A base for a relay comprises a main body (1) which has a central boss (5) bearing a socket for receiving the pins of a relay. A module (6) has a central aperture (7) and is received within a recess surrounding the boss (5). The module (6) contains electronic circuits which are coupled to the main body (1) through complementary electrical coupling means (4). The aperture (7) is dimensioned to receive the boss (5) whilst allowing a relay to be plugged therein.

In another embodiment, the module (6) is provided with raised portions for containing additional circuitry.