Lightphone. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Lightphone. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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A schematic diagram of a miniature light telephone with a range of up to 50 m during the day and up to 200 m at night is shown in the figure.

(click to enlarge)

Lamps 6V/0,5A (up to 2000Hz), 24V/0,2A (up to 3000Hz), 12V/0,1A (up to 4000Hz) can be used as emitting lamps. The upper operating frequency of the lamp is indicated in brackets. As a reflector of the emitter, you can use a headlight from a motorcycle or a car, in which the corrugated glass should be replaced with an ordinary one.

At the transmitter input (Fig.a), the amplified microphone signal must have a voltage of 0,7 ... 1 V at a load of 600 Ohms. Transistor V2 is selected depending on the power of the HI lamp (P302, PZOZ, P304, P201, P217). Transistor V1 type MP25, MP26, KT361, etc. As a preliminary microphone amplifier, you can use the ultrasonic frequency of the radio receiver. The light signal receiver (Fig.b) is a photodiode, photoresistor or phototransistor placed at the focus of a converging lens. A homemade phototransistor can be made from an ordinary low-frequency transistor in a metal case, if you cut off its end part. The input window of the photosensitive element obtained in this way should be covered with a thin transparent Plexiglas or celluloid plate. Light of variable intensity changes the resistance of the photosensitive element. As a result, under the action of the current set by the resistor R3, an alternating voltage appears on the photosensitive element, which, after amplification, is supplied to the headphones. Transistors V1 and V2 type P27A, MP40, MP41, MP39B. The AF amplifier (UZCH) should have a sensitivity of about 10 mV with an output power of 50 mW. The distance between the converging lens and the photoresistor is found experimentally by the maximum output effect.

When adjusting the phototransmitter, the lamp must be placed at the focus of the reflector. To do this, in the dark at a distance of about 10 m, a minimum (no more than 300 mm) diameter of the light spot from a burning lamp is achieved. If the spot diameter is larger, the lamp or reflector must be replaced. In the radiation mode, resistor R1 sets the lamp current to no more than half the nominal. When setting up the receiver, its photosensitive element is placed at the focus of the lens.

The communication range can be significantly increased (1000...1400 m) if the light flux in the light transmitter is modulated by a thin aluminum foil membrane that reflects light. The design of a light phone with a modulating membrane is shown in the following figure.

(click to enlarge)

Under the influence of sound vibrations, the membrane begins to oscillate, as a result of which the brightness of the light radiation reflected by the foil changes. The membrane can be made of thin celluloid with a diameter of 50 mm, on which a circle of aluminum foil with a diameter of 24 mm is glued. Foil can be taken from chocolate wrappers. The membrane is glued to the end of the objective tube cut at an angle. Lens diameter 30 mm, focal length 143 mm. By moving the lamp (3,5 ... 6,3 V / 0,3 A) along the tube, a clear image of the light spot is achieved at a distance of 3 m.

Under the influence of sound (speech, whistling) penetrating into the speech opening, the light flux should change focus. If this does not happen, then you should adjust the length of the tube - the lens with the lens. The tube can be made of cardboard or plastic. Inside it is covered with black matte paint or pasted over with black paper. Adjustment of the optical system of the photodetector is reduced to the selection of the distance between the lens and the phototransistor, which provides maximum volume and intelligibility of reception. In the case of using lenses of a different diameter with a different focal length, the dimensions of the transmitter and receiver of the light telephone should be changed in accordance with the focal length of the lenses. The device uses transistors V1...V5 type KT315D and V6 type KT816A.

The light telephone can be used as a car security device. A light receiver is installed in a window or on a balcony, and a light transmitter is installed in the car, which is triggered when the doors, hood or trunk are opened. The secrecy of the installation can be ensured by installing an infrared filter.

See other articles Section Telephony.

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