The Circuitron DT-1 is a bi-directional detection circuit designed for DC track power that provides an output whenever one of its Opto-Sensors is covered by a piece of rolling stock. Absolutely no modifications to the rolling stock are necessary. Four Opto-Sensors are provided, 2 of which are used for each direction of travel. The two Opto-Sensors for each direction are wired in series, and a single sensitivity control for each direction adjusts the pair to compensate for varying levels of room light. The DT-1 connects to the track to determine track polarity and activates only the Opto-Sensor set for that direction of travel. This ensures that the output of the DT-1 turns off just after the last car clears the crossing in either direction of travel. The DT-1 circuit board measures 2” x 3” and requires a 10 – 18 volt AC or DC input for proper operation, however, the unit will be slightly more stable and easy to adjust if a filtered DC source is used (see POWER SUPPLIES). The output of the DT-1 provides a connection to the common or ground (-) side of the circuit and is capable of controlling DC powered loads up to 250 ma. Please note that the DT-1 has no means of detecting a short train or single locomotive if it falls between the Opto-Sensors., so that while the train is between the sensors, the output will be off. An optional 3rd sensor (see ACCESSORIES section) can be added to each side, if desired, and this will cut the minimum train length in half.