The Circuitron BD-2 is a very sensitive, integrated circuit design block detector which will work with virtually any scale and type of track power, including most forms of command control. The BD-2 works by detecting the current drawn by a locomotive or lighted piece of rolling stock. Detection can be extended to un-powered equipment by using metal wheelsets with resistors connected across the insulator (any value between 4700 and 10,000 ohms should work well). The BD-2 senses detection by “monitoring” the voltage drop across a pair of diodes. These diodes introduce a small voltage drop of 0.7 volts to the track. Although very slight, this voltage drop may be noticed when moving between blocks that have no detection and blocks protected by the BD-2. The simple solution is to install a pair of diodes wired in series with each of the un-detected blocks. An adjustment control is provided to set the sensitivity of the circuit and can be used to compensate for damp conditions, outdoor use, etc. At its most sensitive setting, the BD-2 is capable of detecting 100,000 ohms or less across the rails. The BD-2 also incorporates a turn-off delay of approximately ½ second to prevent loss of detection due to dirty wheels, etc. The BD-2 will directly drive either LED-type or incandescent lamp, 2-color block signals and the outputs have a maximum current capacity of 250 ma. The circuit boards are designed for easy daisy-chain wiring of multiple blocks and will snap into a section of our PCMT (Printed Circuit Mounting Track) for simple installation. The BD-2 is constructed on a 2” x 3” printed circuit board and is available in standard 3 amp track power capacity (which is adequate for HO and smaller scales) and in a heavy duty 6 amp capacity version, the BD-2HD. Both circuits require a 10-18 v. DC power source for proper operation. The signal driving capacity of the BD-2HD is increased to 500 ma. per output and will work well with large scale equipment.