analog direc current and automation

[HEdward]

I've got about a zillion questions on being able to automate my soon to be built HO pike. My locos are all at least 15 years old save on IHC Mogul purchased to pull my collection of Danbury Railway Museum cars. I'd like to be able to run three or four trains on ONE long twisted and folded loop. The layout I'm planning will be six feet by twenty four feet. I'd like to run it as a display mostly since the other half of my basement includes a pool table, bar, game table, sofa, fireplace, etc.

I was thinking of using some sort of superimposed AC curent to detect trains in the blocks and relays to cut off or slow trains in trailing blocks. Will this work? I don't want to burn out any of my locos.

Aside to anyone who remembes my last question...the New Fairfield(CT) Gnome and Garden Railroad failed to break ground as it primary financial backers moved to Womelsdorf PA. I'm hoping to get into garden railroading at my new, larger and nicer home but the HO layout comes frist.

[cacole]

You don't need superimposed AC current, just block detectors and resistor wheel sets.
As you have surmised, a superimposed AC current is going to burn up motors. I don't know of any block control system that needs AC current, so why are you asking this question?

[HEdward]

Block detection...without using IR emitter and detector pairs which would show openly on the track plan? The idea of using a low voltage AC was taken from something I read decades ago on lighting the passenger cars when the train is in the station. Keep in mind that with twins on the way, my budget is extremely minimal. The only reason that the layout will even be started soon is that underneath we will have sixteen cubic yards of storage space. If you saw all my wife's Christmas decorations, you'll nderstand why we need it.

So lets get back to answering the question. If my AC idea won't work, and IR pairs are unsightly, how does one detect blocks even if a train is stopped using only DC?

[Seamonster]

Basically, there's 3 ways to detect trains: IR detectors, visible light detectors and current detectors.

You seem to have ruled out IR detectors because they show. True, but they could be disguised as trackside structures, relay boxes, a pile of ties, bushes, whatever. Advantages: no modification to locos or cars, independent of track power, will detect stopped train. Disadvantages: sometimes difficult to line up emitter and detector, need to have the beam across the track at an angle to avoid dropouts between cars, they stick up beside the track, can detect only the presence of a train, not that the whole block is occupied.

Visible light detectors (photocells). They change state when a train blocks the light from reaching them. Advantages: installed between the rails, so are not very obvious, require only one component as opposed to two for IR detectors. Disadvantages: can release when light reaches them through the gaps between cars unless a delay is built into the circuit, operation may be erratic as the room light changes, don't work well in dim room light.

Current detectors sense the current drawn by the locomotive. Advantages: will detect a locomotive anywhere in the block, minimal modification to the electrical wiring. Disadvantages: detect only the locomotive--once it leaves the block, the detector thinks the block is unoccupied, even though the rest of the train is still in the block, when the locomotive stops, it thinks the block is unoccupied because there is no longer any current flowing.

The disadvantages of the current detectors can be overcome, however. If you are running lighted passenger cars, they will draw current, thus detecting the rest of the train. You can install metal wheelsets with insulated axles on your cars and attach a very tiny resistor to the axle, connected to the wheels on opposite sides, which will draw current all the time and keep the detector activated. You could also paint a stripe of resistive paint along the axle instead of installing a resistor. You don't need to have every car with a resistor--just the first and last ones. If you always run freight trains with cabooses, then equip your cabooses with a resistor wheelset. The locomotive will initially trigger the detector when it enters the block, and the caboose will keep it triggered after the locomotive leaves the block until the caboose leaves the block. If you stop your train in the block, the caboose will keep the detector triggered.

Any detection system will require some electronics. If you don't feel up to "rolling your own" when it comes to electronics, there are a number of manufacturers who make detection systems. Check the Walthers catalogue. If you want to build your own, there have been numerous articles in Model Railroader on detection circuits, and many books on electronics have detection circuits in them, including Peter J. Thorne's series of books.

I would say, as a rule of thumb, if you need to know if a train is in a certain spot, use an IR or photocell detector, depending on how good the room lighting is. This type of detector is best for monitoring fouling points in yards and hidden trackage or detecting trains in tunnels. If you need to protect an entire block, then I'd say to use some form of current detection.

Hopefully I haven't thrown too much stuff at you in this post, and that it has helped you in some way. There are lots of people knowlegable in electronics in this forum and I'm sure you'll get some good ideas from them too. Good luck, and let us know how you're progressing.