Analog IN-13 Bargraph Nixie Tube Thermometer

This is a project for a Russian IN-13 bargraph Nixie tube to use it as an indoor room thermometer.
It is named "NixieTherm" and is also available as a fully complete kit incl. enclosure as shown at www.Nixiekits.eu

The IN-13 is a special construct of a gas discharge Neon display and works similar to the well know Neon bulb in illuminated mains power switches or as Nixie tubes. But this bargraph has a current depending legth of the glow.
As all other cold discharge tubes also the IN-13 needs a "little bit more" high voltage to work; at least 120VDC. The current through the tube must be limited, normally with a resistor. In the NixieTherm this is done with a high voltage transistor, as we need a variable current from 0....4.5mA.

Let's start with the description of the circuity for this thermometer:

Step 1: Circuit Description

Let's first make an overview.
On the top of the schematic the complete voltage supply stages are drawn. On the buttom of the schematic the analogue circuity for the temperature measurement is drawn incl. the variable current source for the IN-13.

As you see, the circuit is powered from 5V through an USB connector. This is possible, as the max. power drawn from the supply is less than 2 watts.
After C9, L1 and C15 which filters glitches from the step-up converter the input voltage is first feeded to a TC962. This is a high current capacitor charge pump and is able to deliver 80 mA max. - the well known ICL660 will not work proper in this circuit!
The TC962 is connected as a voltage doubler and as a simple voltage inverter, so on its "outputs" there are two voltages: +9.5V and -4.5V.
The -4.5V is only used as a negative supply voltage for the operational amplifier IC2, so there's no need no explain any more.
Now we take a closer look to the doubled input voltage:
The parts around the IC4 build a step-up converter ciruity modified from the MC34063 application manual.
First you see the driver stage T3/T4 on its output. This is necessary as we need to convert 5V to 125V and therefore the MosFet transistor T2 needs to switch as fast as possible (very low time for rising and falling edges of the square wave on its gate to get the maximum self-induction voltage from the inductor L2). This can not obtained by the internal MC34063A output stage as it is only a darlington source stage and can not sink current.
I've tried to measure the rising and falling time of the driver stage with my digital oscilloscope but the measuring limit was 40ns... and it will switch faster.
Don't try to substitude the BC639/649 wirth standard NPN/PNP transistors like MPSA42/92 for example. This will not work as they aren't able to drive the excess peak current needed to discharge the MosFet's gate ;-)

Maybe your're wondering why the MC34063A and the driver stage is connected to the 9.5V. This is necessary as the needed voltage swing for the MosFet to completely turn them on for an RDSon value of less than 1 ohm is around 6V.
Finally the output voltage of the HV converter is limited with the voltage devider R20/R21 to the nominal 125VDC.

The 9.5V is also connected via R16 and R17 to a 2 x 3 LED string. These LEDs are 3mm RGB slow colour changing types and are used to illuminate the thermometer's scale like an edgelit display. As the colour changing is provided with the internal chips of the LEDs also with PWM, the six electrolythic capacitors are needed to avoid flickering. A nice feature - not a bug - can sometimes happen when the thermometer is turned on and all three LEDs of a string will start with a "zero PWM". Than no current is present in the string and it needs some seconds before turning on - like to an old tube radio which also will not work immediately after turning on. If you don't like this effect simply connect a 10kohm resistor in parallel to every LED.

Now have a short look to the temperature measuring ciruity.
The sensor, a LM35DZ, has an output voltage of 10mV/°C, so normally around 250mV. As this is a little bit low for the following variable current stage this "ULM35" is amplified by IC2A 18-fold. The exact amplificationfactor can adjusted with TR1.
ZD1 is a variable constant voltage reference (adjustable with TR2) and forms an extra voltage which is finally "substracted" from the amplified "UTEMP". This is needed as the scale of the thermometer will start at 10°C; and at this is also the threshold point of starting current flow through the IN-13 bargraph tube; therefore we need to substract: 10mV * 10°C * 18-fold amplification = around 1.8V from the "temperature". This is provided by IC2B, which is a difference amplifier with a variable output current together with the HV transistor MJE340 (not voltage) as the feedback resistor R12 is connected to the current measuring resistors R13 and R14.

Note the capacitor C16, connected to the non inverting input of IC2B. This limits the rising time for the current through the tube. Without this capacitor the Neon glow will start after turning on not from the bottom of the tube as the slope will exceed the IN-13 maximum.

The solder bridges X1/Y1/Z1 are necessary to "burn-in" the tubes. As all tubes are taken from old stock, they are stored for more than 20 years. Many of the tubes are not able for the first minutes to display the full lenght of the glow. They need to be "formatted" with overcurrent for some minutes. Therefor the solder bridge must be set from X1-Y1 (normally used) to Y1-Z1. This connectes the cathode of the tube direct to the limiting resistors R13/R14 and feeds around 10mA (the voltage drop of the tube is ca. 100V at this current and the HV voltage will also drop to 110V at this current) through the tube.

If the tube displays the full lenght Neon glow, this solderbridge can removed and the "burn in" procedure is finished.

Step 2: Assembling the Board

If you don't want to create your own board you can optain a full electronic kit incl. enclosure and engraved scale at http://www.nixiekits.eu

The following assembly description is taken from the manual of the kit, but even if you're creating your own board, the seqeunce of fitting the electronic parts will be nearly the same.

1. Placing the resistors
We will start with the low profile electronic parts; so let’s start with the resistors and here it is the best to start with those values which are mostly used in the circuit.
Pay special attention with the values of the 2k49 and 249k resistors. They differ only in one colour ring (brown or orange). Therefore first finish assembling one value before you pick up the next.

2. Placing the semiconductors
Start first with assembling the three diodes D1…D3. Take care for correct orientation. The cathode ring is marked on the assembly drawing and also with a circle around the solder pad.

3. Placing the RGB LEDs
Flush-mount now the LEDs to the PCB and adjust them to sit straight. Also take care for the orientation. The anode is the long leg and this pin is marked on the PCB with an “A”, the cathode is marked similarly to the diodes with a ring.
Please solder only one wire first, than adjust and when finished solder the remaining leads.
Do this as fast as possible not to damage the LEDs due to the high soldering temperature.

4. Placing the IC sockets
Now insert all three 8-pole IC sockets. Pay attention to the correct direction marked with a notch.

Please do not insert any IC at this stage!

5. Placing the MosFet transistor IRFD220 (T2)
You should discharge yourself by touching any metal work before placing the transistor.
Note the correct orientation; also the Drain pin has two legs, which are tied together; this is shown also on the PCB.
Tip: To fix the part, solder the drain connection from the top side..

6. Placing the npn transistor MJE3439 or 340 (T1)
Take a pair of flat pliers and hold the transistor around 3 mm from the package. Than bend all the leads straight downward. Now fit the transistor into the three round pads direct above T2 and turn around the PCB. The transistor will adjust itself. Nevertheless solder only the middle leg, check and modify for correct adjustment. Than solder the remaining two legs. Please pay attention of this mounting position as the transistor will otherwise not fit into the thermometer’s case.

7. Placing the ceramic capacitors / EMV-inductor
Start with placing the two 330pF capacitors C12 and C19, than place the six 100nF capacitors. If you’re looking for C9: It is located above the power switch left to the EMV-inductor L1; this inductor we will fit next.

8. Placing potentiometers / switching converter inductor
Now fit the potentiometers TR1, TR2 and the inductor L2. Do not push the supplied shaft in one of the potentiometers yet.

9. Placing the electrolytic capacitors
Fit first both high diameter capacitors C14 and C15. Pay attention not only for correct direction but also not to mismatch them. C14 is the 2µ2 350V capacitor and is located below the blue inductor, C15 is the 470µF 10V capacitor and is located left to IC4. The anode + of all capacitors is marked on the PCB with a corresponding print, the cathode – is marked, as usual, with the ring at it’s pad.
Next fit the remaining eleven 100µF capacitors. For optical reasons take care for a clean straight orientation, therefore solder first only one lead each and align the capacitors. When finished, solder the remaining.

10. Placing small signal transistors / reference diode
Now fit the remaining 3-leg-electronic parts. Because of a different pin spacing a slightly „anti-mismatch guarantee“ is given but nevertheless have a look at the part’s marking (e.g. the LM35 and BC640 have the same pin spacing).
Please pay attention, that the mounting height does not exceed the height of the electrolytic capacitors, so push them slightly lower in the PCB.

11. Placing the USB connector / slide switch / temperature sensor
Fit the USB connector into the board and solder one pad for the package mounting from the top side for fixing them. Do it in the same way with the slide switch.Then turn around the PCB and solder the remaining leads. You may pay attention again to the very narrow spacing of the USB connector’s pads. Take care not to short circuit some pads during soldering.
The temperature sensor IC1 is mounted from the solder side of the pcb. The „flat“ side with the print points to the front of the pcb. The spacing between the top of the pcb and the bottom of the temperature sensor’s body should be 6 mm.
The leads needs not to be cut from the component side but pay attention that they will not make any short circuit with the USB connector’s package.

12. Fitting the ICs
Finally fit the three remaining integrated circuits into their sockets.

Now our assembly of the electronic parts is finished and we will start to do a first function test.

But beforehand, check your work again for assembly mismatch, correct orientation of the electrolytic capacitors, diodes, transistors and also for solder bridges.

Step 3: First Power-up and Voltage Test

13. The function test
Connect now the board to your USB power source (an USB loader power supply is recommended) and turn on the thermometer with the Power switch.
Caution! High voltage is present on the board – don’t touch the converter’s relevant parts.
Now the LEDs should light up with their never ending colour play. Now put your black probe of the multimeter on the GND testpoint, and with the red probe check the voltages on the testpoints below:
Pin 4 of IC2: ca. -4.8V
Pin 1 of IC4:ca. +8.9V
Cathode D3: ca. +127V
Testpoint ULM35: ca. 0.3V (depending on ambient temperature)
Testpoint UTEMP: Output voltage of the measuring amplifier, adjustable with TR1 in the range of ca. 3.8…5V (depending on ambient temperature)
Testpint UREF: Reference voltage, adjustable with TR2 in the range of ca. 2.8…6,6V.

Don’t continue your work until the voltages are within their given range. This may damage the circuit. Check for the fault; mostly a solder bridge or a reverse-mounted part.

14. Adjusting of the measuring amplifier
For an exact temperature display a trimming of the amplifier is necessary.
Therefore plug the shaft loosely on TR1.
Now check again the voltage on testpoint ULM35. It is in the range of 0.3V and represents the actual ambient temperature (10mV/°C). Please write down the voltage reading. The amplifier must now be adjusted for 18-fold amplification. Therefore simply multiply the voltage reading by 18 and adjust for that value on testpoint UTEMP with the potentiometer TR1. When finished carefully remove the shaft.
Example: ULM35 = 0.3V so adjust for 5.4V with TR1 at testpoint UTEMP.

15. Adjusting of the reference voltage:
This is just a coarse adjustment only to give a suitable display on the tube. The final adjustment is done with the complete assembled thermometer.
Therefore plug again slightly the shaft on TR2 and adjust for a voltage reading of 3.7V at testpoint UREF. When finished remove the shaft and keep it for final use.

Now testing and adjustment of the electronics is finished and we will start after “formatting” the tubes to assemble the enclosure.
So first disconnect the board from the power supply.

16. “Formatting” the bargraph tube
Many of the tubes will fail, due the long storage time for more over 20 years, during first powering up to display the full length glow. To archive that, these tubes must be powered with over current for some time as they will show the full display with the following procedure:
Check if he power supply is turned off. Now pull out the operational amplifier IC2 from its socket (this is very important!) and turn around the board.
You’ll see near the position of T1’s pads three solder jumpers marked X1, Y1 and Z1. Connect now pads Y1 and Z1 together. Pick up the tube and connect the lead wires to the pads marked A, K and K1. The outer wire, which is internally connected to the tube’s anode mesh must be soldered to the “A” pad. Now place the tube on your desk and pay attention not to produce any short circuits between the wires and the board. Turn on the power supply again.
Now the tube must light up with a long glow, which should as long as it matches the red printed marking ring at the top of the tube. If this fails and the glow lenght is shorter, keep the circuit powered on as the glow reaches the marking.
This „formatting“ may take up to 15 minutes, so don’t be impatient. The tube will get warm during this procedure.
If the formatting is finished, plug off the power supply, de-solder the solder jumper from Y1 and Z1 and also the tube’s wires.
Re-plug the operational amplifier IC2 into its socket.

Step 4: Assembling the Enclosure

17. Assembling of the enclosure
It is necessary to note that we will mount the NixieTherm Mk II enclosure not only „from the top to the bottom“, but also upside down. Therefore it is necessary to secure the thermometer’s scale e.g. with a small vice. But before you clamp the scale please cover the jaws with any kind of protective material so as not to scratch the scale’s Plexiglas.

Start with peeling off the protective film from the scale. Now apply carefully both tube clamps into the scale’s notches.

Note: The clamp itself has also a small notch on its back side so fit them in that way that this notch is located on the „rear“ of the scale.

Now fit this “scale-tube-combination” to your vice in such a way, that the „Mr.Nixie logo“ is readable upside down and located near to the jaws.
Pay extra attention that the tube doesn’t drop out during mounting and do not tighten the jaws too tightly.

Now fit the top cover to the scale. Remember again, we are working „upside down“ so the milling „NixieTherm“ must be readable mirrored. Futhermore these words must be located at the front of the scale – this is the direction from which you are normally looking at the thermometer. Have a close look at the pictures on left, which will help you to find the correct fitting position.

At this stage it is time to check the alignment of the bargraph tube. If the tube is dropped in the right way round you will see the anode mesh when looking at the scale from the front (non-mirrored digits). When looking from the back you should see a metallic surface in the tube with the print of type and manufacturing date. If not, simply rotate the tube by 180°.

Now take both LED covers and remove the protective films from the mirrored side. Both covers have a deep milling grove for the tube and a slight wider milling grove for the space of the resistor R5, which is located near the LEDs at the position of the back cover. Both deep milling groves must facing “downwards” to the tube.
Have a close look again at the picture on the left for assistance. The mirrored face of the LED covers must face to the scale on both sides as it will reflect the LED lighting.

For mounting the PCB holder pick up two mounting brackets, two M3x16 screws and four washers and assemble as shown in the picture. Now slide this combination though the drilling holes of the LED covers and the scale.

At the threaded end now fit again a mounting bracket and a dome nut and tighten them slightly only by hand.
Now adjust all four brackets for orientation upwards and parallel to the top cover. Now tighten the screws again, but only such that the mounting brackets can slightly adjusted by pushing them.

Now pick the four flat hat M3x8 screws and the M3x12 metal spacers and stick them „from bottom to top“ through the drill holes of the top cover. Only tighten them slightly by hand. Pick up both thicker (8 mm) Plexiglas frame halves and peel off the films. Slide them gently over two distances.
Pick up now the „fully“ Plexiglas frame with the „chamber“ cut-out and bend them carefully over the four distances; pay attention to the orientation of this „thermic chamber“ and the cutout for the USB connector. The picture gives you assistance in this assembly stage.

• Now fit the electronic board. Pay attention at the three tube’s leads. They must be threaded through the milling groove in the middle of the board. Also these leads may not cross match.
Attach the shaft to trimmer TR2 through the solder side’s drilling hole and press it to its mechanical end.

• Pick up the four Philips screws M3x6 and tighten them softly within the tapped holes of the mounting brackets. Maybe the screws must be bend a little bit if the holes of the bracket will not fit to the holes of the board. Now fasten the four screws with your screwdriver in a crosswise fashion.

• Pick up the supplied tube and cut them in the middle. Thread both halves through the outer leads of the tube, the milling groove of the PCB and push them as close as possible to the tube’s glass body. Now cut the leads as they must be soldered to the pads A, K and K1. Pay attention not to stick the leads too far through the drilling holes of these pads as they may produce a short circuit with the parts located on the assembly side.

• Now an additional jumper must be set between the solder pads X1 and Y1.

• Finally pick up the smaller (4 mm) Plexiglas frame halves, remove the films and attach them to the enclosure.

• Now thread the remaining four Philips M3x16 screws through the holes of the four rubber feeds.

• Pick up the black bottom cover, remove the protective films and attach them to the case. If all is well, both bent leads from the tube should a little press on the bottom. This will fix the tube in its final scale position and is intentional.

• Pay attention to correct orientation of the bottom cover. The louvres are now located at the front side, so you should see the soldering pads mentioned before when looking through the louvres. Also only the left potentiometer TR2 may be accessible through the drilled hole. Have a look at the picture on the following side for assistance.

• Now assemble the four rubber feet to the bottom cover. Please don’t over tighten the screws.

• Finally fit the supplied label, unclamp your NixieTherm from the vice and place it down on its feet.

• After tightening the four pan head screws with an appropriate screwdriver we have finished or work.

Step 5: Final Adjustment

Congratulations. Now have a lot of fun with your self-assembled NixieTherm thermometer.

18. Final display range adjustment
As the display remains steady for a while – your ambient temperature must remain also steady of course – you can do the final adjustment of the displayed value with respect to a reference temperature probe with the TR2 trimmer and the supplied shaft.
In the same way the display can adjusted at any time for accuracy. But please consider that the NixieTherm thermometer should be powered up for at least an hour before doing these adjustments due to the self heating of the temperature sensor, which must also be corrected.

Connecting to an USB port / USB-Hub
It is not recommended to connect the NixieTherm to an external USB-Hub when this Hub doesn’t have an extra power supply which is able to supply the needed 450mA for the thermometer.
If your computer is equipped with a modern USB3 port than simply connect the NixieTherm to this port.
However, due to the high quantity of different USB controllers and manufacturers, it is not guarantied, that the NixieTherm will work on every USB port.

The best way is to power the NixieTherm from its own external 5V 500mA supply (USB loader).