Abstract: A system for monitoring the volume of traffic on telephone lines or in a data acquisition system as a function of time using photocells responsive to the aggregate visual light output of pilot light displays utilized in the telephone or data systems. An array of photocells is positioned over the pilot light display so as not to interfere with its visibility by an operator. The output of the photocell array is coupled to a recording device which records the instantaneous output sum of the photocells but which is insensitive to ambient light conditions of the surrounding environment or to transient light conditions of the sources to be monitored. Means are also provided for triggering an alarm when a predetermined voltage level is attained which provides an indication of the condition when all lines are in use.