[sdiy] Oakley S&H clock LED pulse extender

[Jean-Pierre Desrochers]

LM555 will light a led the time you want
from a very small triggering pulse.

JP


Le 2014-06-04 15:23, Dave Brown a écrit :
> Decouple the base drive with a series diode and add a resistor and 
> capacitor
> in parallel to ground from the base.  The drive signal will charge up 
> the
> capacitor and the resistor will bleed off the charge.  You might not 
> even
> need the resistor but I like to add them.
> 
> Dave
> 
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Andre Majorel
> Sent: Wednesday, June 04, 2014 11:59 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Oakley S&H clock LED pulse extender
> 
> 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/
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