I am currently working on a project that involves an arduino mega 2560, one 74154 demux and 16 stepper motors. I want to be able to control which stepper motor turns at a given time. For this, the 74154 "tells" which stepper motor to turn when I throw LOW or HIGH from the arduino to the demux.
The stepper motors I have are rated for 5V and are driven by ULN2003AN driver boards that came with the package. The steppers should turn for about 6-7 seconds (completing one revolution). I want to have a separate 5V supply for my steppers and for this purpose, I am planning to put transistors to act as switches that will only turn on when an output from the demux is given.
Since the 74154 has ACTIVE LOW outputs, I think using PNP transistors would be a good idea (if my understanding of PNP transistors is correct) given that PNPs "turn on" with a negative base voltage.
Regarding the 74154, when I feed 0000 to the demux's ABCD input pins and 00 to both G1 and G2, Y0 produces an ACTIVE LOW output (while all output pins are ACTIVE HIGH), correct? Does this mean that, that ouput pin is producing 0V to the base terminal of the first transistor, and not any voltage at all? I should need any voltage lower than my 5V supply in turning on the transistor and allowing current to flow from the emitter to collector to the VCC pin of the first ULN2003A driver board and turning the first stepper motor. Is my understanding correct, or perhaps not? If I am wrong, can anyone enlighten me on this? :)
Also, I still have a dilemma about using TIP125 (a PNP transistor), if it is the best PNP transistor to use, and the values of my base resistor and whether I should put a resistor before connecting to my load, or not.
Any opinions and answers are truly appreciated! Thank you. ^^
Edit: This is the schematic I drew, hehe.