[time-nuts] GPS for Nixie Clock
cautery at montac.com
Sat Jul 16 09:14:01 EDT 2016
I would run a test and track turn ON/OFF times against varying
intensities to get enough data to chart/graph. THEN, you can make
informed decisions about intensity level, whether you want to consider
turn off time, etc.
This could become quite voluminous... data acquisition-wise and just the
sheer amount of information.
Clay Autery, KY5G
On 7/16/2016 2:08 AM, John Swenson wrote:
> 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:
>>>> 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.
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