Page 11 - November December 2013
P. 11

Signals for the Cascade Division Jonathan Small Part 3 – The relays
When designing the signalling system, I’d decided that I would employ miniature relays. With toggle switches it is possible to display many of the required aspects, but not all. In particular I identified a need to display “all red”, and this required a level of switching I could only achieve with relays. More advanced circuitry employing logic gates would also work, no doubt, for those proficient with such electronics.
Overview of the construction of the relay case
Relay case, front view. The case is ready for installation and wiring in of signals
I used 12V DC miniature relays and mounting bases. These are commonly­available non­latching relays, where the coil defaults to its closed position when the power is off. It opens when voltage is applied, and closes again when voltage is switched off. Unlike a solenoid motor, the coil of a relay is not damaged by voltage being applied continuously.
To make the relay operating circuits I experimented with differ­ ent mounting boards, including home­made printed circuit boards, using copper­coated board and ferric oxide solution to etch the circuit. In the end it proved to be just as efficient (and for me more reliable) simply to fix the relay bases to plain per­ forated board (without copper coating), and solder the various connections using fine insulated wire from below.
I designed the relay modules to accommodate up to four relays each, on equally­sized boards and equally­spaced rows, to be mounted in a sturdy and accessible case that could be bench­ tested thoroughly before installation. Two “simple” modules could be mounted on one board, so there were nine modules to assemble. The mounting board needed some minor work to get the pins of the relay bases to fit through, then I glued these to the boards with Walthers Goo and clamped them overnight. The wiring of each module took under an hour, and once completed and tested, I mounted them in the relay case, and wired the leads into the series of terminal strips in the upper
section of the relay case. I made a test set of signals with con­ necting wires and relay test leads, so I could bench­test the whole thing before installation beneath the layout.
When wiring relays it is important to distinguish the two circuits: the one that controls the relays, from the signal control wires. Again I did this with rigorous colour coding:
To avoid conflicts I found it useful to have two different colours for the 12V DC+ signal feed. Note also that there is no 12V DC–, or common, connection to the signals in the relay case. There is one for the coils of the relays. A common return bus connects all the signals through their terminal blocks under the layout.
Rear view of relay module mounted in case. Here two “simple” modules are mounted on one base, two relays each module
Wiring the relay modules
The various boundaries on the layout are as follows:
 Colour
  Relay circuit
 Signal circuit
   Black
   12vDC+ to relays
      Blue
 12vDC– to relays
  Orange
    12v DC + to signals 1
 Purple
    12v DC + to signals 2
 Red
      Red signal out
   Green
    Green signal out
   Yellow
         Yellow signal out
                     #
  Relay Module
 Block boundary
 Module type
   1
  A
 Interbay (yard) – W. Snohomish
   Simple
   2
  B
 Snohomish – Sno. Tunnel – Monroe
   Compound B
   3
  C
 Monroe – Sultan
   Simple
   4
  E
 Sultan – Gold Bar & Reiter
   Junction E
   5
  D
 Gold Bar – Index
   Simple
   6
  F
 Reiter & Index – Halford
   Junction F
   7
  G
 Halford – Baring
   Simple
   8
  N
 Halford – Snohomish Cut-off
   Simple N
   9
  H
 Baring – Grotto
   Simple
   10
  J
 Grotto – Skykomish
   Simple
   11
  K
 Skykomish – Scenic
   Simple
   12
  L
 Scenic – Cascade Tunnel – Berne
   Compound L
   13
  M
 Berne – Merritt
   Simple
   14
   L
  Merritt – Winton
    Part of L
    december 2013 11
   9   10   11   12   13