[time-nuts] FE5680 missing PPS soln

[Rob Kimberley]

Mine was a green LED out of the junk box, but I did use 330R in series. 1k8
seems rather large.
I will probably put something between the 5680 and the LED later. I have a
bin full of old 74xx, so should be easy.

I've seen lots of threads about the 5680 and tweaks etc., but not much on
actually building them into anything.

I just built a very cheap dual unit (two 5680s bolted in). When I say cheap,
the only thing bought were the 5680s. The rest was out of the junk box.

The case is from an old Technics tape deck
Front panel is cut down and drilled from an old PC side panel.
The 5680s are bolted to the bottom of the case
Main power (+15V) comes from a universal 5A laptop power supply (with
selectable output voltage set to +15V). I used an old 10V brick to drive one
of the 7805 sent by the Chinese supplier with the 5680s. The rest is the odd
resistor and capacitor and wiring. Not pretty but very functional.

Front panel has: 2 switches (+15V to the Rbs): 2 red LEDS - power ON to the
RBs, 2 green LEDs - Lock, and two BNCs - 10 MHZ outout.

The only thing left is where to fit the teddy bear they sent.......

Have fun with your projects!

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 23:20
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] FE5680 missing PPS soln

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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