I am interfacing some simple circuits that run on 9V to my
cpu board which runs at 5-6V.  The optoisolator is a 4N35.
On the led side, I put the signal I want through a 10k resistor
to the base of a 2N2222.  On the transistor side, I tie my
cpu input line to the collector which has a pull-up resistor
of 47k.

It functions OK, but seems VERY slow.  I can detect pulses
that occur at about 2kHz, but not much faster.  Isn't the
rise/fall time of this device, something like 5uS?  I should
be able to detect my target of 40kHz, but I can't get 16kHz.
This is done using wire-wrap and the wires going to the cpu
and to the test pin are about 8 inches long, but I'm not
doing anything high-speed.

In the Art of Electronics, it mentions tying the base of the
phototransistor to ground through a resistor to improve the
speed.  Is that what I need to do?  How do I calculate the
resistor value?


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