74141-e

Written by Kai Chmieleffski on Saturday July 18, 2020

This is a small project which was done basically out of curiosity. I didn't have enough 74141 Nixie driver ICs in the lab to rebuild a vintage piece of metrology equipment, which is why I decided to just try and create a simple component that can mimic the features of the 74141 and 7441A out of modern SMD components, with the goal to have a pin compatible drop in replacement.

To achieve this I started by studying the datasheet for the 74141 and the 7441A, which revealed that those ICs are definitely simmilar, but not identical. There is a very significant difference - The truth tables differ between the models in regards of the "not allowed" Hex values of A to F. 

While the 74141 just simply blanks the tube, which can be usefull in some applications, the 7441 displays pseudo random number combinations and the 7441A just continues with one again.

The truth tables look as shown:

A B C D 74141 7441 7441A
0 0 0 0 0 0 0
0 0 0 1 1 1 1
0 0 1 0 2 2 2
0 0 1 1 3 3 3
0 1 0 0 4 4 4
0 1 0 1 5 5 5
0 1 1 0 6 6 6
0 1 1 1 7 7 7
1 0 0 0 8 8 8
1 0 0 1 9 9 9
1 0 1 0 nothing random* 1
1 0 1 1 nothing random* 2
1 1 0 0 nothing random* 3
1 1 0 1 nothing random* 4
1 1 1 0 nothing random 5
1 1 1 1 nothing random* 6

* two number pairs are alternating

Also, the IC is capable of driving moderately high voltages, which needs to be remembered - modern logic doesn't work well with voltages of around 60 volts ;-)

So I implemented a little driver circuit, a microcontroller and a few passives, neatly arranged on a small PCB the size of a DIP16 IC and created the corresponding PCB. I also added a solder jumper on the bottom to switch the replacement IC between behaving like either a 74141 or a 7441A. I didn't bother to implement the functionality of the 7441 non A, as the alternating pairs of digits didn't seem that usefull to me.

74141-e Top view 3D render

74141-e bottom view

Those PCBs were then ordered, build up and used very successfully. Sadly I didn't take any pictures of a finished one, but I plan on ordering more, building more of those IC replacements and hopefully remember to take pics. I might even offer them at the PCI.Technology shop some day in the future.