Circuitron 800-5401 AR-2 Automatic Reverse Circuit with Adjustable Delay

BD-2 Block Occupancy Detector Current Sensing

Details:

The CIRCUITRON BD-2 is a highly sensitive integrated circuit block occupancy detector designed for reliable operation across virtually all model railway scales and track power systems, including most forms of command control. It detects the presence of locomotives and lighted rolling stock by monitoring current draw through the rails, providing accurate and responsive block occupancy indication.

For unpowered rolling stock, detection can be extended by fitting metal wheelsets with resistors across the axle insulation, typically between 4.7kO and 10kO. The BD-2 is adjustable for sensitivity, making it suitable for indoor layouts as well as more challenging environments such as damp conditions or outdoor use.

The BD-2 uses diode-based voltage drop sensing to monitor occupancy, introducing approximately 0.7V track voltage drop. For layouts requiring uniform running characteristics, this can be balanced by installing matching diodes in non-detection blocks.

A built-in turn-off delay of approximately 0.5 seconds helps prevent false dropouts caused by dirty wheels or intermittent pickup. The circuit supports daisy-chain wiring for multi-block installations and is designed for simple integration into complex layouts.

The BD-2 can directly drive LED or incandescent block signals with a maximum output of 250mA per channel. The BD-2HD variant increases output capacity to 500mA per channel, making it suitable for large-scale signalling systems. Both versions require a 10–18V DC power supply for operation.

The unit is constructed on a compact 2" x 3" printed circuit board and is compatible with CIRCUITRON PCMT (Printed Circuit Mounting Track) for quick snap-in installation.

Features:

• High-sensitivity current sensing block detection for all common model railway systems
• Adjustable sensitivity control for different operating environments
• Compatible with LED and incandescent 2-colour block signalling

Additional Information:

• Input voltage: 10–18V DC
• Detection method: Current sensing via diode voltage drop monitoring
• Sensitivity: Adjustable (detects approx. 100kO or less across rails at maximum sensitivity)
• Delay: Approximately 0.5 second turn-off delay
• Output rating: 250mA per output (BD-2 standard)
• High-power variant: BD-2HD up to 500mA per output
• Track power capacity: 3A standard / 6A HD version
• Signal compatibility: LED or incandescent 2-colour block signals

Measurements:

• Length: 76.2 mm
• Width: 50.8 mm