This paper studies a novel electrophoretic molecular communication (EMC) framework utilizing a piecewise constant electric field. EMC is a particular type of molecular communication that exploits electric fields to induce the movement of charged particles to enhance communication performance. Our previous work proposed an EMC framework utilizing a time-varying electric field that exponentially changes; however, the field with such a complicated shape might be challenging to be implemented in practice. Thus, this paper proposes a new EMC approach exploiting a piecewise constant electric field that can be readily implemented via, e.g., an on/off switch method. We formulate two optimization problems to design the electric field based on different objectives: minimizing a mean squared error and minimizing a bit interval. The solutions of each, such as optimal on-off timings and corresponding strengths of the constant electric fields, are obtained through the Lagrange multiplier approach and the geometric programming, respectively. The Monte Carlo simulation results verify that the proposed piecewise constant electric field significantly reduces the bit error rate relative to the constant field benchmark while performing less well, but not significantly, than the exponential field benchmark.