Bruce,

Instead of using a  phototransistor, use a PIN diode in current mode
followed by a transistor or op-amp. HP and others make photodetectors that
are configured like this. The bandwidth of a phototransistor is limited
because of the HUGE base area needed to get good photo response.

Regards, Jim

At 03:17 PM 10/18/97 -0700, you wrote:
>I would like to design my projects/products so the ubiquitous ON indicator
>LED is serving the secondary purpose of sending debug info serially, to a
>phototransistor or some such which I have placed over it.  This would be
>useful for development, but also for periodic sample testing of production
>devices, don't you think?
>
>I've found that spacing out the bytes, and sending at reasonably high data
>rates results in a visibly steady LED glow (comm feature transparent).  My
>problem is that I can't find phototransistors fast enough to switch fully
>on and off with so little illumination (a few mA through a standard
>brightness LED).
>
>Yet it seems to me that this SHOULD work.  How about you smart folks who
>have done this kind of thing a million times giving me the benefit of your
>wisdom?
>
>Bruce Cannon
>
>
Jim Ham, Porcine Associates
(415)326-2669 fax(415)326-1071
"http://www.porcine.com"