Posted on Feb 7, 2014

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

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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. 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 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. ) 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. 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. When the voltage across...

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