K2 Autodetect keying circuit

[Ward Willats-3]

I know one of you smart folks will easily have some advice off the
top of your head:

I built an opti-copupled keying circuit in a D-Shell, driven by pin
17 of my parallel port and use n3fjp's logging software to drive it.
The coupler is a 4N35 and the current limiting resistor is 1.2K. The
basic schematic is from N3FJP's web site: http://n3fjp.com/rigcw.htm 
-- except I added two 1N5817 shottky diodes to trip the auto detect.

Well, the scope shows everything is wiggling, but the keylines are
only being pulled down about 3 volts (from around 7). I assume I need
to pull the line a lot closer to zero before the K2 will key!

Before I go stabbing around blindly, I thought I'd ask what I need to
do to get more drive. My thought is I can do some math and drop the
current limiting resistance on the LED so it shines more brightly in
there, but maybe there are other factors (such as the maximum current
the parallel port can source) that I should know about before I start
down this road.

Thanks,

-- Ward / KG6HAF
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[Vic K2VCO]

Ward Willats wrote:

> I built an opti-copupled keying circuit in a D-Shell, driven by pin 17
> of my parallel port and use n3fjp's logging software to drive it. The
> coupler is a 4N35 and the current limiting resistor is 1.2K. The basic
> schematic is from N3FJP's web site: http://n3fjp.com/rigcw.htm -- except
> I added two 1N5817 shottky diodes to trip the auto detect.
>
> Well, the scope shows everything is wiggling, but the keylines are only
> being pulled down about 3 volts (from around 7). I assume I need to pull
> the line a lot closer to zero before the K2 will key!

I am doing almost exactly the same thing, although I am using an NTE3040
opticoupler (equivalent to a 4N25).  The specs of the 4N25 and 35 are
slightly different in that the LED has a higher reverse voltage (6V
compared to 3V).  I am using a series resistor of 1K.

The 4N35 is rated at 120 mw maximum dissipation, so you can measure the
voltage output of your parallel port, subtract the 6V drop accross the
4N35 and see if you can reduce your resistor value.  Or, you can replace
the 4N35 with a 4N25.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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