Chapter 5B-1 Page 27
Single Line Acquisition Signals Here is a way to control a single line between two loops. Not prototypical but if you have two operators and the loops are not visible to each then the signals will advise if the single track is being used.
How it Works: The first train to cross the detectors, A or B, will claim the line and turn the signals to Red. The circuit uses two 555 timers that are triggered as the train passes over the spot detectors. Spot A will trigger the top timer in the diagram and it will go into timing mode. However it will not be supplied with timing current until the train passes Spot B and its output (pin 3) goes positive. Now both timers start timing and when timed out reset each other to show green. This is unnecessary with DC power as block switching or the CAB direction controls who can use the line. The signals needed are common cathode. This allows two aspect (separate red and green LEDs) or tri-LEDs (searchlight). The Red leads from both signals connect to the Red terminal, green similarly to the Green terminal. The Red LED drive includes a 270R (270 Ohm) resistor that can be changed to balance the light intensity of the Red and Green LEDs if they are uneven in output. The 560 Ohm (560R) dropping resistors are nominal for 12 volt drive.
When the timer is triggered it can only be reset by the opposite timer being triggered. To stop in the middle and reverse back out would leave the signals at Red until a train passed over the other spot. The delay is adjustable by selection of C1 and C2 capacitors. It should be selected so that -
a train running from A to B will fully pass point B before the signal changes to green. If problems are occasionally experienced and the system is not resetting then the 12 volt power could be routed through a push off switch, to force a reset.
The only other requirement is to train the operators to obey the signals.
