Television projector. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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Many families have photographic films and slides, small photographs. In order to make them easy and convenient to view collectively, the author of the article suggests using a simple television overhead projector. It is also applicable in a number of other cases.

Many hams keep negative black and white and color films and no photographs, or positive films and slides. Sometimes you want to view them alone or with your family. However, often a good overhead projector is not available. If it is present, the room needs to be darkened for viewing. A simple television slide projector, the appearance of which is shown in action on the 1st p., will help to solve these problems. covers, and in disassembled form - in Fig. 1 in the text. You just need to connect it to a black and white or color TV, which is available in almost every family. To connect a slide projector, the device must be equipped with a low-frequency input "Video" for the video signal.

In addition to these applications, the projector can be used for collective viewing in an enlarged form of photographs in the album, drawings, other objects, in particular printed circuit boards, with the necessary illumination, as well as images in a microscope, spotting scope, telescope, etc.

The block diagram of the television projector is shown in fig. 2.

The projector consists of a film or slide illumination device, a device (frame) for their placement, a miniature television camera, a video signal inverter and a power supply unit. An illumination device projects an image on a photographic film or slide into the lens of a video camera, the output of which is a video signal. If the film or slide is positive, the video signal is sent directly to the TV's video input. With negative photo images, the video signal from the video camera must be inverted while maintaining the polarity of the synchronizing pulses. For this, a special video signal inverter is used, which is included in the circuit if necessary.

The schematic diagram of the TV projector is shown in fig. 3. The device uses a miniature frameless video camera SAMSUNG - AV202, usually used in burglar alarm systems, with a lens having a viewing angle of 90°. It provides a video signal with a swing of 1 V and a black and white image with a resolution of 380 TV lines.

There are five LEDs HL1 - HL5 in the projector's illumination device, the brightness of which can be changed by a variable resistor R2, which regulates the current through them. If positive images are being viewed, the signal received at the output of the camcorder passes through the SA1 switch, set to "Non-inv.", to the output of the projector and the "Video" input of the TV connected to it with a coaxial cable.

(click to enlarge)

When viewing negative images, switch SA1 is set to "Inv.". In this case, the video signal from the video camera is fed through the capacitor C2 to the inverter, assembled on transistors VT1-VT3. Circuit R3C1D2 sets the operating point of the inverter. Capacitor C2 and diode VD1 provide binding of the peaks of the negative sync pulses of the video signal based on the transistor VT1 to the voltage level at the operating point.

A cascade with a divided load is assembled on the transistor VT1. Moreover, the operation mode of the transistor VT1 is chosen such that only negative non-inverted synchronizing pulses of the video signal are allocated on the resistor R6. At the collector of transistor VT1, an inverted video signal with positive sync pulses is obtained, which passes through resistor R5 to the input of the emitter follower on transistor VT3. However, as a result of the impact of negative clock pulses from the resistor R6 on the emitter of the transistor VT2 connected to the input of the emitter follower, the positive clock pulses are replaced by negative ones.

At the output of the emitter follower, an inverted video signal proper with non-inverted sync pulses is formed, which is fed to the output of the projector. The trimming resistor R8 sets such a voltage based on the transistor VT2, at which the necessary proportional addition of the inverted video signal and non-inverted sync pulses occurs.

The power supply (adapter) of the projector must provide a stabilized voltage of 12 V at a current of at least 200 mA.

Any resistors and capacitors can be used in the projector. Connector XW1 - type "tulip". Switch SA1 - sliding. Transistors can be used in any of the KT3102, KT3107 series of the corresponding structure.

The design of the TV projector is shown in fig. 4. The unpackaged camera and the inverter board are combined into a common housing (switch SA1 is installed on it), which is located on a rotary (±90° around the lens axis) bracket. The bracket and power supply (the author used homemade, but you can use ready-made) are fixed on a common base.

The backlight device, consisting of a reflector, a light shield, HL1-HL5 LEDs, an adjusting resistor R2 and a constant resistor R1, together with a device (holder) for installing frames with a film or a slide (for example, from a slide projector "Light" or self-made) are made removable. This makes it possible (when they are deleted) to show the photos in the album and other images. The distance between the mounted holder and the lens (about 20 mm) is not critical, as the focus can always be changed.

To view a color image from color positive films and slides, you must use a color television camera with white light illumination and a color television.

Author: L. Kompanenko, Moscow

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