Photomechanical sensor. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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The photomechanical method of obtaining a slow-sweep television image is the most affordable for radio amateurs. Its essence lies in the progressive conversion of an optical image into an electrical signal, supplemented by horizontal and vertical sync pulses.

The quality of the "image" obtained in this case can be no worse than from an SSTV camera made on a vidicon. It depends mainly on the design of the mechanical part of the device, schematically shown in Fig. one.

Ris.1

A sheet of paper with information (text or drawing) to be transmitted, or a photograph is placed on the cylinder 10, which is fixed on the axis 6 with a left-hand thread. The cylinder is rotated by an electric motor 9 (through a belt 11). The carriage 3 with the photosensitive unit is moved along the guide 4 by a backlash-free mechanism consisting of a leash 8 with a threaded half-sleeve 7 attached to it and a clamping flat spring 5. Instead of spring 5, you can use load 16 (in Fig. 1, c it is shown by a dashed line).

A photosensitive unit that reads information line by line consists of a phototransistor 1 placed in a cartridge 13, which allows you to change the distance from the phototransistor to the drum, and a lighting lamp 2. The cartridge is equipped with a miniature lens 12.

In the extreme position of the carriage, corresponding to the beginning of reading information, the contacts SF1 must be closed, forming a personnel sync pulse. Horizontal sync pulses are formed using the reed switch SF2. It closes under the influence of a permanent magnet 15 built into the end of the cylinder at the point where the beginning and end of the sheet with transmitted information (photo) converge. The required duration of the horizontal sync pulse (5 ms) is obtained by adjusting the gap between the magnet and the reed switch. The carriage is returned to its original state, corresponding to the beginning of reading the next frame, manually by lifting the conductor 8.

The electrical signal generated by the phototransistor enters the electronic unit of the sensor, the circuit of which is shown in Fig. 2.

Fig.2 (click to enlarge)

Amplified by the operational amplifier DA1, the video signal from the output of the photosensitive element - the phototransistor VT5 is fed to the amplitude limiter, made on VDI-VD3 diodes. The level of limitation is selected by resistors R9, R10 when adjusting the sensor. From the point of connection of the diodes VD2 and V03, the video signal comes to an electronic key assembled on transistors VT1 and VT2. When the pairs of contacts SF1 and SF2 are open, the transistor VT1 is closed and VT2 is open. In this case, the video signal through the transistor VT2 enters the frequency control circuit of the multivibrator on transistors VT3, VT4.

When one of the pairs of contacts is closed (during the formation of a clock pulse), the transistor VT1 opens, and VT2 closes - the passage of the video signal through VT2 is blocked and the frequency of the signal generated by the multivibrator will depend on the position of the trimming resistor R12 engine. From the output of the multivibrator through an active low-pass filter assembled on the operational amplifier DA2, and the level control - a variable resistor R22 - goes to the output of the sensor.

The cylinder is machined from durable plastic. Its diameter is calculated by the formula: D \u2d 2L / 8p, where L is the length of the threaded part of the axis outside the cylinder. The recommended axle diameter is 1 mm, the thread pitch is 41,4 mm. In this case, the cylinder should have a diameter of 150 mm. The length of the cylinder, taking into account the space for the groove groove for the belt, should be XNUMX mm, while the length of its working part should be equal to the length of the threaded part of the axle.

The diameter d of the working surface of the pulley 14 on the engine axis is calculated based on the rotational speed N (in min-1) of the motor shaft, the diameter D of the cylinder (taking into account the depth of the groove) and the rotational speed n (min-1) of the cylinder, according to the formula: d= nD/N. The value of n is taken equal to 1000 min-1, since the length of the SSTV signal line is 60 ms. If, for example, the speed of the electric motor is 2500 min-1, then the pulley diameter is 16,4 mm.

The carriage is made from any material. Its design is clear from Fig.3. The axis on which the cylinder is fixed must be of high quality thread and minimal runout. It is fixed in ball bearings.

Ris.3

The electric motor is asynchronous, with a power of about 1500 W. The frequency of rotation of its rotor can be in the range of 3000 ... XNUMX min-'.

The remaining parts of the sensor can be made arbitrarily, depending on the capabilities of the radio amateur.

The adjustment of the electronic unit is reduced mainly to focusing the phototransistor and adjusting the frequency modulator. First, with closed contacts SF1 or SF2, resistor R12 sets the frequency of the multivibrator to 1200 Hz. By periodically changing the black and white fragments of the image in front of the phototransistor and sequentially adjusting the resistors R2, R9, R10, the frequency of the multivibrator is changed within 1500 ... 2300 Hz.

The disadvantage of the described sensor can be attributed to the relatively low efficiency. However, the sensor can be used in conjunction with a tape recorder (in this case, the image from the sensor is pre-recorded on the tape), and even with a computer.

It is very tempting to make a similar sensor with increased resolution - to divide the frame not into 128 lines, but into 256. In this case, it will only be necessary to increase the image reading time to 16 s. The line duration can be kept at 60ms, but it is better to increase it to 120ms. All this will only entail a change in the thread pitch on the axis of the cylinder and the frequency of its rotation.

Literature

1. Balabansky P. et al. SSTV technique. - Sofia: Technique, 1985.

Author:E. Sukhoverkhov (UA3AJT), Moscow; Publication: N. Bolshakov, rf.atnn.ru

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