[sdiy] Oakley S&H clock LED pulse extender

[Andre Majorel]

I'm driving an Oakley S&H with a very short pulse (about 23 µs).
To make the flashing of the LED more visible, I've tried adding
a capacitor.

			    +15 V
			      |
			    A +----------+
			     _|_         |
			     _V_  LED    |
			      |          |
			      |          |
			      >          = Ca
			      <  R2      |
			      >  1.8 k   |
			      <          |
			      |          |
			    B +----------+
			      |          |
			    |/  Q1       |
			----|   BC549    = Cb
			    |\  NPN      |
			      |          |
			    C +----------+
			     _|_

A 100 nF at Ca or Cb doesn't seem to do much. With a 2.2 µF at
Ca or Cb, the short pulses don't even light up the LED.

I was hoping that Q1 could supply enough current to "instantly"
load a capacitor large enough to keep the LED on for a tenth of
a second or so.

What am I doing wrong ?

-- 
André Majorel http://www.teaser.fr/~amajorel/