The Crossing Diamond Protection circuit is designed to prevent collisions where one rail line crosses another. Using phototransistor sensors to detect a train the circuit will disconnect the track power from the line that crosses the occupied line.
The tracks are protected by two groups of phototransistors, Q1 – Q4 and Q5 – Q8. When any or all of the sensors in a group is covered by a train the corresponding timer will be triggered, the relay for the opposite track will turn on and disconnect the power from that track.
The 8 volts referred to in the section below is the Reference Voltage for the circuit. This voltage determines the level at which the comparators will switch states and the length of the time delay for a given timing capacitor value. This voltage is determined by the 4.7K and 10K ohm resistors and is approximately 2/3 of the supply voltage.
A train enters the protected section of TRACK 1:
1. When a train covers any or all of the TRACK 1 sensors the voltage at the MINUS input of IC 1A will go above 8 volts. This will cause the output output of IC 1A to go LOW and drain the voltage from the 25uF time delay capacitor at its output.
2. When the voltage cross the 25uF capacitor is below 8 volts the output of IC 1C will go LOW and Q1 will turn ON. This will energize the TRACK 2 power relay and disconnected the gapped section of TRACK 2 from the track power.
3.0Also, when the output of IC 1B is LOW the sensors for TRACK 2, (Q4 – Q8), are effectively shorted to common through D1 and the output of IC 1B. This deactivates the TRACK 2 sensors and a train entering this track will have to stop and cannot take power away from TRACK 1.
4. After the train has left the crossing and all of the TRACK 1 sensors have been uncovered the output of IC 1A will go HIGH.
5. When the output of IC 1A has been HIGH continuously for approximately 15 seconds the voltage across the 25uF timing capacitor will reach 8 volts and the output of IC 1B will go HIGH and the transistor Q1 will turn OFF. The TRACK 2 power relay will be released.
6. The track power is now reconnected to the controlled section of TRACK 2 and the Diamond Protection circuit is ready for the next train on either track.
7. If a train enters the controlled section of TRACK 2 then the above action will occur but for IC 1B and IC 1D instead. The power will be disconnected from TRACK 1 and the Q1 – Q4 phototransistor sensors will be disabled.
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