Thanks particularly to Jerry, W4UK, for a suggestion that led to a fix for
the my problem getting an optoisolator to key my FT-1000 off my computer's
serial port.
Apparently, I wasn't alone in having problems with optoisolators. The
standard suggestion is to use an input resistor of about 1 k ohm. However,
I discovered that such a resistor doesn't allow enough current to really
light up the LED when powered from my serial port. I measured the
short-circuit current from a line on my serial port at about 10 mA. Since
the voltage is around 12 V, the internal resistance must be about 1200
ohms. Since 10 mA is well below the maximum LED current (maybe 50 or 60
mA) for the 4N33 optoisolator, using a 1200 ohm input resistor would allow
only about 5 mA LED current.
The trick was just to eliminate the input resistor and it switches just
fine now. As I had hoped, the optoisolator fixed the 40-meter RFI problem
that I had had with transistor switching.
Glenn W6KNB
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