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Linked Departure signals Flashing yellow signal aspect
Chapter 5c-1 Page 31 Flashing Signal Aspects
Some signal light aspects are steady and some are flashing. Why?
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Well, by making one of the lights What circuit do I use ? If your signals have a flash it is effectively adding a 4th common wire that is aspect to a three lamp head. The positive - use PNP most common is an advance If the common wire is approach light (Yellow flashing) and negative or ground - use this informs the engineer that the NPN second signal ahead is at Red and But not on signals with a speed must be reduced to enable a common dropping resistor complete stop. Necessary in these in the signal. days of long heavy trains. While a flashing divergent advance approach The Green Flashing LED would indicate a turnout ahead and to be prepared to stop at the second This 5mm diameter signal. In some systems it can device contains an IC indicate that the train is approaching oscillator circuit in series an area (junction/loop etc.) where the with an LED. This allows it approach normal speed is changed to an approach limited (higher) speed. I would expect there to be a set of high speed turnouts ahead. A to be driven from a very wide voltage range from 3 to 15VDC without any external components required. The LED itself Red flashing or Marker board produces a bright output of flashing allows a proceed at reduced 500mcd (typical) making it speed which over-rides the stop and suitable as a very notice stay condition of a normal Red ﾭ able warning lamp in aspect. alarm panels, car and boat I can find no real information on dash boards, etc even in dwarf signals flashing but as they are daylight usually only 2 aspect it perhaps doesn't apply although I believe SP flashed Green under some conditions. The series 6K8 ohm resistor reduces the brightness as it is not being used as an indicator in this circuit. Interestingly in the United Kingdom a flashing yellow would indicate that a train is to take a diverging route ahead with a lower line speed than the main route, indicating to the driver to slow the train down in time for the speed limit of the diverging route as opposed to slowing even more in preparation for a stop signal. The micro switch shown in the schematics would typically be connected to a turnout throw bar or possibly be a block detector relay.
Reference these web sites for more signaling information :- http://www.railroadsignals.us/ - for a reference to Color Position Lights. http://www.davros.org/rail/signalling/articles/junctions.html - U.K. systems http://www.railroadsignals.us/rulebooks/rulebooks.htm from a selection of railroads
LED signals come in several configurations. The Common Anode type has all the positive LED leads connected and they will be wired to the 12v (or positive source). This could be via a common dropping resistor. The Common Cathode type has all the negative leads connected and wired to ground (negative voltage) or could have a common dropping resistor that is wired to ground. Other types are less common but there can be signals where all the LED connections are individually made. There will be 6 wires going to the signal. Using a common dropping resistor is an acceptable practice, but LED's of different color have a different brilliance and an individual dropping resistor for each LED is preferable, especially if they are run at a lower intensity for more realism. They can be used on 4 aspect (2 yellow) signal heads where both yellows may be flashing together.There is one circuit to be used with common anode (PNP) and another for common cathode (NPN) signal systems. Both circuits do the same thing but with reversed polarities. The LED is activated by the signal control system as normal and then when the ‘flash’ switch is on, the flashing LED will activate and turn on the transistor which shorts out the LED so it is extinguished. The flash rate is controlled by the flashing LED part of the circuit and is not adjustable. They are spec'd as between a 0.5 second and 2 seconds period.
Components,
1 x 5mm Flashing LED, (any color) Transistor NPN 2N2222A or PNP 2N4403 Resistors, 6,800 (6K8) Ohm 2,200 (2K2) Ohm 220 (220R) Ohm Note the copper track is cut at position row 1, hole 6 ( X on the diagram) The 8 by 11 hole PCB could be expanded to allow for plugs to be installed. In fact you could make the PCB by cutting and drilling a blank piece of copper clad board into 8 strips with row one cut in the middle. Drill 1mm (#60) holes where needed. Mount the components and solder. Note the LED is polarized, get the "+" (longer lead) in the right place or it will destroy when powered on. The transistor leads need to be spread a little so that the emitter is one hole away from the base lead. The 220R resistor is connected to the signal to be controlled. The GR-Y positions shown can be altered so that if the Green is to be controlled then it is wired down row 5. It would even be possible to have two aspects flashing by putting a second resistor from the transistor collector to another signal line. Either would be flashed depending on which was active. Or if a two yellow (4 aspect) head then both yellow lines could be controlled as they would both be active at the same time.
Installation:
The circuit to control a 3 aspect LED signal is inserted into the 4 wires running to the signal head. A 5th wire and switch for a control voltage is added to turn on the flashing aspect when required. If the flash is not activated the signal LED works as normal. The switch could be manual, a microswitch actuated by the turnout position or a control voltage sent from the signaling system. The circuit may be used to control any of the aspects as required.
