I am using an Arduino Uno with a Tamiya dual motor gearbox with the DRV8835 shield. I am using the library available on GitHub with the sample sketch only one motor will run and in forward only, the LED works correctly.
I tested the motors and they work correctly forwards and backwards when connected directly to a battery I also tried a different Arduino and had the same problem.
I played with the sketch and I can set the speed from 0 to 400 using motors.setM1Speed(speed); and I will work perfectly but it will not work with negative numbers to go in reverse. Also M2 will not work at all.
From your description, I suspect there might be a connection issue between the shield and the Arduino. Can you post some pictures of your setup that show all of your connections as well as some close-up pictures of both sides of your DRV8835 Dual Motor Driver Shield?
You were right! I soldered the connections better and that fixed it. Phew! Thanks! I was worried I had fried it. This was my first real soldering job and I have learned a lot.
I am glad touching up your solder joints fixed the issue!
We generally do not recommend connecting batteries in parallel. You might consider using a single battery pack with a higher capacity (which would be more efficient) instead.
