the objective unit with the lens turned inside towards the TV screen and the grooved side of the lens facing the TV screen. Thereafter, place the big projector screen about 1.4m away from, the projector, shutting off the room lights or external lights completely. Picture will be very bright only when the room is fully dark as in a cinema. Adjust the objective unit by sliding until a very sharp picture is projected on the screen. Now the screen can be moved back and forth and the objective unit adjusted again until the desired size and sharpness are obtained.
very important point may be mentioned here. The brightness of the picture will depend on the reflective power, of the projector screen. The screen should be straight without any curve anywhere on its surface. Its bright white surface should be light reflecting type, not light-absorbing. For this reason Kodak’s Ektalit screen has been found to give picture so bright that it could be viewed even in a half-dark room.
In case of non-availability of this material similar results can be achieved by trying out small size (about 30.5 cm x 30.5 cm) smooth surfaces of different materials and painting them with different types of bright white paints. The one that gives the brightest picture can then be selected. Fix everything after the final adjustment. To give it a professional look, a black stripe can be painted on the screen to border the picture size. Allow little time to let your eyes get used to the darkness and enjoy your home cinema. To make it a realistic home cinema, an extra loudspeaker box can be placed behind the projector screen. The outside loudspeaker box and the TV loudspeaker can be easily connected in `either-or’ manner by means of a three pole jack or a slide switch. All these new switches can be fitted together on a small case or even built into the TV case. The inside walls of both objective and projector cases should be painted matt black to arrest diffused light and thereby obtain extra sharpness for the projected picture
