Stand-alone modules or blocks for use in creating low-power sensor-based monitor/control systems. Each module performs a pre-defined function, and when included in a monitor/control network operates as a node on the network and automatically communicates with one or more connected nodes using a uni-directional packet-based protocol. One class of such predefined nodes communicates Boolean values, for example, with input sensor nodes detecting the presence or absence of environmental phenomena (e.g., motion, light, sound, water, button presses, etc.), intermediate nodes transforming those Boolean values using combinational or sequential logic, and output nodes converting the resulting Boolean values to environmental phenomena (e.g., beeping, light, electric relay activation, etc.) or to data for further processing by a computer. Another class of nodes communicates integer or number values.