[sdiy] Oakley S&H clock LED pulse extender

[rburnett at richieburnett.co.uk]

In addition to the 555 timer circuits already suggested, there are some 
discrete and IC monostable circuits here that will extend short pulses 
out to something more suitable to drive an LED:

http://www.electronics-tutorials.ws/waveforms/monostable.html

If you don't want to mess with the base drive of the common-emitter 
transistor that is already there, then i'd suggest that you follow this 
existing common-emitter stage with whatever arrangement you choose to 
use for the monostable, and then finally drive the LED with the output 
of this monostable.

(Just slapping a big capacitor across the LED (Ca) or the transistor 
(Cb) as you showed is asking for trouble as there will necessarily be a 
large current surge when the transistor turns on, in order to either 
quickly charge or quickly discharge the capacitance.)

-Richie,


On 2014-06-04 19:58, Andre Majorel wrote:
> 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 ?