[sdiy] Oakley S&H clock LED pulse extender

Dave Brown davebr at earthlink.net
Wed Jun 4 21:23:23 CEST 2014

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.


-----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

What am I doing wrong ?

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