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.
Detector Circuit Operation
- When a large enough current flows through R1, the optoisolators LED will
turn ON.
- When the LED is ON the optoisolator's output transistor will conduct and
the voltage across it will drop below 6 Volts.
- 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.
- 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.
- 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.
http://home.cogeco.ca/~rpaisley4/OptoBOD.html
