When the LED is ON the optoisolator's output transistor will conduct and the voltage across it will drop below 6 Volts. When a large enough current flows through R1, the optoisolators LED will turn ON.Only one resistor (R7) is needed for the direction LED's as only one of them can be at a time. Optoisolator Type Block Occupancy Detector - Direction Indicating Schematic This detector will only be operate properly with reversible current type direction control such as conventional power packs. Dual and quad optoisolator packages could be used for this circuit to reduce the number of devices in the circuit. 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. ![]() This circuit will indicate not only an occupied block also the direction of the trains travel. The next schematic shows a direction indicating version of the basic detector circuit. ![]() Time Delay Optoisolator Block Occupancy Detector Schematic The circuit would be used to prevent relays from "chattering" during brief losses of current flow due to poor current pick-up for example. 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 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. Basic Optoisolator Block Occupancy Detector Schematic This circuit could be used with DCC control systems. this circuit will indicate an occupied block for a train traveling in either direction. The following schematic shows the basic detector circuit. The circuits are built around the LM339 Quad Voltage Comparator chip and the H11AA4QT or 4N35 optoisolators. 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 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. There is nothing special about these circuits and there are other versions of this type of detector on the web and other places. Three versions of the detector are shown: Basic, Time Delayed and Direction Indicating. ![]() This page is about a block occupancy detector that uses optoisolators to electrically separate the track from the detector output. Optoisolator Occupancy Detector Optoisolator Type Block Occupancy Detectors
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