[time-nuts] Generating a solid PPS from 10Mhz source

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Jan 18 14:54:54 EST 2016


On Monday, January 18, 2016 11:45:00 AM Poul-Henning Kamp wrote:
> --------
> 
> In message <569BC23A.3030400 at arcor.de>, Gerhard Hoffmann writes:
> >LEDs abused as References:
> This is one of the most stupid ideas ever, because LEDs works both ways.
> 
> (Back when LED wrist-watches first came out, people soon discovered
> that they would reset themselves when photographed with flash.)
> 
> If you insist on using LEDs as voltage references, the first thing
> you need to do is to dip the LED in something which shields it 100%
> from incoming light *including infrared*
Actually when forward biased they are not that sensitive.
I used a string of forward biased LEDS to set the voltage of a node in an 
amplifier. To detect 100Hz modulation due to photocurrents in the LEDs the 150W 
incandescent bulb had to be placed within a few cm of the LEDs.
Using a little bit of opaque pain or epoxy is a small price to pay. 
Photosensitivity is also an issue with glass encapsulated low leakage diodes 
if you scratch  the paint.
Even glass encapsulated reference (and other) zeners are photosensitive.
For sensitive measurements a light tight enclosure is often used.



Bruce



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