I'm contemplating having basic interlocking/route setting on my
smallish terminus-to-fiddle yard layout rather than just "electric
pencil" point operation and ordinary switching for the signals (mix of
2 & 3 aspect MAS, some with call-on lights) - any suggestions for good
books or online resources? I was planning to use mainly solid-state
stuff based on logic gates, maybe to mimic OCS or NX installations, as
it's cheaper to build than an all-relay system. Thanks in advance for
I think Roger Amos' book on model railway electronics has some stuff in it.
However, these days, I would jump to something digital, and seriously
consider DCC solutions as well (because people have built in the bits you
need already). An old computer is cheap (since new ones are well under
£300), and contains more logic capabilties than you'll every need.
Would the stuff on
be of use ? Particularly
the signal logic section. It does basic simple interlocking, not full-blown
everything (though you could program your own as an add-in I guess :-) ).
Add that to a DCC system. If not using DCC for the locos yet, investigate if
something low-cost like the SPROG unit could do the control for you as well
as the computer interface via USB.
It would be fun to create the interlocking in electronics, the only obvious
setback is that the interlocking would be hard-wired once created and it
would be more than a trivial task to, for instance, add another turnout
interlocked to others.
You will need to decide whether you want to interlock in a simple way using
just a turnout and its associated signal or whether you want some
super-duper interlocking with everything linking to everything else (more
easily done in SSI or other software).
There is also route setting which is another level above interlocking but is
related where you can define routes which operate various turnouts in
sequence although the individual turnouts will only switch if there is no
corresponding conflict with other routes. It is worth noting that in order
to avoid circular dependencies with route setting and interlocking that all
turnouts and signals should be dependent on signal aspects only, not on
other turnouts. If you made turnouts dependent on other turnouts, signals on
signals etc. then it would be easy to create a situation where everything is
locked out waiting for everything else.
Depending on the turnout motors you are using and how they are operated, the
logic matrix can simply be cascaded CMOS 'AND' gates which can add "aspect
1" AND "aspect 2" AND NOT "aspect 3" between the point stud on the control
panel and the driver power transistor that will pass CDU current to the
turnout motor. Use hex inverters to swap polarity of inputs if you do not
have available inputs for each signal aspect you are dependent on.
If you do not have signals as may be the case in a fiddle yard, you can
mimic simple semaphores with SR Flip Flops. The beauty of these is that
there are separate inputs to set the signal to each pretend aspect and there
are two outputs (1 an inverted form of the other) which you can use to drive
your interlocking logic for your turnouts.
Wish I had a light pen to draw something.
My experience using 2 aspect signals is most of the interlocking can be
achieved with multi pole switches if you use tortoise point motors.
These come with a double pole double throw switch included and is in
effect a relay. Although the initial cost seems high you get the
advantage of using one set of contacts for your frog wiring, the other
is free for interlocking logic, enough for simple 2 aspect signals in
some areas. However you are correct in avoiding relays generally if you
have to pay for them, and for 2 aspect signals my preferred circuit is
based on the 555 timer chip and my 3 aspect circuit uses a cmos 4001
and a led driver chip to drive 2 signals. The 2 aspect circuit can
easily be changed to operate from momentary push buttons or
automatically using light dependent resistors on the input. I have
added the wiring diagrams on to my web page for those interested. To
simplify wiring you could use a timer to run through a time sequence
for a 3 aspect signal, however I have not tried this, it's just an
armchair idea at the moment. Also check out
for lots of
useful model railway circuits including how to do route wiring for
HO wagon weight and locomotive tractive effort estimates
DC control circuit diagrams
HO scale track and wheel standards
Any scale track standard and wheel spread sheet