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
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.