Turnout Anti-Derail Protection Circuits


  The circuits presented here will prevent turnouts from being thrown while a train is on a switch, ideal where the switch cannot be seen by the operator. They use phototransistor sensors to detect the train and have a short time delay to compensate for gaps between cars. The circuit can also be used to protect crossovers and ladder tracks with the addition of more sensors.

  There are two versions, the first uses push buttons and is for twin coil switch machines. The second uses a toggle switch and can be adapted for other types. The main difference between the circuits is that the second version contains a 'memory' function that will allow delayed throwing of the turnout until after the train has cleared the switch.


Push Button Type Protection Circuit with 'Relay' Output

Push Button Type Protection Circuit Schematic with Relay Output


Operation for Push Button Type Protection Circuit

  The Push Button version of the protection circuit simply disconnects the switch machine from its power source when ever a train is on the switch. This circuit could be used for Twin Coil type machines

  The LED in this circuit will be ON when the switch is protected

  1. Anytime one or more of the phototransistor sensors is covered by a train the output of IC 1a will go LOW and drain the voltage off of the 10uF capacitor.

  2. The output of IC 1b will then go LOW and the 2N3906 will conduct and energise the relay coil.

  3. When the relay is on the switch machine power will be disconnected from the protected turnouts push buttons, thus preventing it from being thrown.

  4. After the train has cleared all of the sensors output of IC 1a will go HIGH and the 10uF capacitor will charge through the 1 Megohm resistor.

  5. When the voltage across the capacitor reaches 1/2 of the supply voltage the output of IC 1b will go HIGH and the relay will be turned off. (The time required to charge the capacitor to this voltage is 6 to 8 seconds.)

  6. The power is again connected to the switch machine and it can be thrown normally.

Push Button Type Protection Circuit with 'SCR' Output

Protection Circuit with SCR Output Schematic


Operation for SCR Output Protection Circuit

  This version of the protection circuit replaces the mechanical relay with an SCR, (Silicon Controlled Rectifier). The SCR will allow easier 'PCB' construction of the circuit and may be cheaper to build as well.

  The advantage of using a relay versus an SCR for the output is that the switch machines will still operate if the protection circuit has no power or fails.

  The LED in this circuit will be OFF when the switch is protected

  1. Anytime one or more of the phototransistor sensors is covered by a train the output of IC 1a will go LOW and drain the voltage off of the 10uF capacitor.

  2. The output of IC 1b will then go HIGH and the and no current will flow through the indicator LED or the optoisolator's LED.

  3. When no current is flowing through the optoisolator's LED its output transistor will not conduct and the gate of the SCR will not be able to trigger. When either of the push buttons is activated no current will flow to the coils because the SCR cannot turn on.

  4. After the train has cleared all of the sensors output of IC 1a will go HIGH and the 10uF capacitor will charge through the 1 Megohm resistor.

  5. When the voltage across the capacitor reaches 1/2 of the supply voltage the output of IC 1b will go LOW, the optoisolator will be turned on and the SCR can be triggered. (The time required to charge the capacitor to this voltage is 6 to 8 seconds.)

  6. The switch machine can again be thrown normally.

Toggle Switch Type Protection Circuit

Toggle Switch Type Protection Circuit Schematic


Operation for Toggle Switch Type Protection Circuit

  The Toggle Switch version of the protection circuit places a flip flop type of memory circuit and a relay between S1 and the switch machine. The flip flop will remember the last position of the turnout while the sensors are covered by a train and prevent it from being thrown.

  The LED in this circuit will be OFF when the switch is protected

  1. Anytime one or more of the phototransistor sensors is covered by a train the output of IC 1a will go LOW and drain the voltage off of the 10uF capacitor.

  2. The output of IC 1b will then go HIGH. This will disconnect the center terminal of S1 from the circuit common.

  3. With S1 cutoff from the common it will have no effect on the memory section of the circuit, IC 1c, and the relay cannot change states if S1 is changed.

  4. After the train has cleared all of the sensors output of IC 1a will go HIGH and the 10uF capacitor will charge through the 1 Megohm resistor.

  5. When the voltage across the capacitor reaches 1/2 of the supply voltage the output of IC 1b will go LOW and S1 will again be connected to the circuit common. (The time required to charge the capacitor to this voltage is 6 to 8 seconds.)

  6. If S1 has not been moved when the time delay is runs out the turnout will stay in its original position. If however S1 has been changed the turnout will automatically change to the new position.

Phototransistor Sensor Positioning

Sensor Position Diagram

  The diagram above shows possible sensor positions for a few turnout arrangements.


Protection Circuit Notes


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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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