It has been more than a year since I posted an instructable. I finished high school, got into college to study Electrical Engineering and now with my first year completed and much deserved holidays, I will write a few instructables from projects that I have been making over the year.
I know that Nixie clocks are overdone and projects making them are being covered to death. But with that in mind, I searched for kits, pcbs and anything to assemble one as when I found out about Nixie tubes, I ordered some. I had to make a power supply to make them work, so as impatient as I am, I couldn’t order a kit from china. Then I needed to make the drivers for each Nixie and sooner or later I decided upon making a clock from scratch.
Note, this clock is not the most accurate clock on the world as it doesn’t synch from internet, that can be an addition. So, if you want a good-looking Nixie clock, you are in the wrong place. But if you want to know how to make one from scratch, then keep on reading.
If you want to learn how everything works and understand and upgrade yourself, I urge you to read everything as this is the knowledge I acquired and am sharing it with everyone. Be sure to read on wiki or other sources if you want a more details. The point of this one is to also explain how every part works and how to figure that out with datasheets, so if you want to make this and modify and understand, make sure you read it all (I know it might be a lot).
Also note, that this Instructable isn't step by step how to solder every component. I expect for readers and potential makers to know how to read a schematic along with pictures. It is not complicated and can be simply done on protoboard just like me. I also suggest you to build separate sections of this circuit, which are each individual step, like oscillator, HV power supply, digit drivers and the minute counter. You will learn about different parts better.
Step 1: About Nixies
Now, working with old stuff has mostly in common, that everything has high voltage. From old CRT TVs, test equipment and Nixies. Nixies are cousins of vacuum tubes filled with Neon. They are cold cathode(katode) type as vacuum tubes, they don’t need a heater. They only need high voltage about 170V and around 1-3mA to work. Driving will be described later.
With Nixies you apply high DC voltage to the anode (+) and cathode (-). Anode is the wire mesh in front of the tube. It ensures, that electric field is as equal as much as it can be between it and the letters. Letters are cathodes and must be pulled to ground to make them glow. By design, the small and sharp surface causes electric field around it to be very large, as the electric flux is constant, the electric field gets bigger with smaller surface. That way it excites the gas around it and make it glow. I mentioned that the anode is a large mesh to ensure even electric field around it. If you apply too much voltage or no current limit, you might see random glowing patterns on the mesh. So, if you see that, turn it off!
Same way you can get IC adapters and IC connectors, so you don’t solder them to PCB directly, you can buy adapters for your Nixie tubes for ease of replacement and testing. These Nixie tubes that I have were quite cheap, about 10€ for 6 of them. They are called IN-12a/b.
Step 2: Power Supply
I made my own power supply. It is very inefficient and produces a lot of heat. I am looking into the design and choice of components. It is based around MC34063 IC. It is configured to have an external transistor to handle switching of 170V. Apart from that, it’s a standard boost converter schematic. Observe the schematic and make sure you buy parts that are at the same voltage and current rating or greater.
I need to warn again. This power supply can produce to 300V DC and maybe more if no feedback is connected or not trimmed right. Be very careful and buy component that can withstand that if you plan on experimenting and testing to ensure no component failure like output capacitors, transistors or diodes. The will likely not kill you, but it will give you a big or a surprise shock (these are worse) when testing so be sure to disconnect power when changing components and wires and have everything clear when powering on.
Power supply runs on about 12V DC in and for that use a pre-made mains AC/DC converter to ensure no fatal voltages present in the circuit.
Also, this power supply can have current limit on the input. This must be set so that your supply work fine in normal operation. Mine is very inefficient and has to have such current limit, that if shorted, the rectifier diode blows. That's how I changed more than 5 when testing.
Also make sure that the transistor has as low gate capacitance and ON resistance to minimize switching loss. As well make sure to get a diode with Reverse recovery time lower than 50ns and peak current of at least 1A. Standard among schematics are BAV21 and IRF740 as they are rather inexpensive and often available. If I get my hands on other components that provide better performance, I will share that in the comments.
When selecting an inductor make sure to not use torodial type. They are good for suppressing high frequencies and therefore are used at the output of a power supply. For our main inductor that induces high voltage, we need one designed for high switching frequencies. Look for open styles like the one in the picture. They also leak more electromagnetic radiation, but you can find shielded ones as well for a small premium.
As I said, I will test this supply with different type and size of inductor, output capacitor and switching transistor. For now, I have what was suitable around the workshop and what the schematic required. I will post update in the comments.
Step 3: Backup Power
I also implemented a crude backup power. I had a 5F, 5.5V super capacitor laying around, so I integrated it into the circuit to keep on powering the system when mains power is disconnected. This is useful if you move the clock or the power cuts out for a short period. It will power the counters and oscillator for half hour at least.
Step 4: Clock and Timing
To keep track of time, we must first create a stable timer. a popular timer for clocks is frequency of 32.768kHz. This frequency is used in all real time clock (RTC) application. Why? Because it is 2^15. So if you divide the frequency by two 15 times, then you get a perfect 1Hz. Also note, do not use 555 timers for any critical time keeping applications as they are not accurate.
If you want, you can buy premade 32.768kHz oscillators in these square tin cans, looking like bigger crystals, but as I said in the beginning, I want to make everything. A Duck search later, you find this schematic.
This is a crystal oscillator that uses two inverter gates found in inverter ICs. Most comon As I said, I will use only digital logic ICs to learn about them and see how hard can it be to figure out the schematic by myself. So, the schematic above provides 32.768kHz square clock on the output. The second inverter can be avoided, but it provides a nicer and faster rising and falling signal as each inverter has input Schmitt trigger.
Step 5: Alternative Oscillator
If you observed the datasheet for CD4060, then you have seen the input stage to the counter. We are using ΦI which is an input signal. There is also _Φ0 pin that goes after the input not gate. You can see that the input only has this configurable not gate to control signal with reset. So we can just use this not gate to make the same oscillator. If you scroll down on the datasheet, you can also see a schematic for the same oscillator. So, if you want to optimize your design and loose an IC, go ahead. I only saw this later on a forum and then I went to the datasheet again and saw it, so I’m sharing it now with you.
Step 6: Understanding Clock Dividing
To understand how clock dividing stages work, we must look at the selections of ICs for the job. I had a small knowledge of these ICs and only knew what they could do and that way I combined them to get desired clock signals. I think the clock oscillator is self-explanatory, so I won’t discuss it.
First stage shaves off most of the frequency. I mentioned that 32.768kHz frequency is widely used as it is exactly 2^15 and we can use any simple logic or micro controller to get 1Hz. So, to divide it as close to 1Hz as we can, we can use CD4060 14-bit binary counter. 14 bit represents 2^14 and counter means that it can count up to that. So, if it counts to 2^14, how many loops will it do with our frequency of 2^15? Exactly 2 as it is the difference of 2^1. So, our output of the maximum part of the counter is our 2Hz. That signal comes out on pin 3. Do not forget to tie RESET to ground.
Now that we have the most of division done, we need to get to 1/60Hz. As we divide more and more, we can also get 1Hz to light up an LED, so we can see that the counter is working. To do that, we can use CD74HC390 which is a dual decade counter. It has two x2 and x5 counters. We can connect them together to get all sorts of divisions from this IC, ranging from 2 to 100. We will need to first enter a x2 counter to get our 1Hz signal. We can then take this signal to another x2 counter and get 1/2Hz signal and then take it to a x5 counter to get 1/10Hz. This is very simple. You can these inside counters in any order. I can also mention that it can be easier to debug if you wire counters, so it counts from highest to lowest, so x2^14 is first, then x6, then x5 and then two x2 so you get smaller counters as the signal gets slower. I am not aware if this or inverse way affects signal but in my case is mixed in the order I present them. Don’t forget to tie both MR1 and MR2 to ground for IC to count.
Last, we need to get from 1/10Hz to 1/60Hz. To do that, we need to divide by 6. It cannot be done with any of previous used ICs, but we can use same IC that we will use for driving Nixie tubes and keeping time. It’s the 74HC4017 which is a decade counter with all 10 stages broken out on pins. That way, we can tap off at any point of counting. To divide by 6, we can wire 1/10Hz signal into its input and wire “6th” stage to reset and use it as our output signal. This way, every time the IC counts to 6, it also resets itself and sends out a signal that we can use as our minute signal. Don’t forget to tie MR and CE pins to ground for IC to count.
The illustrations only serve to simplify the explanation. See the schematic for pins and connections, such as reset pins, etc.
Step 7: Time Keeping
Nixies are driven by 74HC4017 ICs. This IC is a decade counter with each stage having its output. That way it counts is from 0 to 9. This is just what we need for our Nixies to display numbers from 0 to 9. We will also configure the pins to have them display 24hr time format. Each stage will be show in the picture.
First digit is the minute digit. This one will go from 0 to 9, so we can leave everything as it is. We pull the RESET and CE to ground for normal operation. Now, the secret ingredient is this pin called TC. It is an output pin, that goes high every decade cycle, so when a count from 0 to 9 completes, pin goes high and after count 4, it goes low. We can use this pin on the next digit signal input.
Second digit gets a signal input every 10 minutes. But as we know, the minutes go only to 59 and then complete an hour. For that, we need to reset second digit when it wants to go from 59 to 60. That way we wire pin for digit “6” to reset in order to reset it when it reaches number 6. No need to worry for digit 1 as it will already resets itself when going from 59 to 60. We again use its TC pin to advance next digit.
Third digit is used for the first hour digit. It will also count from 0 to 9 so for now, we can leave it to count. Its [carry out] can be wired to fourth digit clock input. But, as we know, the first hour digit goes 0-9 (00-09), 0-9 (10-19) and 0-3 (20-23). So, we need to reset it when it reaches the third number 4. To do that, we will help ourselves with fourth digit.
Fourth digit is used as second hour digit. It goes only from 0-2 so we need to reset it on 2. But as I mentioned in the third digit, we will use this digit to reset itself and the third digit. To reset both digits on a special number (not auto reset on 9 to 0), both need to get to a combination of 2 for fourth digit and 4 for third digit. For this, we can use AND gate. And gat will have its output C HIGH only, if both inputs A and B and HIGH. But I didn’t think about this and didn’t have any AND gate ICs around. They are cheap, so I advise you to just purchase one and have them on reserve. But I wanted to test it and to not waste room for a huge IC to only use one AND gate (as they come with 4 or more like our 74HC04). I found this schematic with just two diodes and a resistor and it acts just as an AND gate.
Now, with our makeshift AND gate, we can wire pin “2” of fourth digit and pin “4” to AND gate inputs. AND gate output will be connected to both digit counters to reset them both at once. Also, there is pulldown resistor on both resets to ensure normal operation.
Step 8: Output Driving
Now we discussed how these counters will keep time. I said, that these are also used to drive Nixie tubes. But Nixies will have a large voltage that would kill outputs of the counters. That way we need external transistors. I used MPSA42 as they can handle this high voltage and are very cheap and available everywhere. You can use any high voltage transistor. The roll of these transistor is to pull every cathode / number to ground. The following schematic shows how these are connected. Do this identically for every digit. It's long and boring process that requires lots of attention and time.
Step 9: Extras
If you made all of these sections and connected them together, you have a working Nixie clock (timer). Here are all of the schematics and documents that you will need.
Also, if you see on schematic, there is a diode from M1 to M2 and from M2 to H2. And from M2 and H1 there is a line going to a push button. It advances the count manually to set the time. Just note that you cannot set time for M2 when M1 is 0-4 because of TC pin properties. Read the datasheet to see why. Also I wanted it to be simple and with as less components as possible.
If you have any questions about stuff I didn't clarify or in case I missed something, please point it out in the comments or send me a message.
Thank you for viewing this instructable. Make sure to check out my other instructables about electronics.