Hi this is an led controller there can only be one led on at a time it was on crocodileclips i don't know what its called but if anyone does that would be helpful. when A is high 0 is on if B is high 1 is on if A and B are high 2 is on and when A and C are high 3 is on or somthing like that.
A 74138 is a 3:8 line** A 74138 is a 3:8 line decoder and a 74154 is a 4:16 line decoder. They take binary data and decode it to any of the outputs. Where ** is is the family of chips, most common today are HC so 74HC138 or 74HC154. Different families have different requirements for current and voltage. The HC family is the most versitile.