Circuitron
DT-4 Rolling Stock Detection Unit

Walthers Part # 800-5204, p. 288 Walthers 2012 HO Scale Reference
A scale, $46.95, currently in stock at Walthers

 
Ideal for spotting a hidden train or piece of rolling stock. Four independent detection units combined onto one circuit board, can be used separately or together to control external devices. Supplied with four Opto-Sensors that mount between rails to detect movement. When train covers a sensor, the unit activates any connected DC accessory, such as an indicator light on a control panel. Independent sensitivity controls are provided for adjusting each Opto-Sensor to lighting conditions. Constructed on a 3 x 3" printed circuit board. Requires 10-18V DC. Independent of track power. Compatible with radio or carrier control systems.