what reverses the flow of current
To reverse the direction of current, reverse the voltage. A common circuit to do this under digital control is an H-bridge which consists of 4 transistors, a positve supply input and a negative input supply. and two ouptuts - the polarity of which is swapped by an input control line.
Answer Wiki. To reverse the current, you need to reverse the voltage. It will be more easier if you think in terms of voltage. Now, this can be achieved in many ways. It depends on what is your power requirement. If your current requirement is less than 1A, you can use a motor driver IC like L293D.