Device for checking eyesight. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Medicine
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A diagnostically convenient sign that makes it possible to judge the state of vision is the ability of the eye to notice rapid changes in the test "picture" (we do not touch here, of course, on the medical aspects of this diagnosis).
On fig. 1 shows a schematic diagram of a light generator that forms red-black and green-black meanders (ALS331AM LED) of various frequencies, which is immediately measured.
Fig.1 (click to enlarge)
The master oscillator of the device is made on the elements DD5.3, DD5.4, R3, R4, C1. It is excited at the frequency Fз@1/2(R3+R4)C1. Counter DD2 lowers F by 4 times - this signal (pin 4) goes to the measuring unit DD3, DD4, HG1 - and 64 times (signal to pin 12) - to the operating frequency of the light generator itself.
The time interval on which the frequency is measured is 64 ms. It is formed by an oscillator whose frequency is set and stabilized by a ZQ1 quartz resonator (DD5.1, DD5.2, etc.). The counter-divider DD1 this frequency is reduced to a value,
at which a "single" pulse of the required duration appears at the output of Q12 DDL (in fact, a pulse with a duration of 62,5 ms (1/16 s) would be needed here. But the error (+2,5%) is quite small and we neglect it) .
It is easy to see that the measured frequency is 16 times higher than the repetition rate of the "light" meanders themselves. This made it possible to speed up the measurement, to make it tracking (delay less than 0,25 s).
When the device is turned on, all its counters are set to zero by a pulse, the duration of which is close to R6-C2sO.5 s. Each elementary frequency measurement begins with the appearance of the start combination "12" at the outputs Q13 and Q1 of the counter DD11. Its front counters DD3 and DD4 are set to zero (pulse duration at their R-inputs - R5·C3@10 μs), and at the input of the element DD6.2 there is a signal "1", allowing counting. This will continue until Q12 outputs a "0" (this will happen after 64 ms). From this moment until the beginning of the next starting combination, the HG1 scoreboard will show the result of the measurement just taken. (The introduction of the signal from the Q13 output into the starting combination made it possible to increase the duration of the display of the measurement result on the display three times.)
Switch SA1 sets the color of the meander: either "red-black" or "green-black". The brightness of the LED in both of these modes is set (if it seems necessary) by selecting the resistance of resistors R10 and R11.
All fixed resistors in the device are of the MLT-0,125 type; variable R4 - linear (type A), with a comfortable handle. Capacitors C1 ... C4 - KM-6 or the like, C5 - K50-35, etc. Transistors VT1, VT2 - almost any pnp. Since one of the elements of the DD6 chip is not used here, its inputs must be connected to the "+" power supply or to "ground".
Of the four digits of the IZHTs5-4/8 scoreboard, only two are used here. It can be, of course, any pair, but adjacent digits look better (two lower digits of this scoreboard are switched in Fig. 122).
It is recommended to make the HL1 LED remote by mounting it in the depth of a blackened funnel, which is applied to the eye so that the LED would be in the central field of vision. (Peripheral vision, which, by the way, has much less inertia, belongs to other "spheres" of the visual tract and is of no particular interest here.)
The inertia of a normal eye is rather small: it ceases to notice the red flashes of the LED only at frequencies of 40 ... 42 Hz, green - another 2 ... 3 Hz higher. A decrease in the frequency at which the eye stops noticing flickering to 35 ... 30 Hz and a significant divergence of frequencies for red and green is a reason to see a doctor.
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