An interface circuit for indicating the state of a set of switch contacts having high noise immunity and high reliability. The switch contacts to be monitored are connected in series with the primary winding of a transformer having a magnetic core with a highly rectangular hysteresis loop. A voltage is periodically applied through the switch to the transformer primary. A test pulse of current is then passed through a secondary winding on the transformer. The signal across the secondary winding produced in response to the test pulse current indicates the closed or opened condition of the contacts.