Light telephone with IR beams. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Light telephone with IR beams. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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For infrared radiation, the air environment is a kind of filter, the transparency of which can be judged from the spectrogram shown in Fig. 1. For waves of "near" IR radiation - l=0,8...1,3 μm - its transparency remains quite high.

Light telephone with IR beams
Ris.1

Until very recently, the use of IR for communications has been hampered by the lack of easily modulated emitters. With the advent of IR diodes, this obstacle has disappeared.

On fig. 2 shows a schematic diagram of a transceiver operating at a wavelength of 0,95 μm (lmax IR diode AL107B).

Fig.2 (click to enlarge)

As it turned out, IR diodes work well as receivers of infrared radiation. In this case, power is not supplied to the diode: when the pn junction is illuminated, an EMF arises on it, depending on its illumination. This reversibility of the IR diode makes it possible to significantly simplify the optical-mechanical part of the apparatus.

Since the input impedance of the amplifier that picks up the signal from the IR diode working for reception must be large enough, its first stage is made on a field-effect transistor VT1. The main amplification of the signal occurs in an amplifier assembled on bipolar transistors VT2-VT4. Its gain is Ku@10000.

The output stage of the amplifier, made on transistors VT5-VT8, provides the buildup of the dynamic head BA1 at the reception and a sufficient amplitude of current fluctuations in the IR diode in the transmission mode. When switching to transmission (switch S1 in Fig. 129 is shown in the "reception" position), the dynamic head is connected to the amplifier input and used as a microphone. The signal amplified in the VT2-VT8 path is introduced into the IR diode in the form of an audio frequency current. Its level will obviously depend on the voltage at the output of the amplifier and the resistance of the resistor R8. The radiation of the IR diode is linearly related to this current and will track it even at the highest telephone frequencies (IR diodes are quite fast).

A very important node of the IR transceiver is its optical system. As a lens that concentrates the light flux on the IR diode in the receive mode and "compresses" its diverging radiation (~40°) into a narrow beam in the transmit mode, a lens from the enlarger condenser is used, having a diameter D=70 mm and a focal length F=85 mm. It is recommended to maintain the D/F@1 ratio by using other lenses as well. It is not recommended to use the so-called coated optics here. It is enlightened only for the spectrum of 0,4 ... 0,7 μm ((better, almost without loss, the IR radiation of a mirror with an external coating is focused).

When constructing an optical system, every effort is made to minimize the parasitic illumination of the IR diode. The space between the diode and the lens must be tightly closed with a conical opaque casing, and the external illumination of the lens should be reduced by pulling a hood over it. A hood can be made from a piece of plastic or metal tube with an inner diameter slightly larger than D.

It should be as long as possible, at least not less than 2D. The inner surface of the hood should be blackened; it is better if this coating is matte.

About other details of the IR transceiver. Dynamic head VA1 - type 0,1GD-6, but you can take any other, having a voice coil resistance in the range of 6 ... 16 Ohm. Transistors VT2-VT4 - almost any npn structure - KT315, KT3102, etc. Resistors R2, R3, R5 ... R11 - MLT type; R1 - C3-14 or KIM; R4 - trimmer or adjuster of any type. The IR phone's power supply must be capable of delivering 100 mA.

An avometer is required to set up the device. Turning on the device for reception, measure the voltage Uk on the collectors of transistors VT7, VT8. The Uk = +1,5 V required here is obtained by changing the resistance of the resistor R10. Then check the voltage at the source of the transistor VT1 (+1 V) and its drain (+2 V). This mode is set by changing the resistance of the resistor R3. Now, pointing the phone at a lighted object, you can hear the noise, and if the light is electric, then the background of the alternating current. Street lights in the evening are thus listened to from a distance of several hundred meters.

Switching the transceiver to transmission, measure the current in the IR diode (in order not to break the circuit - by the voltage drop across the resistor R8). It should be within 30 ... 40 mA, maximum - 50 mA. It is regulated by the selection of the resistor R8.

In conclusion, the current consumed by the transceiver in standby mode (10 mA) and when a correspondent signal appears (up to 30 ... 40 mA at high volume) is measured. In the transmission mode, the current consumed by the transceiver must be 30...40 mA. If there is no overmodulation, then it will not depend on the volume of what is said into the microphone. The desired modulation level is set by selecting the resistor R7.

For further experiments, you will need a second IR phone. If the distance between the devices is small, then the amplifier may be overloaded during reception, which will affect the quality of the transmission (there is no AGC in the receiver). In this case, you need to somehow reduce the level of the IR carrier. It is possible, for example, to stop the lens of one of the devices with a ring of black paper.

Since the width of the IR phone radiation pattern is close to 1,5°, aiming them at each other presents certain difficulties. It is useful to equip the apparatus with at least simple sights.

The best pickup will correspond to the highest volume of the received signal.

In the daytime, the range of the IR communication line reaches several hundred meters. It is limited by extraneous illumination (primarily a light background behind the correspondent), which increases the noise level at the reception. In the evening and at night, it increases to 1,5 km.

Author: Polyakov V.

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