Display module

As Base Module can work with many different display types, many of them require dedicated design of display module. Interconnector between Display and Base Module provides eight logic lines, +5V power line and high voltage from step-up converter. More details are provided below.

Display Module designed for Open Nixie project contains 4 Nixie tubes, 74141 decoder and anode drivers for every tube. Tubes that can be used are:
QS30-1 (not tested),

As interconnection provides only power and logic lines – all power drivers for display have to be implemented on the module.
Display is driven with dynamic scan method. This means that corresponding cathodes (digits) from every tube are connected together. Every tube is then powered individually for a period of time in a cycle. This means that high (anode) side and low side (cathode) drivers are required. Frequency is to be determined experimentally – for this project it is about 150Hz. Lower frequencies made flickering visible (as single tube is turned in only for 1/4 time of cycle). Faster frequencies cause ghosting. More than one digit can be seen turned on simultanously on single tube. This is caused by limited speed of drivers.
High side drives are based on a pair of complementary transistors (MMBTA42/MMBTA92). Input accepts large variety of input level standards (everything above B-E polarization voltage ~0,7V should work). Typically base resistor (R1, R5, R9, R13) should be selected to provide about of 0,1mA of base current. Resistors R4, R8, R12 and R16 are used for limiting current that flows through tubes. The values of the resistors should be adjusted experimentally, depending on power voltage, type and wear level of tubes and desired brightness. Let’s see some details. Here is oscillogram which shows voltage between tube’s anode and GND.

Each tube is turned on for about 3ms. The most important part is marked with red circle. There comes more detailed view.

Tube ignites and after about 30us voltage drops and current stabilizes. Voltage drop is about 14V. This drop is across resistor in series with the tube. There is 2k resistor in series so current about 7mA flows through the tube. As duty cycle is 25% the average current would be about 1.75mA. The LC-513 tube is rated for 2.5 to 3mA typical anode current, so it works within its specification. Oscilloscope helps determing proper value of the resistor and allows to check if tube operates within its limits. One more interesting detail – interferences from step-up converter powering Nixies can be seen (40-50kHz).
As the low side driver – integrated circuit 74141 is used. It is an old dedicated TTL IC for driving indicator tubes, not manufactured anymore yet still easily available. It has four input lines and ten outputs and works as BCD to Decimal decoder. For input values greater than 9 all outputs are off. 74141 uses DIP16 case but it has to be mounted on the surface of PCB. Dedicated surface mount socket can be used or standard THT socket with bent and cut legs will also do its job.

Display Module connects with Base with two headers. Header marked as J1 carries logic lines for input of 74141 and +5V with GND for powering the IC. Header J2 carries logic lines for driving anodes and HV for powering tubes.
First version of PCBs has an error – transistors Q2, Q4, Q6 Q8 have swapped Emiter and Collector leads. to make everything work those transistors have to be soldered on their back, in a dead bug way. KiCad projects available for download have this error fixed and are marked as Rev. 2.

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