[sdiy] can I make this circuit change?

[Roy J. Tellason]

On Sunday 18 January 2009 11:28:57 pm db wrote:
> I tried 104 at 1k and 105 at 1k, and still not quite enough oomph to drive
> the LED on.
>
> Maybe I *need* an op amp if I want to do this?

Some years ago I built myself a small circuit that I used as a "logic probe".  
One part of it was a pair of transistors,  each driving an LED,  where +5 at 
the input would light the green one,  no input would light the red one,  and 
it'd be proportional in between (the circuit ran off a 7805 for power).  That 
was just two transistors and some resistors,  and only the one between the 
stages needed a bit of fiddling to get right.

The other portion of it,  more pertinent to what you're looking for here,  was 
a "pulse detector" to tell me that there was signal activity when it was of 
short duration and wouldn't be apparent on the above circuit.  This was 
another two transistors,  some resistors and a cap.  The circuit was 
a "monostable multivibrator",  if you want to google that.  I'm sure there 
are circuits out there...

An awful lot of what I was bumping into doing a quick look seemed to be about 
the use of one chip or another,  the 555 or the logic-family specific parts.  
You don't need a chip to do this.  Maybe something like this would work:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm#mono.gif

Anyhow,  that's what worked for me.  :-)

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