A modular DIN rail mounted base includes a two-part body made of plastic shells and a unitary circuit board assembly having two vertically spaced circuit boards. The edges of the circuit boards fit into interior grooves formed on the interior surfaces of the shells to locate the assembly vertically in the shell. An alignment rib provided in a groove in a shell extends into a recess formed in a circuit board edge in the groove to locate the assembly longitudinally along the groove in the body.