Using Large Motor Drivers with 3.3v Micro Controllers
Pololu and StepStick style stepper motor driver modules are great, but they have limited power. If you want more power, you will need to use larger stepper motor drivers like the one shown above. While the small modules are limited to about 2A. I have seen these stand alone drivers deliver close to 10A.
They often use opto-isolators on the inputs. Opto-isolators are used to block noise and spikes. Large machines with high currents can cause a lot of electrical noise. This noise can damage sensitive electronics and cause false triggers on switches or even false steps on drivers.
Opto-isolators use a non conductive gap. The signal crosses the gap using the light of an LED and an optical receiver. No noise, signals or spikes can cross the other way.
The LEDs have a forward voltage spec. You need to size a resistor to the voltage of your signal. The stepper drivers typically have a resistor built in for a 5V signal. They show that resistor in the simplified schematic on the case.
You can use a larger voltage by adding another resistor in series with their resistor. The operating manual will tell you how to size that resistor. Unfortunately, you cannot use a lower voltage.
Here is a simple circuit I have used to get around this. Just about any NPN transistor should work because the current required by the LEDs is typically less than 15mA.
