[sdiy] Oakley S&H clock LED pulse extender

Roman Sowa modular at go2.pl
Thu Jun 5 10:54:40 CEST 2014


I'd say this:
drive the NPN with the 23us pulse via 10k resistor, 22n cap is placed 
across C and E.
E via 1M resistor to base of PNP (E to +15) driving the LED from C via 
7k5 resistor. That should give blinks in range of 20ms, very visible. I 
use 30ms in all my boards and it's blinding maddness.

If PNP has too low beta or LED needs higher current, then try P-MOSFET. 
I assume 1..3mA of LED current, which is even too much for modern bright 
LEDs

Roman

W dniu 2014-06-04 20:58, Andre Majorel pisze:
> 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 ?
>



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