[time-nuts] GPS for Nixie Clock

Tom Van Baak tvb at LeapSecond.com
Sat Jul 16 07:43:17 EDT 2016


> Yes, I was planning on using a high speed photo diode to actually 
> measure the turn on time of the digits. I hadn't thought of the turn OFF 

Or just measure the anode/cathode current of the tube. The plots are non-linear and wonderful.

At this point, consider moving over to the excellent NeoNixie-L group (was yahoo, now googlegroups) for all the rest of your nixie questions. You'll find plenty of digit response time discussions in the archives, as well as hundreds of messages about using WWVB and GPS to set and/or drive the clocks.

Thanks,
/tvb

----- Original Message ----- 
From: "John Swenson" <johnswenson1 at comcast.net>
To: <time-nuts at febo.com>
Sent: Saturday, July 16, 2016 12:08 AM
Subject: Re: [time-nuts] GPS for Nixie Clock


Yes, I was planning on using a high speed photo diode to actually 
measure the turn on time of the digits. I hadn't thought of the turn OFF 
time, do I want the old digit to be turned off before the new one lights 
up or for them to be overlapping? I have been thinking about what 
threshold to use, 50% intensity is probably about as good as any other. 
It might turn out that different digits turn on differently, so I will 
have to calibrate each one separately.

John S.



On 7/15/2016 4:57 PM, Chris Albertson wrote:
> If you are going for the sawtooth correction then you also might want
> to add some kind of forward correction for the delay in the tubes and
> the drivers.  Your MOSFET gates the nixie tube itself have capacitance
> and switch times that will delay the switch of the display and of
> course the digital processing in the FPGA takes some number of
> nanoseconds.   I think you might need some way to actually measure all
> of these as any estimate might be your single largest source of error.
>    I don't know how to measure it.  Perhaps a pair of phototransistors
> one aimed at a PPS LED and one at the nixie tube.  This unknown delay
> is likely larger than the sawtooth correction.  at this level you
> might have to define when a digital is actually "on" as there is
> likely some thermal constant and the numbers don't light up instantly.
>    I'd bet the turn on time is larger than the sawtooth correction.
> What is "on"?  50% brightness?
>
> It gets hard when you start caring about tiny increments of time.   I
> have a mechanical clock, about 14 inches in diameter that is slaved to
> NTP.  The designer took a big short cut.  Time is kept internally at
> the hundreds of microseconds level and the pulse goes off to the
> stepper motor at the correct time well at least at the 100+
> microsecond level but the hands don't move instantly because (1)
> slight gear backlash and (2) they have mass.  I can actually SEE the
> delay with my eyes.  The designer must have forgotten that a "move"
> command requires some milliseconds to execute (I'm thinking about
> 100ms or more).  I don't care but it's fun to think the actual display
> is 10,000 times less accurate then the internal timekeeping.   You
> don't want this to happen to happen nixie clock
>
> BTW I did not build my mechanical NTP clock.  I got a free broken
> clock and had to fix it, cut and soldered a few traces, fixed some
> cracked parts and learned how it works in the process.
>
> Finding which PPS to use is easy, you can do that by eye.  Compare the
> serial data stream to the time on your NTP sync'd computer.  A full
> second off problem is easy to see.
>
>
> On Fri, Jul 15, 2016 at 3:53 PM, John Swenson <johnswenson1 at comcast.net> wrote:
>> Yep, that is theory. The fun part is going to be getting the right edge for
>> the new PPS. Half the time it will the one before the PPS from the GPS and
>> half the time it will be the one after. From the sawtooth data I should be
>> able to figure out which is which to align it to the new LO.
>>
>> John S.
>>
>>
>> On 7/15/2016 3:17 PM, Bob Camp wrote:
>>>
>>> Hi
>>>
>>> If you are going to go “full boat” then you probably should get the
>>> sawtooth correction out of
>>> the GPS and feed that into your control loop. You will need something you
>>> can run out at the
>>> “few hundred seconds” sort of time constant.
>>>
>>> Bob
>>
>>
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