We explore and explain how inexpensive current sensors and current transformers work. We show how to use them on your model railroad or any other use, and how they work with Digital Command Control (DCC) for occupancy detection. We also develop a model in LTSpice and simulate the NCE BD-20 to see how it works, and do oscilloscope bench testing to see how good the model is. This inexpensive device can then be used with an Arduino to replace a very expensive DCC system using JMRI and DCC++.