Trains4Africa
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Optoisolator Type Block Occupancy Detectors

Optoisolator Type Block Occupancy Detectors

  This page is about a block occupancy detector that uses optoisolators to electrically separate the track from the detector output. Three versions of the detector are shown: Basic, Time Delayed and Direction Indicating.

There is nothing special about these circuits and there are other versions of this type of detector on the web and other places. The circuits have simplified outputs that use a minimum number of parts and can be configured to control just about any load device from LED’s to relays.

With the track isolated from the detector output there can be a great deal more flexibility in building control circuitry for a layout as there is no concerns about connecting systems with different power supplies.

The circuits are built around the LM339 Quad Voltage Comparator chip and the H11AA4QT or 4N35 optoisolators.


Basic Detector

  The following schematic shows the basic detector circuit. this circuit will indicate an occupied block for a train traveling in either direction.

This circuit could be used with DCC control systems.

Basic Optoisolator Block Occupancy Detector Schematic


Time Delay Detector

  The next diagram shows the basic detector but with a built-in time delay via C1 and IC 1B to prevent the indicating LED from flickering during brief drops in current flow. In this circuit the output will turn on instantly by will not turn off until approximately 2 second after a train is no longer detected.

The circuit would be used to prevent relays from “chattering” during brief losses of current flow due to poor current pick-up for example.

Time Delay Optoisolator Block Occupancy Detector Schematic


Direction Indicating Detector

  The next schematic shows a direction indicating version of the basic detector circuit. This circuit will indicate not only an occupied block also the direction of the trains travel.

The circuit uses two optoisolators and detector circuits, one for each direction, to determine which way the current is flowing through the bridge and thus the direction of the train. Dual and quad optoisolator packages could be used for this circuit to reduce the number of devices in the circuit.

This detector will only be operate properly with reversible current type direction control such as conventional power packs.

Optoisolator Type Block Occupancy Detector – Direction Indicating Schematic

  Only one resistor (R7) is needed for the direction LED’s as only one of them can be at a time.


Detector Circuit Operation

  1. When a large enough current flows through R1, the optoisolators LED will turn ON.
  2. When the LED is ON the optoisolator’s output transistor will conduct and the voltage across it will drop below 6 Volts.
  3. This will cause the output of IC 1A to go LOW and the LED will turn ON or the voltage from C1 will be drained off.

    In the Time Delay version, when the voltage across C1 drops below 6 volts the output of IC 1B will go LOW and D1 will turn ON and indicate the presence of a train in the block.

  4. When sufficient current no longer flows through R1 and the optoisolators LED the transistor will turn off and the voltage across it will go above 6 Volts. The output of IC 1A to go HIGH and the LED will turn OFF.

    In the time delayed circuit the C1 will charge and approximately 2 seconds after the train leaves the block the LED will turn OFF.

  5. In the direction indicating circuit the the operation is the same as in the Basic version except that two isolators are used to sense which way the current is flowing through R1. The two optoisolators drive corresponding comparators that have their own LED’s to indicate the direction.

Optoisolator Type Detector With A LM555 Timer Output

  The following circuit uses an LM555 timer to provide both an open collector and bipolar outputs. The LM555 timer is used as an output device for some of the other block occupancy detectors on this site.

Optoisolator Type Block Occupancy Detector – LM555 Timer Output Schematic

Optoisolator Type Block Occupancy Detector – With Crossing Flahsers


Optoisolator Type Detector Notes

  • Resistor R1 sets the minimum current level for detection. A value of 470 ohms will give a detection threshold of approximately 4 milliamps. Higher resistances will give a lower current threshold.
  • The rectifier bridge needs a current rating high enough to handle the maximum short circuit that the throttle can deliver. A rating of four amps will be sufficient for most uses.
  • Four individual diodes could be used but this usually more expensive than a bridge with the same current rating when currents greater than 1 amp are involved.
  • If higher current loads are to be controlled by the detector an external transistor can be added to the comparators output this will allow loads of up to 300Ma. to be controlled.
  • For more information on comparator operation please refer to the Comparator information page at the link shown below.
  • One of the advantages of optoisolater type detectors is that the output of the circuit does not have an electrical connection to the track or its wiring and every detector bridge can be an independent unit.

    This means that the detector could be connected to the NORTH rail in one block and the SOUTH rail in the next. Although this is not the best way to do things it would make it easier to add block indication on an existing layout that could not withstand a lot of wiring modifications.

  • The disadvantage of this circuit is that there is an approximately two volt drop across the the rectifier bridge. But if a lower maximum track voltage and a higher throttle output voltage needed to start the train is acceptable then there should be no problems.
  • None of these circuits is set up to detect a train in the block when there is no track power applied. The first two circuits would give an indication if they were used with DCC systems. The Direction Indicating detector can only be used with conventional power pack systems.