[sdiy] Oakley S&H clock LED pulse extender

Neil Johnson neil.johnson71 at gmail.com
Thu Jun 5 00:23:19 CEST 2014


> 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 ?

23us is a _very_ short time to discharge a large capacitor through a
simple NPN transistor.   If you don't want to futz with the input side
of things then the right place to put a capacitor is across the C-E of
the transistor, so the capacitor keeps current flowing through the LED
while it charges back up.  The problem then becomes one of discharging
the capacitor within your 23us period, and that depends on how hard
you drive the base.

It's a bit late for soldering now, but a quick LTSpice simulation
suggests 10u across the C-E legs, and drive the base through a series
100R (pulling lots of current from the input though) might work when
driven by a 5V pulse (you didn't state what voltage your input pulse
is, so I took a guess at 5V).


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