A combination of a light source and a photosensitive detector.

A light-emitting diode and a light-detecting device sealed together in an integrated package. The input circuit and Output circuit are electrically isolated since they are only connected with light.

A type of connector that removes any risk of ground fault or other disruption. Normally installed just upstream of the timers, these devices optically isolate (filter) everything but actual timing impulses. A type of firewall for your timing system.