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ArcticMyst Security by Avery

Laser Transmitter Circuit, need help

Joined
Apr 27, 2011
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Hello, I'm trying to make my first laser circuit.
I'm trying to beam information with a data stream, i.e off/on

I found this circuit in a book, but i don't understand how to determine the proper resistances and bias voltage.

http://i1140.photobucket.com/albums/...vercircuit.jpg

I'm using a hitachi HL6335G 635nm, 5mw diode

Any circuit theory help would be greatly appreciated!

Frumpz

I'm also highly interested in the proper term for this type of circuit so i can google more efficiently.

Is it modulated? laser driver? i'm not really sure what to call it
 





Joined
Jan 14, 2011
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You are most likely looking for a 'Modulated Driver'.
Modulating the signal inteligence into the beam is the easy part. Demodulation is a bit more complicated. I cann't look at photobucket on this connection, so can not comment on your circuit. A cheap and dirty solution is to feed your LASER supply + into the collector of a BJT (or maybe the drain of a FET/MOSFET), tie the emmiter (or FET/MOSFET source) to the LASER + , and feed your signal intelligence to the base (or FET Gate). then you only need to attenuate the signal to avoid over driving the LASER if you are sending an analog signal (you said "on/off", so I'll assume that you are talking about digital signal modulation, in which case you do not need to worry about over driving the modulation mixer (transistor/FET).

Rambling on... there are a couple of LASER diode driver boards out there that have 'TTL modulation' inputs (Axiz for one).

Have fun and good luck!
 

anselm

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You could try this schematic, even if it is a bit more complicated than what you need.
Die4Drive

To get the proper (very low, I guess) current for your particular diode, you will have to
substitute some of the R3 R4 R5 resistors....
 
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It would help to know what type of data/speed you want
to transmit.....

Jerry
 

anselm

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So that's what, 100-500MHz modulation frequency, no?
I actually have no idea if the circuit or the diode or the detector can handle this much.:yabbem:
The diode at least should be able to cope fine...
 
Last edited:

Benm

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Well, it would seem almost possible. A dvd writer diode can be pulsed (at least) at the data rate the dvd can be written, and the detector used in playback is fast enough to handle the dvd read speed. For a 16x drive thats about 20 mbyte/sec (160 mbit).

To preserve clock lock you may have to XOR the data against a carrier of double the data rate, which would end up at at least 80 mbit transfer speed.

We know for a fact that lasers and detectors are at least this fast, although their speed is probably not the limiting factor in a dvd drive at all.

The problem will be s/n ratio on the link. If you couple it with fiber optics, that should be no problem, but if you want to go through open space, and have to deal with noise from ambient light, this will change.

Apart from that it will probably prove pretty difficult to practially construct a diode driver and detector circuit this fast that can be connected to a pc on either end.
 
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So that's what, 100-500MHz modulation frequency, no?
I actually have no idea if the circuit or the diode or the detector can handle this much.:yabbem:
The diode at least should be able to cope fine...

Well, what would be a fairly easy speed to build around?
And how does that determine resistive values?
 

Benm

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I have my doubts about the fairly easy part of such a build. Finding, for example, an opamp with decent performace at 500 MHz will be a challenge. You're looking at RF components here, not your average radio shack supply!
 
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Unless you go for a bandwidth of only a few MHz, you're gonna need A LOT of expensive test equipment en A LOT of knowhow, and choosing the right resistor value will be the least of your problems. If you have a hard time finding the right resistor values you definately can't get up to 100 MHz.
 

Benm

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Nothing wrong with a bit of experimenting though. I'd start at much lower frequencies just to get a feel for it. One interesting project you could do is modulate an 808 diode with the signals from a remote control (usually 37 kHz or so).

If it works, you can change the channels from a considerable distance :)
 
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Imagine irritating the neighboors down the street with that :)
May not be safe though.

But starting a lower frequencies is a good idea, then work your way up in speed and complexity.
 

strik3

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i always wondered if you could use a 808nm diode as a long range tv remote.
......
to the interwebz to i go to do some research on that
 




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