circuitron 5521 block occupancy detector

Circuitron -- BD-1HD Optical Detector -- Tiny Opto-Sensors detect train movement. Sensors mount between the rails and activate circuitry when shaded from a passing train. Independent of track power, rolling stock requires no modifications. Sensors fit between ties and may be ballasted over in most cases. Circuitry is extremely sensitive and will operate properly under very low levels of room light. Positive indication whenever a section of a layout is occupied by rolling stock. Contains all circuitry to power two-color block signals at each end of the protected block. Will give proper indications if a train leaves a block by backing out. No modifications to rolling stock needed. Completely bi-directional. No additional driver boards or relays are needed. 250 ma. output can power lamp or LED-type signals. Requires 6-20V DC input. Independent of track power. Compatible with radio or command control systems. Same as #800-5501, but with 500 mA. output.