Project: Nixie Clock (upgrade) – Using 74HC595 Shift Registers

My Nixie tubes have 11 active pins each: a common anode and one cathode per digit (ten in total). The anode is connected to +180V via a 47k current-limiting resistor and each cathode is connected to the collector of a high voltage bipolar transistor (MPSA42) so that current can be controlled through each of them via the base of the transistor. This gives a total of 29 transistors that need to be individually controlled (24 hour clock requires 3 possible numbers for the first digit, 10 for the second, 6 for the third and 10 for the fourth). I chose to do this by using four 8-bit shift registers, connected in series to make one, 32-bit shift register.

IN-12A_IN-12B-pin-draw.jpg

Datasheet for Nixie IN-12a tubes (not in English!) shows one anode and 10 cathodes

Using a shift register I am able to shift in all the data via a single data line and a single clock line, and then control all 29 transistors using the outputs of the shift registers. The only issue with this is that as the data is shifted in, the displays will scramble as the data will change every time a bit is added. This can be overcome using a combined shift-register and latch such as the 74HC595.

Using the 74HC595, an extra input is provided that allows all the data to latch at the same time (on a rising edge) so the display will only change once per update.

sn74hc595.png

Logic diagram for a 74HC595 taken from the datasheet

You can see from the logic diagram that all outputs will change at once on the rise edge of RCLK. Furthermore we have CLEAR and OUTPUT ENABLE lines for more control. I tied OUTPUT ENABLE to ground so the outputs are always enabled.

shift_reg_signals.PNG

Scope capture of the signals going to the shift registers. Yellow = Clock, Green = Data, Orange = Latch

Here you can see that a new bit is shifted onto the data line on each rising edge of the clock. Finally the latch line is toggled so that all the data appears on the outputs at once.

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