Since there are only 3 input bits (0 to 7) it can only select one of 8 output lines and thus does not do conversion of binary coded decimal (BCD) into 10 line one-hot. You need a 4 bit binary coded decimal word to do 0 to 9.
Yes, so my real question with 74138 was if (any) DECODER/DEMULTIPLEXER IC can be used as BCD. And viceversa, any BCD as a DECODER/DEMULTIPLEXER. For you these things are trivial but they are very new to me. I really dont see the similarities/diferences between these 2 types. Very interesting.... thanks. I will have to pay more attention to my 74138 and see what you've seen/explained.
Also thanks for checking the cascade IC's for me.
I am currently hitting some very hard wall of errors with my boards I make. 1-from the scrappy components, 2-from very intricate connections, even if they are 4 to make, but when 1 goes to 3, in words sounds ok and easy, but there with those tiny wires, aaah, is hard. 3-not very, but too small spaces between the rails and the leg of the diode must hit 'in the zone' and not too much left or right. Get me? So there are even much more details like these, I will not enumerate all, but they are making this entire component extremly hard to make. I had to unsolder over 10 times the same board(s) and search and repair what was buggy and still I didnt figure it out. I did figure out 1 board, 1 diode that is functioning ok alone when measured but not in the cct, very strange. After I replace it, all got to normal working. But until I discovered the -------, took me 1 day. And the fun part, now when I put the second board on the rail, everything goes to hell again...aaaaah. I was VERY lucky with the 1234 module. No errors whatsoever. This one Im working right now is 5678. Im thinking to change tactics and make everything even bigger, to be able to repair and test and check and solder in the same time. Like I did on the breadboard. Well see. Very challenging. But I will make it work, one way or another. I dont like it BIG, but if this is what it wants to be...