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
>

What color is your power LED? Most phototransistors have peak sensitivity
at near infra-red. They will only respond well to red and infra-red LEDs.
Something like a green or blue LED will probably not cause much of a
reaction by the phototransistor. I don't know if there is a wavelength
dependence on speed, but what you might be experiencing is just low
sensitivity to whatever color you are using. Also, are you shielding the
system from external light? A phototransistor can be easily saturated by
incandescent or flourescent ambient light. A small solar cell might be a
better solution. It has a wider wavelength response and is very fast (I
think up to a couple hundred megahertz).

Sean


Sean Breheny,KA3YXM
Electrical Engineering Student