Rail Crossing Diamond Protection Circuits

  The Crossing Diamond Protection circuits are designed to prevent collisions where one rail line crosses another. Using transformer type BODs or phototransistor sensors to detect a train, the circuits will disconnect the track power from the line that crosses the occupied line.

  The circuits have time delays to allow short trains to fully clear the diamond and to compensate for gaps between cars.

  This circuit are meant to be a safety devices only, therefore They do not slow the train gradually but stop them dead by removing the power to the track.


DCC BOD - Diamond Protection Circuit

  The circuit in the first schematic is for a diamond protection circuit that used two transformer type DCC Block Occupancy Detectors.

DCC BOD - Diamond Protection Circuit

  NOTE: if there is a car in the protected section but it is not drawing any current the diamond will not be protected. With photosensors, if a car is on the diamond it will be seen by the circuit at all times.

  The train must have detectable cars at least at both ends and the block must be as long as the longest train.


LM339/LM556 - Diamond Protection Circuit

  See the 4 Photo-Detector page for another light activated - Diamond Protection circuit.


L. M. R. G. Diamond Protection Circuit

  The original Crossing Diamond Protection circuit is used in several places on the London Model Railway Group's - O Scale layout. The first installation was at the main line and trolley crossing. In this location the main line cab operators cannot see a trolley approaching until it is just inches from the diamond.

  The circuit shown here is a later and simpler version of the protection circuit. Included are signal lights, these are optional but do indicate who has the diamond.


Rail Crossing Diamond Protection Circuit Schematic


Circuit Operation

  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. Also, 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.


Signal Lights for The Diamond Protection Circuit

  The mainline and trolley crossing for the London model Railroad Group also had a set of larger than scale signal lights mounted well above the track and near the diamond. One set of lights can be seen from the mainline cabs and the other from the trolley station.

Signal Lights Diagram

  When no trains are in the controlled sections the GREEN lights will be on for both tracks. If TRACK 1 has the diamond he will get a YELLOW light and TRACK 2 will have a RED light. The opposite is true for when TRACK 2 has the diamond.

  The lights have coloured lenses and are about 1/2 an inch in diameter. While they are not used for operational control the lights indicate who has the diamond so that the other train can slow down.

  The operation of the signal lights is very simple. If none of the track power relays is energized then Q4 will conduct and the GREEN signals will be lit. When a relay is on current can flow to the base of Q3 or Q5 and the corresponding RED or Yellow signals will be lit. When either Q3 or Q5 is conducting the base current for Q4 will be drawn away through D3 or D5 respectively and Q4 will be turned off.


Phototransistor Sensor Placement

Protection Circuit Sensor Placement Diagram

  The phototransistors are mounted between the rails and through the roadbed. The groups for each track are connected in series as shown in the schematic.

  The "SLIDE DISTANCE" shown on the sensor placement diagram is the distance that the longest locomotive consist would take to stop when the last engine has entered the gapped section of track. If this distance is too short the lead engine could coast or be pushed onto the diamond even though the track power has been disconnected.


Circuit Notes


Bonus Section

  This schematic is equivalent to the original Diamond Protection circuit used at the London Model Railway Group's club layout. It is functionally the same as the circuit shown above but slightly more complicated

  The major difference between this circuit and the one above is that in the newer version the LM 555 timer chips have been replaced with two LM 339 comparator sections and a different method has been used interlock the circuit when the diamond is occupied.

L.M.R.G. Diamond Protection Circuit Schematic


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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

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21 October, 2009