DIY electronic interlocking suggestions?

Hi all,
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
David Belcher
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I would consider an old computer, SSI from Graham Plowman
and RPC hardware from MERG member Gordon Hopkins
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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
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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.
- Nigel
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Nigel Cliffe
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.
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Luke Briner
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
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for lots of useful model railway circuits including how to do route wiring for point control.
Terry Flynn
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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
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