[time-nuts] FE5680 missing PPS soln

Rob Kimberley robkimberley at btinternet.com
Thu Feb 2 21:10:47 UTC 2012

I've used an LED direct to the lock output. 

330R resistor in series and the other end of the LED to the +5V rail. 

Works fine for me, but then, I'm not interested in the 1PPS (actually not
even sure my bricks have that feature).

Rob Kimberley

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of bob grant
Sent: 02 February 2012 20:46
To: time-nuts at febo.com
Subject: [time-nuts] FE5680 missing PPS soln

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
This logic low is needed to enable one half of the 74AC240 buffer (pin
1) that gates the PPS signal.

Don't directly drive LEDs from the LOCK signal on the DB9 and, voila, the
PPS signal reappears.

  bob grant
  bobgrant at fastmail.fm

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