32.768kHz Pierce Crystal Oscillator?? For Nixie clock.

[greenpossum]

Or is 32kHz low enough where I won't have to worry about that?

It's just that the tolerable stray capacitance is lower for 32768 crystal circuits. You could make the oscillator on a separate perfboard for testing.

[Mighty Burger]

Or is 32kHz low enough where I won't have to worry about that?

It's just that the tolerable stray capacitance is lower for 32768 crystal circuits. You could make the oscillator on a separate perfboard for testing.

Will do. I'd imagine the stray capacitance would be about the same on a perfboard as the finished PCB.

Well, I ordered the parts! They should arrive Tuesday/Wednesday. I want to jump right into designing the PCB but I'm a little reluctant in case I find an issue with the schematic. But April 13 is coming fast and JLCPCB ships from China ..

[greenpossum]

I also used keep out zones on both sides of the PCB around the crystal and the traces to it to minimise stray capacitance.

[Mighty Burger]

I've built the circuit (most of it) on the breadboard! I've ran into three issues while building it.

1. The seconds would not carry over to the minutes, and the minutes would not carry over to the hours. I've figured out why. The outputs of U6 and U12 are connected to both their own reset pins and the clock pin of the next chip. The issue is that each chip resets itself before the next chip receives the signal to advance by one. I fixed this by adding a resistor and capacitor on the reset pins of U6 and U12 so that they reset a couple hundred microseconds later. This seems to work, and I've ordered enough additional parts that I can implement this.

2. When powering on the whole circuit, the state of the binary counters seems unpredictable. Usually they start at zero, but occasionally they will start with multiple random outputs on. It fixes itself once each chip counts over. I figured that this could probably be solved by signaling the reset pins right after the circuit receives power. I would probably do this by having a 100nF capacitor in series with a 10k resistor. The resistor connected to GND, and the capacitor connected to the +5V rail. The junction between the two could then go to each reset pin through a diode. But there's a few issues. I haven't bought enough extra diodes to make this happen (and I don't want to pay another shipping fee from Digikey, wasted money), I didn't test this solution out, and if I place a diode after the NPN AND gate that resets the pins of U2 and U4, that would probably bring the issue around again of the voltage dropping too low for CMOS. I could use an IC for the AND gate but it's starting to get a little convoluted for something due too soon.
I think for now I'll just deal with it and do nothing. I'll fix it in the polished version I want to make after this senior project is in the past. There's an advance button for the hours and the minutes, and a switch that resets the seconds, so if I plug it in and the counters go whack I'll just cycle through all the numbers until it's fixed. I will need to adjust the time anyways every time I plug it back in. I don't think the transistor arrays or the nixie tubes themselves will be damaged if multiple digits are lit simultaneously in the same tube, correct?

3. Some weird stuff happened while I was testing out the nixie tube driving circuitry, but it seems to have fixed itself except for one minor issue. Little bits of the different digits start to glow when no cathode is being driven. This won't be problematic for me because each tube should always have one cathode being driven when the clock is operating as it should. It's just an interesting issue that I might want to figure out in case I want to take this nixie driver design and apply it in other projects, but that's not something I will deal with right now.
Now, because of limited time and jumper wires, I only tested out one nixie tube (connected to the last digit in the seconds), and only four digits of said nixie tube. It concerns me a little bit because I couldn't test the whole entire circuit at the same time but I think it's good enough to design a PCB around and get this thing built.

[greenpossum]

I found that blog page has a lot of info about nixies and their power supplies. The glow tends to happen when all cathodes are turned off.

https://threeneurons.wordpress.com/nixie-power-supply/

In your design you have at least one cathode on so that's not a problem. Some careful designs turn off the anode before the cathode. This is easier to do when the anode is also driven, for multiplexing.

[Mighty Burger]

I found that blog page has a lot of info about nixies and their power supplies. The glow tends to happen when all cathodes are turned off.

https://threeneurons.wordpress.com/nixie-power-supply/

In your design you have at least one cathode on so that's not a problem. Some careful designs turn off the anode before the cathode. This is easier to do when the anode is also driven, for multiplexing.

This is an extremely helpful blog. Thank you, I bookmarked that page. It explains a lot of the behavior I've been seeing with these tubes.



Also, very random question, does anybody know where I can get these brass fittings seen in this picture? They look very nice and I would love to use something similar in my clock. I'm using the same IN-14 tubes, a little over 18mm diameter. I'd assume I'd have to space them a little further apart if I use them.

[greenpossum]

Maybe you can make friends with a tame machinist with a lathe. How about hardware shops? Some kind of decorative fitting?

One nice effect I've seen is to put a blue led at a suitable brightness under the nixie. Good contrast with the amber.

[Mighty Burger]

Maybe you can make friends with a tame machinist with a lathe. How about hardware shops? Some kind of decorative fitting?

One nice effect I've seen is to put a blue led at a suitable brightness under the nixie. Good contrast with the amber.

I'm friends with some people with lathes, only problem is they're all wood lathes  Ah well, I'm sure it'll still look good without the brass pieces

I've seen lots of videos and pictures of nixie clocks with blue LEDs, and to be honest I really do not like that look at all. If I make a polished clock to sell sometime in the future, I'll add it but personally I would end up just turning the blue LEDs off on mine all of the time. Thanks for the idea, though!

[greenpossum]

Post pix when you're done!

[Mighty Burger]

Is there any way I could ask you guys to review my circuit board? This is my first time making these things. I don't really know what I'm doing but I don't think I need to worry too much about high-frequency impedance or anything with the majority of this circuit.
Is the gerber file suitable for submitting to JLCPCB?

I used a keepout zone for copper fills around the oscillator area to minimize stray capacitance as per greenpossum's recommendation (thank you!). I also placed some vias to "stitch" the ground copper pours at different points on the board (is that the correct term?)

Thank you




Also, I am using the "NCH8200HV" module from Amazon as a 170V power supply. That is what that square is for.

I plan on mounting this right to the top wooden piece, so most of the components are on the bottom. The ones that are on the top of the PCB are thinner than the nuts for the bolts I plan on using.

[greenpossum]

JLCPCB has a page on the settings you should use when generating Gerbers.

I recommend you use the highlight net function in Kicad to check that all the connections in the schematic actually correspond to tracks on the PCB. Better to find out at this stage than later. Also generate a 3D view so that you can check clearances. Especially important for large components like modules, capacitors and connectors.

[T3sl4co1l]

I'd like to see more distributed via stitching (what are those rows of vias, are those all GND? but they're so close together?), but other than that, at a glance it looks reasonable.

I would recommend ignoring the oscillator keepout suggestion.  No ground makes it hugely more susceptible to ambient fields.  If you're running too much capacitance from strays, just reduce the damn shunt capacitors, I mean duh..?

Tim

[Mighty Burger]

Was checking through some things and I realized I got my nixie tube footprint backwards. As it turns out, the datasheets I was looking at were a bottom-up view 

[greenpossum]

I would recommend ignoring the oscillator keepout suggestion.  No ground makes it hugely more susceptible to ambient fields.  If you're running too much capacitance from strays, just reduce the damn shunt capacitors, I mean duh..?

If you look at RTC crystals on PC boards this is what they do. Nobody in their right mind will run a trace to the other end of the PCB. All the recommendations say keep the crystal close to the IC pins. So there will be hardly any induced signal for a trace of what, 10mm? 

[T3sl4co1l]

Short trace length yeah, that's good.  But induced signal is induced signal... and the pins are huge and exposed.

You can reduce susceptibility here by dB if you use short traces and smaller pins and components.
By 10s of dB by pouring ground around it.
By 100s of dB if you put a shield can over it.

In exchange for no performance change in the oscillator?  That's not even a discussion!

Sure -- I'll look at three motherboards I've got in my junk bin right now --
Dell Inspiron 5160 I believe -- Intel SuperIO chip, BGA top side, RTC crystal bottom side.  Narrow 4-pin SO style plastic case.  No shunt capacitors (probably enough internal on the pins).  No evidence of removed internal planes.  8 layer board if I'm not mistaken.


Compaq 8510W workstation -- actually three tuning-fork (very narrow body, leadless(?) 4 pin) crystals I've spotted, one looks to be for the TPM (which, haha, Infineon SLB9635TT12 is a TPM controller from 2006 apparently!), has two flanking capacitors and a parallel resistor, solid ground underneath.  One beside the Intel SuperIO chip, has capacitors and resistors nearby but unclear if they're connected.  One beside the PCI bridge(?).  Interestingly, this board doesn't seem to have a backup battery, though the others do.

Desktop motherboard, K7T Turbo2 -- VIA SuperIO, metal-can watch crystal, THT.  Sort-of stapled to the board with a jumper wire.  Seems they stopped short of actually soldering it, but the can is very likely grounded.  Two connected capacitors, and a... 5.6M resistor?

So yeah, ground under all of them.  Removed grounds would be just absurd in high density, high speed builds like these, and would have no effect on the poor crystal.

Tim

[Mighty Burger]

Made some changes here.

- I fixed the IN-14 Nixie footprint. If anyone else ends up doing something similar, please ensure you get the footprint the right way around the first time to save some frustration
Because the digits are now in reverse order of the pins of the ICs, I had to do some very claustrophobic routing. Of course, I ensured the clearance was fine. I have the 170V nets at a clearance of 50 mils and the 50V nets at a clearance of 25 mils. Running the DRC, the only problems it detected were around the two pads of each of the anode resistors and the two pads of R1 (the resistor that pre-biases the zener diode). The anode resistors will only have a drop of 50V across the pads, and R1 will have a drop of 120V, looking online the acceptable clearances for those voltage ranges according to some safety standard are both 25 mil. KiCad's rudimentary netclass clearance system doesn't have a way to deal with that, so just to check, I temporarily changed the clearance of those netclasses to 25mil. Then when I ran the DRC, there were no problems! (Yes, I changed it back to 50mil and refreshed the copper pours)

- I changed up the via stitching a little bit. Distributed it a little more. It's not perfect but I think it'll do for this project.

Not sure what to do as for the GND plane underneath the oscillator part. You two seem to disagree, and with some googling I could only find conflicting answers. I figure that, since while testing the circuit my oscillator I lousily soldered together with unnecessarily long wires (I wasn't aware of the impact it had), with my garage space heater running that pumps out tons of electrical emissions enough to show up in large scales on my scope - since that worked just fine, I figure the oscillator on the circuit board should work OK with or without the GND copper pour. It isn't super high frequency stuff anyways. So I stuck with the status quo and didn't change anything. I might want to do more research on this if I plan on making a polished version. If this doesn't work somehow, I'll just throw in the board I've already soldered to perfboard that does work and wire it to the board just for the senior project grade.




With these renders I seem to only have models for one component, C2 (the electrolytic cap). I think the clearances for everything should be fine, if not I can just fab up a fix and take note of it if I make a polished version down the road.
Thank you guys for the help.
I'd figure I'd ask for one final overview to see if I've made any problems my inexperienced eye didn't catch before I order the boards.

[greenpossum]

So yeah, ground under all of them.  Removed grounds would be just absurd in high density, high speed builds like these

Since when is 32.768 KHz high speed? You have got it confused with CPU clocks. There is a reason many of those are SPXO.

Also OP took it much further than I normally do. My keepout is just in the vicinity of the crystal and also small ones between pins to prevent any trace from going in between. That said, either way is unlikely to make any difference to this low speed design.

[T3sl4co1l]

So yeah, ground under all of them.  Removed grounds would be just absurd in high density, high speed builds like these

Since when is 32.768 KHz high speed? You have got it confused with CPU clocks. There is a reason many of those are SPXO.

Everything else is. PCI(e) buses routed past a super high impedance crystal with removed ground, would destroy both the crystal and the high speed signals. Sorry that was unclear.

Not sure what to do as for the GND plane underneath the oscillator part. You two seem to disagree, and with some googling I could only find conflicting answers.

It's regrettable that I don't have any source of authority to hand -- supportive and consistent references, or analyses, for example.  Or, even if I did, that they would necessarily be understandable.  My signature seems to be more authoritative than his, but that's still just a handful of words, and if they have no meaning to you, it's not much help.

You're welcome to go either way.  It will probably work.  Your environment may not be all that noisy afterall.  Maybe it will merely manifest as slightly out-of-spec or variable timekeeping, and that will be an acceptable burden.

Tim

[Mighty Burger]

Well, I've ordered the boards. Apologies if I've offended anyone over the whole GND plane debacle. Thanks for the help everyone.

[Mighty Burger]

Quick update. The Idaho State Department of Education has decided to let school districts opt out of the senior project requirement because of le Corona. Whether my school actually cancels the project is to be seen. Either way I have been wanting to make a nixie tube clock for a very long time apart from anything school-related, and I'm very excited to see how this project turns out.

[greenpossum]

Well you've got an attractive project going here so you would really get a lot of satisfaction getting it working. You might later like to write up your project at hackaday.io to show it off.

[schmitt trigger]

Since your board is long and the relatively tall tubes con apply significant leverage on the board, I would strongly suggest that you add mechanical support between V2 and V3, and between V4 and V5.

[Mighty Burger]

Since your board is long and the relatively tall tubes con apply significant leverage on the board, I would strongly suggest that you add mechanical support between V2 and V3, and between V4 and V5.

You do bring up an interesting point I hadn't thought much about - mechanical design. It reminds me of one of Linus's videos he made recently where they were reviewing a product, and a member of the crew mentioned how he could tell it was made by electrical engineers because the structure was terrible 
Unfortunately I've already ordered the boards so it is too late to adjust the mechanical support. I will add this to the ever-growing list of things I should refine if I make a polished version in the future. Thank you for the advice.

I wonder what the best way to mechanically attach my circuit board to the enclosure. I'm going for a look similar to the picture I uploaded a few posts up. At first I thought I would simply have small bolts going through the top wooden piece and attach the board using bolts and nuts, but thinking back on it that would mean having the bolt heads visible on the top. Since I'm going for aesthetics (otherwise I would've made a simple clock with 7-seg LEDs) I don't think this is a good option. I could use standoffs and attach it to the bottom board, but it would be less structurally sound than having it attached right to the top piece of wood. This is because, with the exception of the nixie tubes which will poke through holes, all components on the top of the board are very thin and so the board can be attached very close to the top of the clock, distanced by the width of a nut. Attaching it to the top would also give me leeway with the wooden enclosure, as the amount the nixie tubes pop out the top would not be dependent on how tall the entire enclosure is. Everything considered, though, it seems like the way to go is to attach it to the bottom piece of wood, unless there was a way to indiscreetly and strongly attach it to the top. I could potentially thread the wood, but that idea just seems sketchy to me, as I don't think wood would thread well with the fine thread pitch of M3 bolts. I also don't have any threading equipment.

Edit: Maybe I can use a threaded insert. Would those be strong enough? Am I overthinking this?

[T3sl4co1l]

6 screws is fine.  You can by the way get press-fit and/or soldered nuts, which can be used on the bottom side as blind mounts.  Though they're weaker than a clamping joint is.  Or swaged nuts/standoffs, which grip the laminate and don't have much top side profile.

Mechanically I would be much more concerned about the tubes being loose in their sockets, that is, put in some kind of bracket and shock mount for them.  Then you can worry about the PCB.  At that point you can drop it off a ladder and I'm not sure what more you'd really worry about.

Tim

[Mighty Burger]

6 screws is fine.  You can by the way get press-fit and/or soldered nuts, which can be used on the bottom side as blind mounts.  Though they're weaker than a clamping joint is.  Or swaged nuts/standoffs, which grip the laminate and don't have much top side profile.

Mechanically I would be much more concerned about the tubes being loose in their sockets, that is, put in some kind of bracket and shock mount for them.  Then you can worry about the PCB.  At that point you can drop it off a ladder and I'm not sure what more you'd really worry about.

Tim

Thank you for the advice! I've never heard of lots of those technologies, they all seem super useful. Are they used in industry often?

With this PCB I actually shied away from the idea of using a socket for the IN-14 nixie tubes. I was planning on simply soldering the nixie tubes to the circuit board with the white spacers they came with. I designed the footprint around it, though if it turns out socketing is a better option I could probably use socket pins..? (Sometimes I wish I had an existing nixie tube clock to my side so I can see what they did. But they're extremely expensive!) Here is a picture of what those spacers look like (not my picture). I figured since the nixie tube leads were very similar to that of through hole component leads, like resistors, they would not be suitable for a socket, and it would be better to simply solder the leads of the nixie to the board. I've read concerns about soldering the tubes directly to the board and some suggested soldering the leads a little further out because the thermal shock could crack the tubes, but I figured the spacers should create enough distance between the nixie's glass body and the board that it shouldn't be an issue. I do not think the tubes should fail often enough in operation that the time and effort required to desolder one tube and solder a new one would be problematic, but if I want to make a polished version in the future and I wanted to sell it, I might want to switch to a socket so the user can easily swap out failed tubes themselves.


When making that decision, though, I did not consider the factor of mechanical shock. I can see how the tubes would be very vulnerable if the device were to be dropped. Since the tubes will be directly soldered to the board and therefore mechanically attached to it, my inexperienced guess would be that I should shock-proof mount the entire circuit board to protect the nixie tubes. If I were to use simple standoffs, could this be done with a rubber washer on either side of the board at each mounting point? I honestly do not have much of an idea of what I'm doing here (as you can probably tell )

Please be patient with all of my questions, this is the very first time I've designed a circuit board so I'm learning a lot here, thank you all for the help.