32.768kHz Pierce Crystal Oscillator?? For Nixie clock.

[Mighty Burger]

"I am once again asking for your technical support."

Hey guys, I have a question I hope you can help me out with. My high school has a senior project program, and I've agreed to make a nixie tube clock. Yes, I've reached that milestone in my electronics journey. The days are flying by and I need to whip up something really fast, project due April 13
I could just slap in an Arduino, a 170V power module, some bits and pieces and call it a day but I wouldn't learn much. I want to design something a little more discrete, something I actually have to design, which means figuring out how to make a clock, how to use some ICs to do counting, decimal output and such, how to make a 170V power supply and how to drive the tubes off the IC outpus. (Side note, anyone have suggestions for an SMD transistor that can handle this voltage? I found a couple through-hole parts but there's gonna be a lot of them and I'd like to make the whole thing smaller)

Right now I'm working on the clock circuit. I can't use the standard 555 timer circuits I'm familiar with because I'd need something more accurate. Quarts crystals just seem like the right solution. I want to create a 32.768kHz signal and divide it down to 1Hz using a couple ICs. I found a fantastic video by "devttys0" on YouTube that explained everything about how a Pierce Crystal oscillator works. I'll be honest, some of it is a little high-level for me to understand at this point, so some of it went over my head.
He links to a tool he made that helps calculate the values for an oscillator (here: http://www.analogzoo.com/lab/pierce_bjt.html) but I messed around with the numbers for a while and just couldn't get what I needed. Looking back it seems like the circuit was designed for higher frequencies (e.g. 16MHz). So I looked elsewhere for another circuit that might work better.

I resulted to google images and found this picture:


I tried to follow through to the website it was hosted on but it seems to be down so I couldn't find any more info on this circuit.

The circuit on the right seems like a good solution for what I want. It uses a simple transistor and not much else. I have some questions:

- Is this (circuit on the right) actually a good solution for my application?

- What is up with that adjustable capacitor? Are they pricey, and will I need to hold my tongue at an angle while adjusting it to get the correct frequency? Is there a simple solution like this that doesn't use an adjustable capacitor?

- How "generic" is this circuit? Can I throw in just about any transistor (I might want to use an SMD transistor) or 32kHz crystal and just adjust the capacitors to match?

Sorry for the word salad. I'm stressing out a little bit about this and need to figure it out very soon. I'm trying to figure out as much of this as I can by myself but I'm quickly reaching barriers with my limited electronics knowledge and need some outside help. Thank you guys for the help.

[greenpossum]

Why not use an oscillator and divider IC like the CMOS CD4060 or descendants? Readily available and cheap and gets you a 2 Hz signal.

[Gyro]

Yes, a CD4060 or 74HC4060 would be a good choice because it gets you an (up to) 14 stage binary divider.

With reference to the two circuits shown, the logic inverter based one should use an unbuffered inverter, a 74HCU04 would be a good candidate. For the transistor one, a 2N3904 is about as generic as it gets, yes a surface mount equivalent (MMBT3904) is fine.

[Mighty Burger]

Thank you guys. The CD4060 seems like a great solution - combination of a frequency divider and a circuit that handles the quarts oscillator. I found an appropriate circuit and slapped a flip flop on the output and now I believe I have a functioning 1Hz clock. Thank you guys for the help.

While I'm here, I do have one question for something I'm going to run into later on in the circuit. I'm planning on using six nixie tubes in my clock. Two for the hours, two for the minutes, two for seconds. The first digits of the minutes and seconds will not go higher than 5 and that the first digit of the hours will not go higher than 1 (its a 12hr clock).

Considering everything, I will end up needing to drive 44 individual digits. If I decide to exclude the seconds, I will need to drive 28 digits. Every digit will need its own high-voltage transistor. With some googling I found the MMBTA42, which seems to be a high voltage, tiny SMD transistor which could work. I tried looking for a transistor array to save space and dollar, but the results seem to quickly dwindle down as the max output voltage increases. Is there an NPN transistor array that can handle the voltage of these nixie tubes (170V)?

[greenpossum]

The transistor does not need to handle the supply voltage of the nixies. What it needs to handle is the difference between the supply voltage and the sustaining voltage of the nixie. The latter is typically around 130V. So the transistor has only to hold off 40V. Add a bit of margin and a 60V transistor will do.

You can check the datasheet of the 74141, a BCD to decade nixie driver. The output is rated to 60V. Unfortunately the 74141 is out of production, but there is a Russian clone, the K155ID1, that some people use.

[T3sl4co1l]

OP can have a little piece of help.

Yes -- but therein lies the problem, more or less.  The CMOS oscillator is probably better behaved, whereas the BJT you need to account for its impedances, and the crystal and capacitors will act somewhat as impedance matching elements as well as resonant elements.  Which means the two capacitors may end up skewed one way or another, to get reasonably high gain (for low phase noise and current consumption) and frequency within tolerance.

Note that the crystal's frequency is pulled by the capacitors thus attached; the equivalent capacitance seen by the crystal must match its spec, to obtain the specified frequency tolerance.  The two caps act in series, so the series equivalent, plus whatever other capacitances the circuit has to offer (e.g. don't neglect CMOS input pin capacitance, or stray (trace / layout) capacitance).

If the trimmer cap doesn't affect stability -- that is, the oscillator has sufficient gain over the cap's range -- then you would adjust it to trim the frequency precisely.  If you don't have a precision frequency counter, you'll probably do this by small adjustment as you find the clock runs ahead or behind.

The impedance is generally around the same as the reactance of the capacitors, hence the resistors serving the filter network are also quite large -- circa 300kohms.  If you figure similar value capacitors but many times higher frequency, you'll similarly see why 10-30MHz oscillators use lower impedances (100-1000 ohms).  Simply a matter of scale.

Tim

[pqass]

See Dave's nixie video series part 2 #950 here:
He shows that you can use a regular ULN2003 or ULN2803 with a 47V zener (or combination <50V) on the COM pin as the low-side driver.

Therefore, you should be able to implement your clock mostly without discrete components; eg. 6xCD4017, 6xULN2803, 1xCD4060, 1x32765 Hz xtal, 6x22Kohm anode resistors, and a few bypass and clock caps+resistors+zener(s).

[james_s]

I used MPSA42 transistors in most of my nixie clocks, in a few I used some octal transistor arrays. The downside of many of the lower voltage parts is that you cannot turn the tube off completely, if you have no cathodes lit you will get some leakage that causes a blurry glow as I recall. Supertex also made some HV shift register ICs, I don't know if they still do.

[Mighty Burger]

Getting lots of fantastic help from you all. Thank you so much! It took me a while to understand the solution with the Zener diode and the 50V transistor array but I think I understand it now. It's actually a pretty brilliant solution. I had concerns about the digits not turning off fully, but as Dave demonstrated that isn't an issue. It makes sense anyways - the sustaining voltage for the tubes is 130V (thanks greenpossum), and with the 50V zener in use with 170V applied to the nixies, each digit will only see 120V when it's turned off, which is not enough to keep it on. If 120V is too close to the sustaining voltage and some strange leakage happens, I could probably cut off the 170V supply for some microseconds every time the 1Hz clock fires, so each digit that is supposed to be lit up needs to reach the striking voltage. 120V is significantly lower than the striking voltage of 170V. But I'll test it because it probably won't be necessary.

Learning a lot here, thank you guys for helping.



I have a quick question. Could I connect all of the COM pins across the six ULN2803 chips to one zener for the whole circuit? Or should I just use one zener per chip?
Also, reading the datasheet for the ULN2803, it seems like it already has a built-in base resistor, so I can just feed the outputs from the CD4017 decade counters (will use 5V power supply) directly into the chip's inputs, am I reading this correctly?

One more question if you guys don't mind. At this point I think I don't have enough time to really dive into the details about boost converters and nixie power supplies, and rushing the design for a 170V power supply seems like a bad idea. (I want to make a polished version once I receive my HS senior project grade, there I will spend the time to learn everything about boost converters). For now, to present the project I think I'll just go with one of those nixie power supply modules you can get off ebay/amazon, but I'd thought I'd try asking if you guys know of any circuits that would boost a modest 5V or 12V or so up to 170V? I hate asking these sorts of questions before putting lots of effort into trying to figure it out myself, but the deadline is fast approaching.

[greenpossum]

I don't think putting the zener in series with the common pin of the transistor array will work but you need to draw us a schematic.

[Mighty Burger]

I don't think putting the zener in series with the common pin of the transistor array will work but you need to draw us a schematic.

Here is what Dave and pqass was talking about, I think


Simplified, treating the Darlington like one transistor:

[T3sl4co1l]

I would think they can be common, yes. Total current will be, what, mA if that?

Tim

[greenpossum]

Ok, that should be fine. It really is protection for the transistor, but you still need to be able to hold off the difference in voltage otherwise the tube will not turn off. So now the condition becomes:

Vstrike < V+ < Vsus + 47.6

instead of Vceo(max) in the place of 47.6.

As for the V+ supply if I were doing it I would end up designing something similar to those prefab modules that use a boost controller IC. So using a module is fine in my book.

[tkamiya]

My personal feel is that you have enough on your plate already.  I wouldn't be afraid to use a module for high voltage part.  Once you are building, you will have plenty of debugging to do. 

I experimented with Nixies in late 1970s.  Most tubes doesn't necessary require 170 volts.  120 volt transformer output into a simple rectifier will do.  Just don't use wall supply and rectify it.  Use a SMALL transformer and use current limiting.  I just don't recommend doing so much the first time.

[Mighty Burger]

Update! I fussed around in KiCad for a little while. This program takes some getting used to for sure.

Anyways, I made a schematic! I was wondering whether you guys could check my work and offer some advice.
It's slightly unfinished, first because there's certainly going to be some terribly incorrect things my very inexperienced self messed up, second because I couldn't find any footprints or symbols for an IN-14 Nixie for KiCad.
Here it is:


It's a lot to tackle so rather than just throwing it at you guys I'll explain my thought process with everything.



Oscillator Circuit
This is the section on the bottom.


- I took your guys's advice and used the CD4060 (thanks greenpossum and Gyro). I found a random schematic off Google Images (probably not a very good way to find a schematic, but it looked legit). I'm not entirely sure how the resistor values near the crystal were derived.

- The CD4060 divides the 32kHz signal down to 2Hz. The CD4027 takes that 2Hz signal and halves it to get that golden 1Hz.

- Looking on DigiKey, there seems to be an abundance of 32.768kHz crystals with a capacitance load of 12.5pF, I think I'll use one of these. So I would think I should use 25pF caps, but I looked at the CD4060 datasheet and there was an input capacitance listed. Should I take this into consideration when calculating these capacitance values? (I misread the datasheet as 5pF, it's actually 7.5pF so the caps should be only 10pF, that way the total capacitance would be 12.5pF.)

Should I throw in an adjustable cap?



Decade Counter
This is the middle part of the schematic. I did lots of crazy things here that I'm really not sure will work.


- The second digit of every section (hours, minutes and seconds) uses all digits zero through nine. When it counts past nine, it resets back to zero and then sends a clock signal to the next decade counter.

- The first digit of the minutes and seconds sections only use the digits zero through five. So, when these counters reach six, it has a wire running to it's reset pin to reset itself and it sends a signal to the next decade counter to increase by one.

- There's some funky stuff happening with the hours section. Here's why. I'm making a 12-Hour clock. Unlike the 24-hour clock, which goes from 0 to 23, the 12-hour clock starts at 1 and goes to 12 before turning back to 1. There's no such thing as "zero-o'clock" with 12-hour time. To tackle this issue, I just advanced the numbers by one. Let me explain.
With all the other CD4017 counters, Q0 controls the digit 0 for that Nixie Tube, Q1 controls digit 1, Q2 controls digit 2, and so on. But with U4 (the counter that controls the second digit in the hours), Q0 will actually control digit 1, Q1 controls 2, Q2 controls 3, etc.. and Q9 controls 0. This way, it starts out at one-o'clock and ends at twelve-o'clock.

- Once it reaches 12-o'clock I want the clock to reset. So, I made a simple AND gate with some NPN transistors. When the first hours digit is on 1 and the second hours digit turns to 3, it resets.



- I wanted to be able to adjust the time. So I added some things. First, I added a disable switch (SW3, underneath U14 on the far right). I want two things to happen when this switch is hit: 1) It stops the clock from counting up so I can adjust the time, and 2) It resets the seconds part to 00. I achieved this by sending the high voltage to the Enable pin of U14 and the Reset pin of both U14 and U12.

- To adjust time, I want to be able to press a button to increment the minutes or the hours by one. That's what SW1 and SW2 are for. I connected them to a little de-bouncer circuit using some resistors and capacitors, and added D2 and D3 to prevent the signal from being backfed to the previous counter's output.




Nixie Drivers
It's the top section.


I'm hoping this part is self-explanatory. Each of those pins on the different ICs will go to one of the cathode digits of the nixie tubes. I'm using the little trick with the zener diode to clamp the voltage and keep the magic smoke inside the IC packages (thanks greenpossum and pqass!). I'm hoping it'll work out with just one zener for everything (thanks for the advice Tim!). And crap, looking at it I just realized I put the zener in backwards!! oops!

Each nixie will receive 170V through a 20K resistor on each anode. (Just had a thought. Will I need a high-voltage resistor?) I'll go ahead and use a power supply module. Thank you tkamiya and greenpossum for advising me with this




I need to figure out how to put Nixie tubes in KiCad. I think at this point I'm just about ready to start troubleshooting.
Please let me know what you think of this circuit and if you see any issues

[Mighty Burger]

Fixed the zener guys don't worry

[schmitt trigger]

On the decade counters, you are using a pair of transistors as a discrete AND.
My only comment here is that running from 5 volts and with two Vbe drops, the reset pulse’s high level could be borderline to meet CMOS levels.

I could be wrong, but please check it with a scope.

[greenpossum]

- I took your guys's advice and used the CD4060 (thanks greenpossum and Gyro). I found a random schematic off Google Images (probably not a very good way to find a schematic, but it looked legit). I'm not entirely sure how the resistor values near the crystal were derived.

...

I need to figure out how to put Nixie tubes in KiCad. I think at this point I'm just about ready to start troubleshooting.

Have a look at https://hackaday.io/project/167443-crystal-tester where I discuss the values of the Rs and Cs around the crystal. I used 22pF myself. CD4060 datasheets are easy to find.

I think Kicad has a footprint wizard that can create a circle of pads.

As for the reset at 13 why not use CMOS logic to generate the reset signal? You just need a NAND gate (says he without looking too closely). Easier than futzing with resistors and transistors.

[Mighty Burger]

On the decade counters, you are using a pair of transistors as a discrete AND.
My only comment here is that running from 5 volts and with two Vbe drops, the reset pulse’s high level could be borderline to meet CMOS levels.

I could be wrong, but please check it with a scope.

Good point. I'll have to see if it works. If not, what are my options? Is there a transistor that has a lower voltage drop?

Now that I think about it. I'm using four different types of ICs. Three of them, the CD4060, CD4027 and CD4017 all seem to have a supply range of 3 to 18V according to the datasheet. (The fourth one, ULN2803, doesn't need a power supply). So maybe I can just switch the whole power rail to 6V. Would this damage anything? My one concern would be about the outputs from the decade counters (CD4017) going into the darlington arrays (ULN2803). The ULN2803 datasheet says it has a series base resistor for every darlington transistor pair that makes it directly compatible with 5V and 3.3V logic. Is 6V too much without adding any additional resistance?

Maybe I could just put it a notch above 5V. Maybe 5.3V.

[T3sl4co1l]

Q1-E can only pull up to ~4.3V, but Q2-E can pull up to V(Q1-E) - Vce(sat).  It's fine.

Regarding the zener, I wonder if it's worth biasing on slightly?  A 100k (or even more) from +HV to it would do.  This ensures its leakage current doesn't idle any segments.

Tim

[Mighty Burger]

- I took your guys's advice and used the CD4060 (thanks greenpossum and Gyro). I found a random schematic off Google Images (probably not a very good way to find a schematic, but it looked legit). I'm not entirely sure how the resistor values near the crystal were derived.

...

I need to figure out how to put Nixie tubes in KiCad. I think at this point I'm just about ready to start troubleshooting.

Have a look at https://hackaday.io/project/167443-crystal-tester where I discuss the values of the Rs and Cs around the crystal. I used 22pF myself. CD4060 datasheets are easy to find.

I think Kicad has a footprint wizard that can create a circle of pads.

As for the reset at 13 why not use CMOS logic to generate the reset signal? You just need a NAND gate (says he without looking too closely). Easier than futzing with resistors and transistors.

This was a helpful post. Thank you. Reading through it, seems like I should be OK with a plain 10M resistor (no need for this strange 13.6M value one). I'll experiment a little bit with the capacitance. I'll try out 10pF caps and 22pF caps, and maybe an adjustable cap, and I'll measure the frequency output and see how it changes.

I'll see if anyone else has made an IN-14 footprint, if not I'll go ahead and design one

For the reset-at-13 circuit I was hoping to use the most jellybean parts possible. (I think it would be an AND gate, if I'm not mistaken the reset pins are active high) Maybe it's better if I just go ahead and use CMOS logic, it is lower power after all

[Mighty Burger]

Q1-E can only pull up to ~4.3V, but Q2-E can pull up to V(Q1-E) - Vce(sat).  It's fine.

Thank you, I'm still learning about transistors so I wasn't aware of this

Regarding the zener, I wonder if it's worth biasing on slightly?  A 100k (or even more) from +HV to it would do.  This ensures its leakage current doesn't idle any segments.

You mean like this?
Excuse the crudity of the model 



Also, just had a thought. I don't have any bypass capacitors anywhere! Are they needed?

[T3sl4co1l]

Yes, and yes.  A few 0.1uF scattered around the circuit will do, particularly the 4060. Probably, power will be routed in a chain from chip to chip, don't worry about having a cap for each chip, a few can share.  Something to dampen medium frequency resonance, maybe 10-100uF electrolytic, at the inlet or the end of the chain.  CD4000 isn't fast, especially at 5V.

Tim

[schmitt trigger]

Thanks, Tim, he has explained correctly my concern.

But returning to your question about the supply voltage, since your are planning to use CD40xx devices, you do have significant leeway with the supply voltage.

[Mighty Burger]

Tadaa

Here's a semi-final schematic


Time to build this thing up!
I'm probably going to order both SMD and THT versions of these so I can test it out on the breadboard before I design a PCB and solder everything on. This is probably fine for everything but the crystal part, maybe I can just solder Y1, C4, C6, R9 and R11 in a skeleton shape dealio, because breadboard capacitance might mess things up? Or is 32kHz low enough where I won't have to worry about that?

Also I put C4 and C6 as "TBD" because I want to test out these capacitor values and see what works best.

I was able to make the symbol and the footprint for the IN-14 without any problems. Thanks greenpossum for letting me know about that circular wizard