Desktop model of laser communication on 10 Mbit. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Computers

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Desktop - means on wires "on the table, not framed in a case. The distance between the LED and the photodiode is 5 cm, but the light intensity is the same as from a distance of 250 meters from a 5 milliwatt pointer.

The layout is very crude "you can not bomb it, I myself see all its squalor, but he did the main thing - he earned it. Through it, I sat on the Internet all evening, and sent letters.

In essence, the amplifier is assembled from two identical modules connected by an emitter follower. The construction of two modules separated by the constant component reduced the effect of direct current from the LED from solar illumination, and made it possible not to separate the photodiode and the amplifier input with a capacitor, otherwise it would be necessary to put a load on the photodiode, and this would shunt it, i.e. would reduce its return, would affect the frequency response, etc.

This layout has at least two highlights:

1. The photodiode is low-frequency, but operates at a high frequency, it has a large window, which will make it easier to point it at the emitter, it costs 10 rubles at the time of writing this text.

2. All parts, as promised, are ours, and transistors are 3 rubles a piece.

Among the shortcomings, it can be indicated that it doubles the frequency, i.e. it amplifies a 5 MHz impulse twice as much as a 10 MHz one. Apparently, from a distance of 250 meters, he will receive a signal (if any) on a string - i.e. barely. No power filters were supplied, but four independent and stabilized power supplies were used, but all the same it works on the verge of excitation.

But still, there is a starting point, whether the layout is bad or good - it can be finalized, and the general bells and whistles and their sizes are now clear. I have another modification on the way, improved.

The layout was included in the gap in the twisted pair, while the modulator was triggered by a signal from the hub, the distance to it was 40 meters, the receiver worked through a 10-meter segment to receive in the network card of the computer, and the transmission from the computer to the hub went through the wire. So the connection turned out to be one-sided - for reception from the hub through the light, and for transmission through the wire.

Structurally, the layout was made in a shielding box from the UHF TV unit, although this box turned out to be cramped for such a number of parts, you can use a box from SCM or from SKB. As racks, I used 0.1uF capacitors with one end, I soldered them to the case, and soldered the leads of the elements of the cold "nodes (where there is no high-frequency component) to the other end, thus, I solved two problems: filtering and fasteners. I did not mark these racks on the diagram, " but the more there are, the better. You can also use megohm resistors as racks - a circuit with average currents and megohms will not affect the modes. In general, it is necessary to apply maximum measures against self-excitation, and tie chokes to the power outputs (good tone !!!). Otherwise, when an excitation occurs, a lot of time and effort will be spent - this is one of the most nasty parasitic phenomena after the periodic occurrence of defects (periodics).

Instead of multiple power supplies, you can try filters, but that's another story.

"Pitfalls" on which the white sailboat of your hope can break:

1. low beta transistors.

2. poor shielding.

3. too sweeping installation, especially in the emitter circuits of transistors.

4. excitation through the outputs of the power supplies (set chokes)

5. poor quality photodiode.

6. bad optics in the pointer.

7. etc. etc.

Publication: cxem.net

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