arduinoh-bridge
So I have an Arduino and an L293D. The L293D logic is powered by the +5V output of the Arduino, while the DC motor is powered by an external power supply.
Here's the part I don't get. If I connect the Arduino and the power supply grounds together the circuit works, otherwise it doesn't. Can you explain me why?
The error message you're getting "simply" means that the Arduino board isn't responding.
Regardless of the type of shield used, I've often seen this type of problems arise with power hungry shields, when the USB port is not able to provide enough current for both boards simultaneously (because of cheap cables, hub wich is not self-powered, ...).
A possible workaround (at least to test the validity of this answer) could be :
Your NPN power transistor seems to be connected up wrongly.
I'd expect something like this ...
simulate this circuit – Schematic created using CircuitLab
where
Perhaps your LED is actually an 18V LED module that has current-limiting built in?
Note: I believe piezo buzzers don't draw much current, you probably don't need a TIP31 to drive it. Check the specs and choose a more suitable transistor. To be connected as shown, the piezo buzzer should be rated for more than 18V.
Best Answer
While a schematic would have helped describe the problem statement better, one key concept might help in clarifying this matter:
How this applies:
The control side of the L293D is actually powered by the +5V from the Arduino only when the +5V has a reference ground available, that corresponds to that particular +5V supply, in other words, the ground of the Arduino board.
The L293D does not have a separate drive-side ground pin, just the "Heat sink and Ground" pins, which are also the ground reference for the \$V_{cc1}\$ pin.
If you note the schematic on page 3 of the datasheet, the ground references for \$V_{cc1}\$ and \$V_{cc2}\$ are one and the same, the GND pin(s). Thus, that reference needs to be connected to the Arduino's reference ground.