The VFD shield power supply circuit consists of a voltage tripler built around a 555 timer IC. Together with C1 and R5 the 555 forms an astable multivibrator with a 50% duty cycle output.
As the output of the 555 is not capable of delivering enough current, a driver stage around R1, C4, T26 and T27 is added. The circuit consisting of T26 and T27 is also called a push pull driver.
The push pull driver powers a voltage tripler circuit around D2-D5 and C5-C8. The output voltage of this circuit is somewhat less than three times the input voltage. With a 12V input voltage, an output voltage of 32-34 is achievable which is high enough to drive the VFD tubes. A disadvantage of this rather "low" voltage is that when multiplexing the VFD tubes, a very high VFD brightness is not achievable but the overall performance of the circuit is quite acceptable. In order to get higher voltages a switch mode booster circuit would be more appropriate but we decided not to do this as we wanted to keep the design as simple as possible using only standard parts.
A disadvantage of a classic 555 circuit with a push pull stage is that the output voltage of the 555 on pin 3 gets not high enough to drive the NPN transistor (T26, a BC639 in this case) into saturation. As a result, the efficiency of the circuit drops and the NPN transistor may become hot under heavy load conditions. To avoid this, the 555 in our circuit is powered by a supply voltage somewhat higher than the voltage on the push pull driver. When the circuit is powered, the 555 gets power via D2, D3 and R4. As the 555 starts oscillating, a voltage is build up on the electrolytic capacitors of the voltage tripler stage, with C6 as first stage of the tripler charging to around twice the supply voltage. As a result the 555 is powered via R4 with around 22V. Due to the voltage drop across R4, the final working voltage for the 555 is around 14 to 15V. As such it is very important to use a CMOS variant of the 555 as a regular 555 draws more current which may require R4 to be adjusted downwards in value.
The filaments of the VFD tubes are connected in series and an AC voltage is generated by connecting the filaments between the emitters of the push pull driver and C2 and C3. C2 and C3 limit the current through the filament wires with their impedance. The value of C2 and C3 is somewhat critical as the filaments should only glow faintly, nearly invisible under normal light conditions. If the filament current is too high, tube life may be reduced. On the other hand, when the current is too low, VFD brightness is affected negatively.
Finally, D1, a standard rectifier diode lifts the entire power supply around 0.7 V above ground level. When the segment and grid drivers (see below) pull down the voltage to ground level, the voltage on that segment/grid will be slightly negative in respect to the filament. As such a very faint glow of the segments (especially in the vicinity of the filament wire) when they are turned off is avoided.
The tube segment and grid drivers consist of a NPN/PNP transistor pair including the necessary resistors. When a logic level 1 (5V) from the Arduino is applied at the input, the corresponding VFD tube segment/grid is pulled up to 32-34 V, otherwise the segments/grids are tied to ground by the 100K resistors.
We could have used high side (source) drivers here like the Allegro A2982 or a Supertex VFD driver with shift register input. Many of these are unfortunately not easily available, expensive or come in SMD packages only. As this circuit is intended for educational purposes, we decided not to use them. The disadvantage is that quite some soldering work will be necessary to mount all the transistors and resistors.
The tubes are connected with all 7 segments and the decimal dot indicator wired in parallel. The filaments are connected in series as mentioned earlier.
To control the display, you turn first the segments for the first tube on. Then the grid of the first tube is turned on and the segments start to glow. Then the grid of the first tube is turned off again after some time and the segment pattern for the second tube is turned on. Then the grid of the second tube is turned on for some time and turned off again. This process is repeated for tube 3 and tube 4 after which the whole cycle is repeated. Due to the persistence of vision of the human eye, we see a complete display of 4 digits.