one thing i was unsure about, maybe that's it: do the arduino and the motor power supply have to share ground? currently the arduino only is connected to the bridge via pins 3 and 4, no gnd at all. should i try to connect motor and arduino grounds (similar to schematic for L298 below)?
L298 DC Motor Control - Schematic | PyroElectro - News, Projects & Tutorials
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Yes Yes Yes the GND must be common otherwise you can't be sure you are switching the transistors fully on.