I made a simple test circuit to help me understand the L293D H-bridge that comes with the Arduino starter kit.
Digital output pins of the Arduino are wired to the input pins on the H-bridge. Arduino code alternately sends 1/0 and 0/1 down those two output pins, basically (as I understand it) creating an extremely low frequency alternating current in the two output pins on the H bridge. Two LEDs are wired in parallel to the output pins, in opposite directions, so they flash, demonstrating the alternating nature of the current.
The project I'm working on from the starter kit right now is the Zoetrope (page 102), and in that project the L293D is used to control a motor. The Arduino's 5V is connected to Vss and a 9V battery is connected to Vs.
So I'm a little surprised that I got it to work without using the Vs and Vss pins at all. What are they for? I did try swapping the LEDs out for the motor and it doesn't work (not enough power, I guess). But if I hook the 5V on the Arduino up to the Vs pin, it does! Hooking the 5V up to only the Vss pin, however, is not enough to power the motor.
So I sort of get the idea that without Vs and Vss current, the current from the input pins is enough to light the LEDs, but what I don't understand is:
If the voltage on the input pins isn't enough to drive the motor, how come connecting the 5V to the Vs pin is? It's the same voltage as is on the input pins.
What is the Vss pin for?