Orientation device for the blind. Encyclopedia of radio electronics and electrical engineering

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The device, which will be discussed, will help a visually impaired person not only to detect an obstacle in time and estimate the distance to it “by ear”, but also to determine the level of illumination where it is located.

Among the devices that facilitate the orientation of the blind, portable active obstacle locators give the best results. They emit probing ultrasonic or electromagnetic signals in the direction of a possible obstacle. The locator converts the received signals reflected from obstacles into a form accessible for perception by the blind - sound or vibration.

Devices using IR radiation as probing signals were proposed quite a long time ago [1]. One of the options for such a device, suitable for self-production, was described in [2]. The disadvantages of this design include a short range (only 1,5 m) and poor noise immunity. In the same way as to an obstacle, the device reacts to a conventional incandescent lamp located much further away. In the design proposed below, these shortcomings are eliminated by using a selective amplifier in the receiving part. A node has been added that evaluates the overall illumination, and the sound signals indicating the presence of an obstacle and characterizing the illumination are easily distinguishable by ear.

The scheme of the device is shown in fig. 1. The choice of transistors as an element base is due to a small assortment of microcircuits operating at a supply voltage of 2 ... 3 V. In addition, it is easier to achieve a minimum current consumption in a design of discrete elements. In this case, it does not exceed 5 mA.

(click to enlarge)

Every 0,5 s, the emitting diode VD3 sends a burst of IR radiation pulses with a duration of 20 ms. Rejection of the continuous emission of the probing signal is another measure to reduce the average current consumption. The generator that sets the duration of bursts and pauses between them is assembled on transistors VT3 and VT4. The pulses from its output are fed to the base of the transistor VT5, which turns on and off the multivibrator on transistors VT6 and VT7, and it generates pulses with a duration of 58 μs. Trimmer resistor R15 set the pulse repetition rate equal to the center frequency of the passband of the selective amplifier in the receiving part of the device (2800 Hz). To achieve the necessary frequency stability, capacitors C6 and C7 must have a small TKE. It is unacceptable to use ceramic capacitors of groups H30-H90 here.

Pulses with a frequency of 2800 Hz are fed to a power amplifier - a transistor VT8, in the collector circuit of which a radiating diode VD3 is included. The diode current per pulse reaches 300 mA. To quickly absorb the generated heat, the emitting diode needs a heat sink from a material with high thermal conductivity. In this case, copper with an area of ​​​​3 cm2 is used.

The IR pulses reflected from the obstacle, received by the VD1 photodiode and amplified by a selective amplifier based on VT9-VT12 transistors, are heard in the BF1 earpiece from the hearing aid. The louder the signals are, the closer the reflecting object is. With the duration of the burst indicated above, the human ear subjectively perceives it as colored in a certain sound tone, and not just as an unpleasant click.

Receiver gain - 2300, bandwidth (0,5 level) - 300 Hz. The greatest contribution to selectivity is made by the oscillatory circuit L1C11 with a resonant frequency of 2800 Hz. In order not to worsen its quality factor, the VT10 transistor is connected according to a common collector circuit. An oscillatory circuit of a small quality factor, tuned to the same frequency, is formed by the BF1 headphone coil and capacitor C19.

The high input impedance of the low-noise field-effect transistor VT9 serves as an optimal load for the VD1 photodiode. When the photodiode is dimmed, the noise voltage applied to the input of the amplifier does not exceed 0,9 µV. Approximately the same threshold of audibility of the reflected signal.

The sensitivity of the receiver is regulated by a variable resistor R25.

The multivibrator on transistors VT1 and VT2 generates pulses, the frequency of which is the higher, the greater the illumination of the photo resistor R2 included in the base circuit of the transistor VT1, which is sensitive to visible light. The pulses are fed to the base of the transistor VT12. As a result, the signals reflected from obstacles are heard against the background of low-frequency sound - from 100 Hz with an illumination of 1 lk (almost complete darkness) to 1000 Hz with an illumination of 1000 lx (75 W incandescent lamp at a distance of several tens of centimeters). The background volume is regulated by a variable resistor R32. If necessary, the illumination evaluation node can be turned off by the SA1 switch.

The device is assembled in a case with dimensions of 120x90x30 mm. Its mass together with the power supply - two galvanic cells of size AA - 250 g. The emitting diode VD3, the photodiode VD1 and the photoresistor R1 are equipped with organic glass lenses. The width of the zone in which an obstacle can be detected is approximately 20°. Details marked in fig. 1 asterisks, select if necessary during the adjustment of the device.

The solid line in the graph in Fig. Figure 2 shows the experimental dependence of the signal voltage U at the outputs of the BF1 earphone on the distance R to the reflecting IR radiation of a person at the maximum sensitivity of the receiver and the nominal supply voltage (3 V).

Subjectively, according to the average assessment of several people with normal hearing, the level of the sound signal in the indicated in Fig. 2 range interval changed from very loud (close to the pain threshold) to quiet. The dashed line is the result of averaging the experimental data. With the GB2,2 battery discharged to 1 V, the signal voltage decreased by no more than two times.

Literature

1. Mims F. M. LED range-finders can even provide eyes for the blind. - Electronic Design, 1972, No. 11 (May 25), pp. 48-50.
2. Nechaev I. IR locator for the blind. - Radio, 1989, No. 11, p. 85, 86.

Authors: A.Gavrilov, A.Teresk, Tallinn, Estonia

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