> Thanks to all who responded.    Rick, I'm not sure how your original
suggestion (Collector to +, LED/RES to Emitter) answers the inversion
question -- unless I'm not understanding.  Seems that would just convert the
circuit from a common emitter to a common collector circuit.  I'll try it,
of course.
>

You are right. But it may be what you want. The original common collector
circuit inverts, and the common emitter one doesn't.

However, if you are getting some flickering in cct 2 when the LED is mainly
off, this means that the IC is not holding the transistor fully on. At the
cost of one more component, you could add a resistor from V+ to either
transistor base or IC output. You'd want to look at the IC data sheet to
size the resistor and find best location OR you could play. I'd guess that a
10k from +5 to transistor base would make a difference. Whether it's enough
is hard to tell with the data available.

BTW - your 2nd circuit is a cunning one - its a perfectly legitimate but
unusual approach to what you are trying to do. It's main drawback is that
you draw current via the collector resistor at all times whether the LED is
off or on. This may not be a major issue. If it is, then using a PNP and
inverting the circuit, as Matthew suggested, would be an easy fix. This may
still have problems with occasional unwanted on-flicker depending on what is
really happening. Resistor as above should help.

If the IC is in fact producing small output transients when there is no
wanted input then you may have to either eliminate them (why are they
happening?), or filter the inverter input (suitable cap from base to ground
or possibly IC output to ground).

If we knew the IR IC type we could make more accurate comments.
If available, an oscilloscope on the IC output would be useful.

Dangerous assertion: An oscilloscope is THE single most useful tool that an
electrical engineer can have.



        Russell McMahon

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics