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Model Railroading > Signaling (Part 18) – Turnout Controller – Test LEDs

Date: 09/18/23 06:08
Signaling (Part 18) – Turnout Controller – Test LEDs
Author: tmotor

This is Part 18 of a series on Signaling.  (Parts 1 thru 17 were posted earlier.)
I’m pretty big on being able to make troubleshooting as easy as possible.  That way I will not be as reluctant to perform it when it is needed.  If I can push a button, and some LEDs can let me know what the circuit is thinking, hopefully that will narrow the scope of what needs attention.  Manufacturers are very reluctant to incorporate Test LEDs since it will significantly increase the complexity and cost of the PCB.  However, since I’m the customer AND the manufacturer, Test LEDs are a design priority.
It is possible to add Test LEDs to the MP10 Buddy (similar to what was done with the Switch LCC Buddy).  The circuits are all low voltage, however they are not trying to just drive LEDs.  This means their voltages will range from 0v to 5v, or 0v to 12v, depending on the function.  It would significantly increase the size of the MP10 Buddy to have room for 9 LEDs, plus the silk screen descriptions.  Instead, the Test LEDs will be installed on the Turnout Controller.  The wires going to the MP10 only tell part of the story.  Having the Test LEDs on the Turnout Controller instead will be more of a “one stop shopping” experience for gathering information.
The Test LEDs will be grouped by function.  That way the specific problem can be addressed.  For example, if the Turnout Points are failing to move, start with the group of circuits that send and receive the commands to throw the Turnout.
There are 5 basic components on the Turnout Board.
  1) The 10-Pin connector, which receives the Ribbon Cable from the Switch LCC Node.
  2) The relay that drives the MP10’s motor, which determines whether the Points are thrown for the Mainline or the Siding.
  3) The relay that controls the Frog polarity.  The relay receives commands from the MP10, based on the position of the Points.
  4) The RJ-12 Quad Jacks for the Detectors.  They send occupancy data to the Switch LCC Node.
  5) A small circuit to reduce 12v DC to 5v DC.  This provides power for the Detectors, as well as the circuits that sends the position of the Points to the Switch LCC to confirm the Turnout was thrown.
Each of these will have a set of Test LEDs grouped near the component.  Next to each LED will be a description of what condition is indicated by the LED.  This will necessarily increase the amount of real estate needed on the PCB, driving-up the cost.  However, the combination of the LEDs and the associated descriptions will nearly self-document the troubleshooting process.   (Of course, I will need to restrain myself from verbose descriptions, lest the PCB be larger than the layout.)
Tail Wagging the Dog
The concept of Test LEDs mounted to a PCB is easy to wrap my head around.  However, getting those LEDs to illuminate based on the presence of power in the circuit was more complex than anticipated.  The 5v lines were fine.  Adding a resistor was easy enough.  That takes care of the lines sending data to and from the Switch LCC 10-Pin connector.  The 12v lines for the MP10 motor required a Voltage Regulator to reduce the voltage to 5v, and then a resistor.  So far so good. 
Then there are the Frog-related circuits.  They have DCC track voltage, in the form of the North and South Rails.  These Rails will switch polarity based on the direction the train is running.  The Frog could be either polarity at any time.  The trick was to not only determine how to display their polarities, but how to implement it as a circuit.  (At one point I considered using some needle meters, with straight-up being “0 Volts”, negative to the left, positive to the right.  The meters would indicate the polarity of each Rail, as well as the Frog.  Though I LOVE meters, I wanted something more compact.)  A pair of LEDs per Rail/Frog to indicate the polarity would work, but implementing the circuit is easier said than done.
Mountain Out of a Molehill
The number of components for the Test LEDs soon eclipsed the original number on the Turnout Controller.  The design became much more involved and complex.  The good news is the Eagle software can run the Traces on the PCB no matter where I place the components.  This is a HUGE time saver, vs having to manually run the Traces (and NOT forget any, or run the Trace to the wrong Pad).  As long as the schematic is drawn correctly, I am confident Eagle will implement the Traces on the PCB.
The Usual Suspects
When it comes to trackwork, the majority of issues occur at a Turnout.  The same can be said of wiring issues.  Throw-in Signaling, and things can spiral out of control in short order.  The Test LEDs will (hopefully) make the initial installation of a Turnout easier and more accurate.  In the event troubleshooting is necessary, they provide a wealth of information to help narrow-down the area that is misbehaving.  In theory, this means less time will be needed to chase-down wiring gremlins, and leave more time to run trains.  :-D
Mind Reading
When I write a piece of computer code, I add a TON of display statements to let me know what each variable is set to at various places in the program.  This allows me to confirm the logic is flowing properly, or not.  Similarly, the Test LEDs are the hardware version of this approach.  If power is present on a line (or not) the LED will give a visual indication.  A quick scan confirms all is well, or that something is amiss. 
Granted, the same information could be determined with a multi-meter and some probes.  However, it requires memorizing which pair of Pads on the PCB will generate the desired reading.  It also risks the probe bridging between 2 adjacent Pads, which might cause a short, or fry a component.  The Deal Breaker is it will take me 50 times longer to perform troubleshooting with a multi-meter.  Figuring-out the circuits necessary to add the Test LEDs takes time.  However, I would rather spend the time to design it now, instead of spending even more time being frustrated by seemingly endless troubleshooting in the future.
Switching Transistors
When it comes to routing power, I’m comfortable using old-school relays with internal contacts.  I can imagine the small set of contacts moving as the coil is energized, confirmed with a satisfying “clack”.  However, one of the most revolutionary inventions of the 20th Century is the transistor, which acts like an ON-OFF switch.  In relay terminology, a transistor is a Single-Pole, Single-Throw (SPST) device, which can only turn a circuit ON or OFF.  When no power is applied to the transistor, it is OFF.  In relay-lingo, this is a Normally Open (NO) state, where there is no connection and the circuit is dead.   One of the advantages of the transistor over a relay is there are no moving parts.  It is Solid State, will last much longer, and generally, will cost less.
There are 3 different flavors of voltage on the Turnout Controller.  They can’t all be routed to a single push-button to enabled/disable the Test LEDs.  However, a single push-button can command 3 different relays, each relay controlling a separate voltage.  It occurred to me that this would be an opportunity to try substituting transistors for the SPST-NO relays.  This will be an interesting experiment, since I have not attempted this before.  As usual, the circuit will be tested on a breadboard before incorporating the design into a PCB.  If this works as advertised, it will be a welcome addition to the list of components available as design options for future circuits. 

Edited 3 time(s). Last edit at 09/19/23 10:18 by tmotor.

Date: 09/18/23 06:09
Re: Signaling (Part 18) – Turnout Controller – Test LEDs
Author: tmotor

Frog Polarity
The Test LEDs for the Frog was the most challenging.  The Voltage Regulator needs to have a connection to both the (+) Rail and (-) Rail, so it can Reference the total voltage it needs to reduce to get 5v.  This can be easily determined between the North and South Rails (since they are opposites).  However, for the Frog it is less predictable, since it can be either polarity at any time.  This required some fancy footwork to accomplish. 
The theory is that the North Rail will only be (+) or (-) at any one time, and the South Rail will always be the opposite polarity (of the North Rail).  Attach the Frog’s Voltage Regulator to the (-) for BOTH North and South, since only one will be active as the (-) Rail.  Diodes ensure the circuit won’t feedback (which might cause a short) to the Rails.
Of course, this is still theory at this point.  It needs to be tested by implementing it on a breadboard prior to committing the design to a PCB.

Edited 2 time(s). Last edit at 09/18/23 06:51 by tmotor.

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