Light curtain controllers send stop signals to machines that are guarded by light curtains, photoelectric transmitters that project an array of synchronized, parallel infrared light beams to a receiver. When an opaque object interrupts one or more of the beams, the light curtain controller signals the stop. If a lockout condition occurs, an operator may need to reset a switch on the light curtain controller in order to return the machine to a run condition. Most light curtain controllers contain three relays: two control or safety relays and one auxiliary or status relay. They also include a power supply that receives AC or DC inputs. Integral light curtain controllers are packaged with a light curtain transmitter and receiver. External light curtain controllers are stand-alone units that are often positioned near the transmitter and receiver in order to provide a central wiring location. Typically, external light curtain controllers are housed in metal enclosures and mounted in racks or cabinets, or on walls or DIN rails.