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From: sam@colossus.stdavids.picker.com (Sam Goldwasser)
Subject: Re: Laser pointer question
In-Reply-To: colinc@sa-cgy.valmet.com's message of 11 Oct 1994 19:15:49 GMT
Message-ID: <SAM.94Oct12082043@colossus.stdavids.picker.com>
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References: <37eo95$deh@oasys.sa-cgy.valmet.com>
Date: Wed, 12 Oct 1994 13:20:43 GMT
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In article <37eo95$deh@oasys.sa-cgy.valmet.com> colinc@sa-cgy.valmet.com (Colin Constant) writes:

>   Hi,

>   I just noticed that Radio Shack sells a laser pointing device 
>   that gives you (if I remember) a 1/2 in. dot at up to 50 ft.

>   I'd like to modulate this for remote control.  Anyone have any
>   experience with this kind of thing?

I have never done this but here is some info:

Typical laser pointers use visible 670 nm (approx.) laser diodes.  Depending
on model, these put out anywhere from 1-3 mW.  The beam from the diode
is a highly divergent wedge shape (10x35 degree typical).  A collimating
lens in the pointer turns this into a more-or-less parallel beam.  Since
the output is visible, you should be safe as long as you don't stare into
the beam.  However, be careful with IR laser diodes (from CD players, CDROMs,
or optical drives) where the main beam is invisible but there are weak
spurious red emissions.

How they are driven:

Typical currents are in the 30-150 mA range at 2-2.5V.  However, the power
curve is extremely non-linear.  There is a lasing threshold below which
there will be no output.  For a diode rated at a typical current of 85 mA,
the threshold current may be 75 mA.  This is one reason why all applications
of laser diodes include optical sensing (there is a built in photodiode
in the same case as the laser emitter) to regulate beam power.  You can
easily destroy a laser diode by exceeding the safe current even for an
instant.  It is critical to the life of the laser diode that under no
circumstances do you exceed the safe current limit even for a microsecond!

Laser diodes are also supposed to be extremely static sensitive, so use
appropriate precautions.  Also, do not try to test them with a VOM which
could on the low ohms scale supply too much current.

It is possible to drive laser diodes with a DC supply and resistor, but unless
you know the precise value needed, you can easily exceed the ratings.  They
are not forgiving like LEDs.

One approach that works for testing is to use a 0-10 VDC supply with a,
say 100 ohm resistor in series with the diode, and slowly bring the current
up until you get a beam.  If you get the polarity backwards, the voltage
across the diode will go above 3 volts.  Then, turn power off, discharge
the filter capacitor in the power supply, and reverse the leads.

For an actual application, you should use the optical feedback to regulate
beam power.

Assuming that you can get inside one of these laser pointers, you should
be able to find some point in the circuit where the beam current can easily
be modulated.  However, the circuits may have filter caps across various
strategic points (like across the diode itself) which would limit
frequency response.  Simply controlling the current from the battery
probably will not work well due to the internal regulation though it may
be worth a shot.

Again, do not exceed the ratings of the diodes even for a microsecond!

--- sam
