Trains4Africa
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Comparator Based DCC or AC Block Occupancy Detector

Block Occupancy Detector For DCC Or AC

With Isolated Output

  This detector uses the “Diode – Voltage Drop” method and has been designed to use a minimum of parts and use as little current as possible when no trains are detected. The circuit takes its operating power from the booster system supply to the rails.

This circuit has an optoisolated output that can directly control loads of up to five milliamps.

Circuit Design Features

  •   1 Milliamp or less detection current sensitivity.
  •   No sensitivity adjustment needed.
  •   Instant detection with a two second release delay.
  •   3 Amp track current rating.
  •   Operating power taken from the booster system.
  •   Optoisolated output to control external circuits up to 5 Ma.
  •   Uses less than 2 milliamps when no trains are detected.
  •   Uses about 7 milliamps when a train has been detected.
  •   Minimal number of parts required.

NOTE: The Current Sensitivity and Release Delay time can be altered if desired. The light emitting diode, D5, can be left out of the circuit if not needed.


Comparator Based DCC or AC Block Occupancy DetectorSchematic

  This detector was used for the – Grouped DCC Block Occupancy Detectors Installation Test – page located at this site.

There is no need for the detector to sense current flow in both directions as the longest time that a DCC system can be in the wrong polarity for detection is about 12 milliseconds when zero stretching is at its maximum. The time delay and supply capacitors in the circuit can more than compensate for this gap.

The detector will work with variable voltage AC systems if a separate 12 Volt DC supply is used to power the detector. If the AC system is a fixed voltage and carrier system the circuit can be connected like the DCC detector but C1 should be increased to 47uF.