Nixie Clock (non Microcontroller)

This project is a non-microcontroller 24 Hour Nixie Clock. The tubes used are Z573M for Hours and Minutes, and a NE-2 neon lamp for the seconds indication. Z573M tubes have the regular number 5, not the upside down 2.

Keep in mind, this circuit uses high voltage DC power, so understand and follow all safety precautions when dealing with high voltage DC power. If you are not comfortable around high voltage DC, this may not be the project for you, although I will detail steps when building so everything is done in a safe manner.

Full credit goes to Dan, danyk.cz for the design, although I modified the circuit slightly in regards to power. The original circuit used mains power (240 VAC 50 Hz) and a basic one diode half wave rectifier to get a pulsed DC voltage of about 185 volts, and there is no mains isolation. I opted to go with a 12 VDC to 230 VDC booster module, adjusted to 170 VDC and I will be powering the clock with a simple 12 VDC wall adapter.

This clock is built in two sections, the Clock Board (1), and the Nixie Boards (4). The nixie boards contain only 1 tube and 1 dual header (2x6) and they plug into the clock board.

A battery backup feature is in this clock, this does not charge the batteries, so regular AA batteries are used. The logic power is 5 volts DC, and is split into two circuits, the reason for this is when the 12 volts DC supply is turned off, the clock circuit will continue to function but the nixie tubes and their decoders will not be powered until the 12 volts DC supply is restored.

This project assumes you have knowledge of reading schematics and basic soldering skills.

Circuit/component descriptions will be described in the assembly steps.

Keep in mind, I made this project for my own use, I have encountered an error on the clock board where the clock does not count as it is designed, BUT I HAVE A WORK AROUND FOR THIS. On the bottom of the board, simply solder a short jumper wire from U5 pin 3 to U2 pin 1 and the clock will run as designed, however, this bypasses the time set buttons. When I power the clock the time always reads 1:38 (am) and starts counting from there, so to set the clock, I had to turn the clock on at 1:38 am, a minor inconvenience but worth it for me. If anyone wants to verify/update the schematic, feel free.

Parts Required:

Note 1. All resistors are 1/4 watt unless noted, 1% or 5% is fine as nothing is critical in the circuit.

Note 2. Header pins are all 2.54mm pitch, double and single rows will be stated. If you have colored pins, that is a bonus.

Clock Board, x1 (Gerber are available here on my proton drive and here on PCBWay.)

Nixie Board, x4 (Gerber available here on my proton drive and here on PCBWay.)

12v to 235v high voltage boost module, x1 (NCH6100HV)

Z573M nixie tubes, x4 (I bought mine on Ebay, NOS, which is New Old Stock)

NE-2 neon lamp, x1

IRF830 power MOSFET, x3

1N5819 Schottky diode, 8

DM74141N nixie tube driver, x3

SN74HC390 dual decade ripple counter, x3

CD4060 binary ripple counter, x1

LM7805 +5 volt regulator, x1

TO-220 heatsink plus screw, washer, and thermal pad, x1

16 pin DIP sockets, x7

32.768 kHz oscillator, x1

10 uF 25v electrolytic capacitor, x2

100 nF 50v disc capacitor, x9

1 nF 50v disc capacitor, x1

18 pF 50v disc capacitor, x2

10M resistor, x1

100K resistor, x1

10K resistor, x2

1K resistor, x1

470K resistor, x3

470K 1 watt resistor, x1

15K 1 watt resistor, x4

Tactile switch, 2 pin model, 6x6mm, SPST NO, x2

Header pins, double row male, 2x6, x4

Header pins, single row male, 1x4, x1

Header pins, single row male, 1x1, x4

Header pins, double row female, 2x6, x4

Test pins, I use colored bead type pins that you connect to with a hook probe, red x2, black x1, white x1

Battery holder for 3x AA batteries, x1

PCB mount screw terminal block, 2 pole, x1

Jumper wires (22 AWG) with female Dupont connectors, as well as some insulated wire, solid and stranded, with no connectors.

Nylon Standoffs and Screws, I used M3x6, x6, (3mm screw size, 6mm length standoffs) for the bottom of the clock board plus 3mm pan head screws, x4. To mount the boost module to the clock board I used M2 screws, x2 and some 2mm nuts. To mount the battery holder to the clock board I used M3, x2, countersunk screws.

Nylon Spacers, I used M3x11, used to support the nixie boards, plus super glue as needed. 11 is the spacer length, and is the distance from the clock board up to the bottom of the nixie board.

Display case, I used a store bought clear acrylic case that is secured closed with 2 screws, remember, there is a 170 volt DC potential on the board so you do not want anyone to access the board at anytime when it is powered on.

Tools:

Schematic, you can download those here on my proton drive or here on PCBWay.

Soldering iron and solder, and for when mistakes are made, liquid flux and solder wick and/or a solder extractor.

Flush cutters, for trimming leads.

Electrical tape, used for holding components in place while soldering.

Breadboard, or Pin Header Soldering Tool, used when soldering in header pins.

Helping hands, the small device with 2 alligator clips on flexible arms, useful to hold the small nixie board.

Needle nose pliers, used for straightening IC leads.

Round nose pliers, used for bending the 1 watt resistor leads.

Small Philips screw driver, for attaching the voltage regulator to the heatsink and for tightening screws on the screw terminal block.

Small ceramic or plastic standard screwdriver (a tweaker (trim pot adjustment tool) would be ideal), used for adjusting the booster module, important!

Drill (cordless is ideal) and various bits, used for drilling holes in the clock board as well as the display case.

Combination square.

Super Glue.

22 ga solid wire, black preferred, the insulation will be used to cover the neon bulb leads.

Isopropyl alcohol and a toothbrush, for cleaning the boards when soldering is completed.

Multimeter with hook probes, used for basic power checks and high voltage adjustment, important!

First thing to get out of the way is drilling the mounting holes for the battery holder. There is many versions of the battery holder so I did not have the holes drilled when the boards were manufactured, but this is very easy. This part will not be installed yet, we just need to get the holes drilled so when the time comes, the part will be installed.

Place a blank clock board on your workbench, place the battery holder over the battery holder silkscreen, do not cover the two holes on the right side of the battery holder silkscreen, those are where the power and ground wires go eventually. Use a pencil and mark the mounting holes in the battery holder.

Remove the battery holder and place the board on a piece of scrap wood to prevent drilling into your workbench, drill the two holes using a 3.5mm bit, I use a Star-M self centering bit since I am using 3mm screws to mount the holder. Clean off any residue from the drilling process.

First photo shows the completed mounting holes.

Second photo shows exactly where the battery holder will be installed. Now you can set the battery holder aside for now, it will get mounted near the end of the assembly.

Normally I like to add parts from lowest profile to tallest, but for male header pins, I like to get them done when the board has no other components on it, much easier that way.

Easiest way to solder in male headers is to use a breadboard to hold the headers, or as I am going to do, I will be using my pin header soldering tool.

For H1, I used a 1x2 yellow header for the +12 volts and a 1x2 black header for the ground. Feel free to use an all black 1x4 header or color of your preference.

Place the H1 header pins in your breadboard or header soldering tool, with the pins downward, turn the clock board over and position over the H1 pins, support the board with a strip of header pins on its side under the board on the breadboard or header soldering tool. Solder the pins in place.

Add standoffs in the 4 corners of the board, I used nylon M3x6mm standoffs with 3mm screws.

Functions:

H1, is main power, which is 12 volts DC from a wall adapter power supply. There are 4 pins, pins 1 and 2 are connected together and is for +12 volts DC, pins 3 and 4 are connected together and are for ground. The 12 volt DC power supply (wall adapter), through a connecter in the case, will connect to pin 1 with a female Dupont connector and ground will connect the same way to pin 3. Pin 2 will connect +12 volts DC to the booster module, and pin 4 will connect ground to the booster module.

Now we are going to add the low profile components, the 1/4 watt resistors and the Schottky diodes. Schottky diodes are fast switching and low power.

This is straightforward. Using the schematics, place a resistor as stated, tape in place to hold in position, solder in place, and trim leads. Put a pencil mark on that component on the schematic to mark off what you have completed, repeat this with the rest of the resistors, then repeat the process with the diodes, paying attention to the polarity of the diodes when installing them.

Functions:

R1 and R2, are part of the timing circuit for the 32.768 kHz oscillator.

R3, R4, and R5, are part of the timing output circuit on U5.

R11, R12, and R13, are part of the logic for resetting the number count.

D1, prevents current flow into the backup battery when main power is on yet allows the battery to power logic in the clock when main power (+12 volts DC is removed).

D12, prevents current flow from the battery backup in to the 5 volt regulator.

D2 to D7, makes up some of the logic, in conjunction with R11 to R13, for resetting the number count.

Now we are going to install the disc capacitors (I use mostly monolithic), the 1 watt resistors for use with the nixies and neon bulb, and the test points.

Using the schematic, install a disc capacitor, hold in place with tape, solder, trim leads, mark off what you have done, repeat with the next.

Install the 1 watt resistors, then the test points.

Functions:

C3 and C4, are part of the filtering for the 5 volt regulator.

C5, is decoupling for U5.

C6 and C7, are part of the timing circuit for the oscillator.

C8, is debounce for SW1 and SW2 (the minutes and hours switches).

C9 to C14, are decoupling for U2, U3, U7 to U10.

R6, R7, R8, and R9, are current limiting series resistors for the 4 nixie tubes.

R10, is the current limiting resistor for the NE-2 neon bulb, which is the seconds indicator.

TP1, is ground, marked as GND, and is a black bead if using colored test points.

TP2, is 170 volts DC, marked as 170V, and is a white test point.

TP3-A, is 5 volts DC, marked as 5V, and is a red test point. This test point is connected directly to the output of the 5 volt regulator and is +5Va on the schematic, this powers U3, U7, and U9, which are the nixie decoder ICs. When the clock main power is disconnected, this will drop to 0 volts.

TP4-B, is 5 volts DC, marked as 5V, and is a red test point. This test point is connected directly after D1 and D12, and is +5Vb on the schematic. This powers the master clock (2 Hz) and the decade counters. When the clock main power is disconnected, and you have the batteries installed, this will read about 4.5 volts and the clock will continue to operate, you just will not see the clock digits until the clock main power (12 volts) is restored.

Installing the 16 pin DIP sockets and female headers is easy to do. I used machined pin sockets as personal preference, but regular sockets will work just as well.

Keep in mind the silkscreen for U3, U7, and U9 only show the dot for pin 1, not the notch, so when installing ensure you install the sockets in the correct direction with the notch on the socket at the end with the dot. U2, U5, U8, and U10 indicate pin 1 with the notch and a dot.

When installing a socket, insert in the correct orientation and tape in place, solder only 2 pins on opposite corners, remove the tape and check to see if the socket is flat on the board, if it is, solder the the other 2 opposite corners, then the remaining pins.

If the socket is not flat, hold the board on its side, and press on the socket with a finger (not over a pin in either corner), and reheat the solder joint, and you may hear the socket click into place when the solder flows, repeat with the other corner if needed, then after it is flat on the board, solder the 2 other opposite corners then the rest of the pins. Repeat with remaining sockets.

The 2x6 female headers are H4 to H7. Install similar to a DIP socket, using the opposite corner method and reheat if you need to get them flat on the board, then solder the remaining pins.

Functions:

U5, is a CD4060 CMOS 14-Stage Ripple-Carry Binary Counter/Divider. Using X1 which is a 32.768 kHz oscillator as the clock input on pin 11, we are going to take the 14th stage output on pin 3, which divides the oscillator frequency by 16,384 for an output at 2 Hz, which is now our Master Clock.

U2, U8, and U10, are SN74HC390N Dual 4 bit Counter/Dividers, with each divider consisting of a divide by 2 and a divide by 5, and they can be used separately. We are going to take our 2 Hz Master clock on the clock input pin 1 of U2, then take the first output of divide by 2, on pin 3, to get our 1 Hz clock, and send that over to U10 pin 15 for the clock input on the minutes counter, and that also flashes the NE-2 neon bulb to indicate our seconds (heartbeat). U10 is minutes 1s, U8 is minutes 10s, and U10 hours 1s. Hours 10s does not need a decoder as we are only going to use numerals 1 and 2 for the hours 10s so we are going to drive those directly from the first divider, U2.

U3, U7, and U9, are DM74141N nixie drivers, these decode BCD from the counters to base 10, and they are intended for direct nixie driving. Their outputs are open collector NPN transistors with 60 volt Zener diode protection.

H4 to H7, are the female headers and provide power to the nixies and logic from U3, U7, and U9. Pin 1 is marked for the starting point reference, and there is an A marked, that is the anode, or the 170 volts DC sitting on the nixie.

Now we are going to fill in the rest of the parts, except N2 which is the neon bulb for seconds and will be installed in Step 8.

Install U1, U11, and U12, these are the power MOSFETS.

Then install X1, the pads for this component are very close so pay attention when soldering that part in.

Now install U6, which is the 2 pole screw terminal.

Then you can install the two electrolytic capacitors, C1 and C2, then assemble U4, the 7805 voltage regulator, with a heatsink, insulating washer, thermal pad and screw, then solder that part in place.

Install the two tactile switches.

To install the battery holder, BT1, trim the leads to an appropriate amount, strip the end of the wires, and tin them. Insert the leads into the correct holes, tape the wires and holder in place, then turn over and solder the wires in place. Now you can use two countersunk nylon screws and insert through the holder and mounting holes you previously drilled, and screw on the same length standoffs as you used on the four corners of the board.

The soldering, except for N2, is complete, so at this time you can clean the back of the board. I normally spray with isopropyl alcohol, scrub with a toothbrush, spray again, and let every thing drip off and allow to dry thoroughly.

After the board is completely dry, now you can install the booster module, simply insert nylon screws through the module and through the clock board, secure in place with nylon nuts.

Install the ICs, paying attention to the notch on the IC same orientation as the notch on the sockets.

Functions:

U1, is an IRF830 power MOSFET and is used to switch on and off the N2 to flash for the seconds indicator.

U11 and U12, are also IRF830 MOSFETS, they are used to enable hours 10s, 1 and 2 only, they are driven directly from U2, no nixie driver needed.

X1, the 32.768 kHz oscillator that will provide the clock to U5, U5 will reduce that down to 2 Hz.

U6, is a 2 pole screw terminal, I am using this to apply the 170 volts DC to board.

C1 and C2, are filter capacitors for the 5 volt regulator, with C1 is on the 5 volt DC output side and C2 is on the 12 volt DC input side.

U4, is the 5 volt DC regulator, the output is split into 2 sections, +5Va and +5Vb on the schematics. The +5Va powers the nixie drivers, U3, U7, and U9, and when main power is lost (12 volts DC), the nixie drivers stop working along with the 170 volts DC, so the nixies will not light. The +5Vb powers the master clock and the counters, when main power is lost, the master clock and counters will continue to work using the backup battery.

BT1, is the 3 AA battery holder, which provides 4.5 volts when the 12 volt main supply is lost, like during a power outage. Regular batteries can be used, as this is not a charging circuit for these batteries, they are just providing a backup source. Schottky diode D1 prevents current flow to the battery, and in the event of a power loss, D1 will allow power to the logic circuit of the clock while the nixie drivers and tubes will be off.

Booster Module, provides the 170 volts DC to power the nixie tubes. This module is powered by the 12 volt DC primary power. The module is adjustable, mine 85 to 235 volts DC and will need to adjusted prior to using the nixie tubes, this will be shown as well.

SW1, is used to advance Minutes when setting the time, pressing this increases the clock rate.

SW2, is used to advance Hours when setting the time, pressing this increases the clock rate.

N2, is a neon bulb used as the seconds indicator, this is pulsed by the 1 Hz clock from U2 pin 3.

This step describes assembling a nixie module, of which you will need 4 of these. The parts needed are 4 nixie module boards, 4 2x6 male pin headers, and 4 Z573M nixie tubes. A breadboard or Pin Header Soldering Tool is needed to solder in the headers.

First thing is to solder in the headers. Place a set of headers, male pin side into your breadboard or soldering tool, place an extra row of single pin headers on the breadboard or tool on its side, then place the board, with the words HEADER THIS SIDE, facing down, as shown in Photo 2 and I place a large nut on it to hold it in place. Solder 2 pins in opposite corners, just like you do with DIP sockets, check to ensure the header row is flat and square to the board, then solder the other 2 corners, then the remaining pins. Repeat to complete 3 more boards.

Photo 3 shows the pinout of the Z573M nixie tube as viewed from the bottom of the tube. Note the gap between pins 1 and 13, they are not marked on the tube, you have to tell the position from the gap. Pin 1 is cathode 1, or the number 1, the rest of the pins and the numbers the correspond with are shown in the table next to the pin out diagram. Pin 13 is anode, and will be your 170 volts DC. Pins 4 and 10 are NC (not connected) so you can snip those off if you wish from your tubes. Pin 7 is the front of the tube and is the viewing direction.

Now to get the leads into the board, an easy method (applies to NOS as the leads are long) is to start with lead 1, leave that lead as is, lead 2 cut about 2mm from it, next lead cut off about 2mm, and continue, making each each lead about 2mm shorter, as shown in Photo 4.

Then take lead 1 and insert into hole 1 on the side of the board that says TUBE THIS SIDE (as marked on the board, then you just move the tube and line up lead 2 and insert into hole 2, and so on, skipping holes 4 and 10 if you clipped off 4 and 10 before starting, until you have all the leads in the holes. When you look at the bottom of the board, count the leads from the marked 1 to lead 7, which is the front of the board, marked with an arrow.

For soldering, I have a spool of solder with paper over each side of the center of the spool, I poked a hole in one side and could insert the nixie into that to so the board is facing upwards. Solder 2 opposite leads, then check to see if the tube is square to the board, if not, adjust the solder on either leg to square it up, then solder another lead in between the soldered leads then check square again, and adjust as needed. When you have it square, trim the soldered leads and solder a few more leads, trim those, and repeat until all the leads are soldered in place (and of course, skip holes 4 and 10 as there are no leads if you already trimmed those out of the way). Repeat for the remaining boards and tubes until you have 4 completed modules as shown in Photo 6.

Turn a tube module upside down so the bottom of the board is facing upwards. Apply a small dot of super glue in the center of the circle of pads and place a M3x11 spacer on the glue, as shown in Photo 7. Repeat for the remaining modules. The spacer keeps the module from sagging.

Insert each nixie module into the female headers on the clock board as shown in Photo 1.

Then, without clipping the leads on the neon bulb, just set that into the holes for N2, mine was right about in the middle of the tubes on either side of it. Then I measured the length from the top of the clock board to the bottom of the neon bulb, my measurement was 23mm.

I then took a piece of black solid core 22 AWG wire and stripped a two lengths of insulation from the wire, cut the two pieces of insulation the same length of 23mm, which is the distance between the top of the clock board and the bottom of the neon bulb. Insert each lead on the bulb into a piece of insulation.

Remove the nixie modules from the clock board, insert the neon bulb into N2, as shown in Photo 2. Now using a spare hook probe, grab one of the leads under the board to hold the bulb in place, then turn the board over and prop up the side with the neon bulb, I used a roll of solder to do this, as shown in Photo 3. Solder the lead with no hook probe, then remove the probe and solder the other lead, trim the excess leads.

Turn the board over and reinsert the nixie modules, as shown in Photo 4, now you have the neon bulb soldered in place and the leads insulated.

This is a simple process to make the connections, colored wire is ideal but not necessary. I used a yellow jumper wire, plugged into one of the + pins on H1, run to the left of the battery holder, across the top of that, then cut the wire to length and strip the end, and secure in the VIN on the input side of your voltage booster module, yours may not look exactly like mine but these are relatively generic devices that operate in similar fashion. Mine has tabs that are pressed in order to insert the wire, others may have screw terminals, other may have pins.

Repeat the same with another wire, I used green, connected to one of the - pins on H1 and run the same way as the yellow wire, and connect that to GND on the input side of the module.

The SHDN terminal is left disconnected for the booster always on, that is a terminal used to control the booster using digital logic from a MCU.

The output side of the booster module is going to connect to U6, which is a 2 pole screw terminal.

I used a white wire from the HV terminal on the output side to the + terminal on U6, and a black wire from the GND to the ground terminal on U6.

Resistance Checks:

Remove the nixie modules from the clock board.

Using a multimeter, set to Ohms and a mid range scale, like 20K, turn it on, and connect the ground lead to the GND test point. All we are going to do here is verify the board is assembled properly to ensure nothing is shorted from ground to any power rails.

Now connect the meter red lead to TP2 (170V), meter should indicate an open, then connect to one of the 5V test points, meter should indicate an open, then check the other 5V test point, meter should indicate an open.

Testing headers H4 to H7. Leave the ground lead of the meter connected to GND test point. Now using a jumper wire with female and male Dupont connectors, (or a piece of male header pin), insert that into the pin marked A on H4, connect the red lead of the meter to the jumper or header pin, meter should indicate open. Repeat for H5, H6, and H7.

Now move the ground lead of the meter to the 170V test point, then rechecking pin A on H4 to H7, the reading should be 0 ohms. You can also check the A pin on N2 as well from the bottom board, which will also read 0 ohms. Remove the jumper wire or header you used.

Now connect the ground lead from the meter to the GND test point, then connect the meter red lead to one of the - (minus) pins on H1, marked as 12VDC, reading should be 0 ohms. Then check the other - pin for the same reading of 0 ohms. Now connect the red lead to one of the + (positive) pins, reading should show an open, and check the other + pin for the same reading.

Power Check: (This involves turning the clock board on, pay attention to where you are connecting probes.)

First, ensure all nixie modules are removed from the clock board.

Using a multimeter, connect the black lead to the GND test point, and connect the red lead to either 5V test point. Set the meter to DC Volts and set to the 20 volt range

Connect a +12 volt DC power supply (turned off at this point) to the + and - pins on H1.

Turn on the power supply and read the meter, it should be reading 5 volts. Turn the power supply off and wait a minute or two, this is to allow power on the HV booster module to return to zero.

HV Power Adjustment (170 volts DC):

For this you will need a small ceramic or plastic standard screwdriver, a tweaker (trim pot adjustment tool) is perfect, that will fit the slot on the trim pot on the booster module. Do NOT use an all metal jewelers screwdriver!

With power still off, connect the meter red lead to the 170V test point, black lead should still be on the GND test point. Set the meter to DC Volts and a range of 200 volts. No connecting or disconnecting the probes from GND or 170 volts when power is on to the clock.

Turn the 12 volt power supply on and read the meter, you are looking for a reading of 170 volts. Using one hand only, carefully use the ceramic or plastic screwdriver and slowly adjust the trim pot on the booster module until you get a reading of 170 volts. The neon lamp should be flashing at this point.

Turn off the 12 volt power supply and wait for the meter to read 0 volts before touching anything on the board, keep the meter connected to the ground and 170 volt test points.

Turn off the 12 volt supply and add the nixie modules.

Now turn on the 12 volt supply, you are looking for the nixies to light and for the neon lamp to flash at a 1 Hz rate, the heart beat.

*** Here is the caveat if there is no heart beat on the neon lamp (the lamp is not flashing). ***

On the bottom of the board, solder a short length of jumper wire between U5 pin 3 and U2 pin 1. This will enable the clock to run but disables the time set buttons.

To set the time, write down the time displayed when you turn on the clock and the neon lamp is flashing. Do this several times, mine is always 1:38 (am) so I have to turn the clock on then. After the clock is running when you "set" it, now install the batteries, then if you have a short power outage, the time will remain counting but the nixies will be out, when power comes back on, the nixies will light and will show the correct time.

I used a acrylic case I bought, I drilled three 7.5mm holes along the top back of the case, for heat to escape, and drilled a hole in the side and mounted a panel mount DC jack. Connected the jack to some jumper wire and those wires have Dupont connecters on the other ends and those plug into H1

How you want to display the clock is totally up to you. Main thing to keep in mind is you want an enclose to keep anyone from touching the 170 volt DC power.