[time-nuts] FE5680 missing PPS soln

Attila Kinali attila at kinali.ch
Thu Feb 2 22:10:28 UTC 2012


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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