Below is a schematic of the window comparator interfaced to the Eltec 442 Pyroelectric detector. In this schematic, R2 could be replaced with a variable resistor to adjust the size of the voltage window. This means that one or the other of the digital outputs from this circuit will go from low to high when the motion occurs depending on the direction of the motion.
Experimentation is the best way to learn how the detector behaves.
This circuit uses the LM139 Quad Comparator. This chip is easy to locate and costs typically less than a dollar. Resistors R1, R2, and R3 create a voltage divider. By reducing the R2, the size of the window is reduced. Larger values of R2 will increase this window size.