Model Railroading > Signaling (Part 17) – Turnout Controller – The Design

This is Part 17 of a series on Signaling.  (Parts 1 thru 16 were posted earlier.)
 
The next PCB on the drawing board will be the interface between the LCC network and the Turnout.  There are several features that it will provide.
   -Control the motor in the MP10 “switch machine”.
   -Changes the polarity of the Frog based on the position of the Turnout (thrown for the mainline, or the siding).
   -Sends the position of the Points to the Switch LCC to confirm the Turnout was thrown.
   -Collects the inputs of up to 4 Detectors.
   -Enable/Disable Test LEDs to expedite initial installation, and troubleshooting.
 
Servos
I considered using servos as the “motor” to throw the Turnout.  However, none have built-in relays, and would require a set of external micro-switches to act as relays.  Though the brackets to align the micro-switches with the servo can be 3D printed, it made the design more complex and bulkier.  Moreover, the servos cannot run on simple DC power.  They require the power to be Pulse Width Modulated (PWM).  This requires a whole other piece of specialized equipment.  An Arduino has several PWM output pins, so that is the ideal solution for a PWM source.  The issue is the Arduino + servo + micro-switches will cost more than the MP10.  Though I like the idea of using a servo, the MP10 was a more turn-key solution. 
 
Poor Track Record
Several TrainOrders members have reported that they purchased and implemented the Walthers switching system, and the onboard servo fails after about 30 cycles.  This gave me great pause, and caused serious 2nd thoughts about using a servo to thrown Turnouts.
 
Short Fry
The internal relays in a switch machine (such as a Tortoise) are routinely used to change the Frog polarity.  However, if there is a short, and there are a lot of locomotives in the block, the 1A rating of the relay’s contacts may be exceeded.  It is conceivable that if the power district has a 5A breaker, that the breaker won’t trip during a short that is less than 5A, which will fry the contacts in the switch machine.  The solution is to use external relays with 10A-rated contacts.  The relay in the switch machine sends commands to the external relay.  The switch machine relay contacts never deal with the short directly, and the (higher-rated) contacts of the external relay will be able to handle any short thrown at it.
 
Relay Race
Relays pre-mounted to PCBs are popular with the Arduino-crowd, since it is a relatively easy way to have the Arduino control equipment with much higher voltage and amperage needs.  Initially, this was going to be the relays used.  However, they require 10 hard-wire connections.  There was no clean way to make them plug-n-play connections, and I want to keep connections requiring set screws to a bare minimum.
 
The Traces on a PCB are electrical connections between components.  They function like a wire.  If the relays were mounted to a PCB, Traces would make the connections to the relays and would eliminate 10 hard-wire connections.  I’m IN! 
 
The automotive industry has many applications that use 12v DC relays with 10A+ ratings.  This means they are more common, and therefore cheaper, than a 5v DC relay.  However, this requires an external 12v DC power source, since the LCC Network only provides 5v DC.  Moreover, the LCC 5v DC is only 0.5A, which is too wimpy to be driving a bunch of relays.
 
No Direct Drive
It would be nice if the I/O Port Pins (from the Switch LCC) that control the Turnout throw, could drive the MP10 directly.  This would eliminate the need for the 2nd relay all together.  The issue is the Switch LCC provides 5v DC, but the motor in the MP10 needs 12v DC.  Hence, the need for a relay.  Since the MP10’s motor only draws 150mA at the most, it doesn’t require a relay with 10A-rated contacts.  However, relays with 1A ratings are less common, and therefore as expensive, if not more, than the 10A relays.  In order to simplify the parts inventory, both relays will be (at least) 10A-rated. 

Dave
 



Edited 4 time(s). Last edit at 09/12/23 10:25 by tmotor.

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I got to hand it to you Dave. Your going way deeper then I'd ever want to go for a hobby, but it IS fascinating!

Greetings Chris!

> I got to hand it to you Dave. Your going way
> deeper then I'd ever want to go for a hobby, but
> it IS fascinating!

LOL!  Glad you find them of value.  :-D

It is a hobby within a hobby.  I have always been interested in creating circuits and PCBs.  Now that I have a reason to design some, I've gone pretty far down the rabbit hole...

When I investigate a new (to me) technology, part of my process is to document ideas along the way.  When I see them on the screen, I no longer have to "remember" them, but will be thinking of how to improve on it.  After a few days, a modification comes to mind, and then that gets documented.  For my TO posts, I take a summary of the ideas and share them.  Frankly, I wasn't sure how much interest this would be to folks, but has been consistently over 400 views.  I'm happy to share my journey with others.  If it helps some folks to avoid some of the pitfalls I have encountered, so much the better!

Take care and God bless!
Dave