[Elecraft] K2 Autodetect keying circuit

[Vic Rosenthal]

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