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Another benefit can be the use of an H-bridge to control a stepper motor. This tutorial shows how to do this.
Example
1x Arduino
1x H-Brücke Bridge SN754410 (Datasheet)
1x DC Motor
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Further Information:
H-Bridge - Wikipedia
Using Two Motors
Here is an example with two motors. Remember, you can supply 9 volts to the VIN pin on the Arduino to run everything with the 9-volt battery.
