I was reading this tutorial and I was confused on how I would control a motor with a 9v battery while powering my arduino with just the USB cable plugged in.
Electronic – arduino – Controlling a motor with arduino and H bridge and external power supply
arduino
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From the comments @ sparkfun (Specifically superbrad):
The L293D has internal flyback diodes, which catch what would otherwise be high voltage inductive noise (sometimes even in the kilovolts). With the SN754410, you must use external flyback diodes in your H-Bridge.
Even though you might feel like you can get away with it, it's a VERY bad idea to use the SN754410 without a flyback diode. It's not just the driver you have to worry about, it's your entire circuit (MCU, accelerometers, expensive XBee, etc.). Eventually, your luck will run out, even if you don't turn it on and off frequently.
Do you have the DC supplies and Arduino grounds connected together? If not then any PWM from the Arduino will not work.
Most small plug type DC supplies are floating, so their grounds will not be connected to USB ground (of course the USB ground may be floating if e.g. on laptop battery)
As Russell mentioned a diagram of your connections would be useful.
Thanks for the diagrams:
Your problem is as stated above, you need to connect the grounds together. Otherwise, the Arduino PWM signal has no return path (remember a voltage is always relative to something) to Arduino ground since the grounds are not connected.
It's okay to connect all the grounds together (arduino and DC supplies)
The only time is isn't is in the very rare case where you might have a supply "ground" that is above earth ground relative to supply grounds (e.g. a 20 V supply with V+ at 50V and V- at 30V relative to earth/AC ground could not be connected to a 20V supply with V+ at 20V and V- at 0V)
If in doubt you can test for any voltage between the grounds with a multimeter (e.g. one probe to one ground, one to the other ground) but since most DC supplies are floating (not referenced to earth ground) then essentially the outputs are just like batteries.
Hope I haven't confused things here :-) Connect them together and let us know how it goes. Here's an altered version of your diagram (note extra black ground lines):
Best Answer
The USB connection provides 5V or 3.3V power for the Arduino. The 9V battery provides power to the H-bridge, which drives the motor. The connection between the two sources is made with the H-bridge chip using transistor driver circuits.
This is a basic H-bridge:
V+ is 9V. The P-channel FETs require ~9V gate drivers to turn off, while the N-channel ones may only require a 5V signal to turn on. In reality, then both likely have similar transistor driver circuits, both of which only require 5V inputs, which the Arduino provides, to fully react.
The parts indicated in the tutorial, SN754410 or L293, are half H-bridges and use BJTs, otherwise known as push-pull drivers. They only have the connection on one side of the classic H-bridge (one NPN, one PNP).