[time-nuts] FE5680 missing PPS soln

bob grant bobgrant at fastmail.fm
Thu Feb 2 23:20:28 UTC 2012

The LEDs are common garden variety, tied to +5V and inline with 1k8...I
was expecting 2ma or so. 
Note, whereas the PPS signal is buffered the LOCK signal for the DB9 is
not passed though the 74ACT240 buffer. 

On Thu, Feb 2, 2012, at 11:10 PM, Attila Kinali wrote:
> On Thu, 02 Feb 2012 12:46:01 -0800
> bob grant <bobgrant at fastmail.fm> wrote:
> > Some info...
> > 
> > Its tempting to attach an LED to the /LOCK signal on the DB9.
> > However this signal is very weak and the LED does not seem very bright
> > and PPS signal does not pulse...Hmm
> > 
> > Internally the /LOCK pin is connected the 74AC240 buffer, but with an
> > LED helping to keep the signal voltage high (2.3V) a logic low is never
> > asserted.
> May i ask what kind of LED you connected and at which current you
> drove it? I don't know which 74AC240 the FE5680 units use, but
> Fairchild lists theirs with an absolute maximum rating of +/-50mA.
> I wouldn't use it to drive more than 20mA, which means that you
> need at least a current generation LED, or better a high efficiency LED.
> Such as John Lofgren used. Alternatively, use a small logic level 
> P channel FET like the FDV302P or a small PNP.
> 			Attila Kinali
> [1] For those who don't know, Absolute Maximum Ratings should read
> as "If you exceede these levels, your device will be dead for sure!"
> -- 
> Why does it take years to find the answers to
> the questions one should have asked long ago?
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