Building an electrically isolated swing gate

Well, it’s been a busy summer! I managed to quit my day job for a few months and happily self-employed! Only thing was, that left me with very little time to do railway things – Plus, shipping cheap electronics from China took until earlier this week!

My problem was this: Grand Forks, the western end of the railway, was supposed to be a good place to turn the wayfreight and whatnot. In practice, it messed up all operation in the yard and adjacent sawmill! The solution was simple, I already had staging designed. The issue was getting there, as it would have to zip under my duckunder. Time to build a swing gate!

Mechanical stuff first:

Swing gate design

The gate is simple enough. It’s a pair of 2x4s mounted in an L shape with curved plywood on either side to brace. Two hefty hinges anchor it to another 2×4 anchored to the floor and benchwork above. I added a few touches to make it operate smoothly. The hinge side is really simple, but the other end has some mating features. The subroadbed is 3/4″ plywood with a cut made at about 30°  and fastened to the gate and the far side of the benchwork – This gives it a good striking surface to stop at with the rails just in line. The bottom has a slider of hardboard with the leading edge sanded down that sits on a projection of plywood, lubricated with graphite from a pencil. Super tight, and super smooth! Finish it off with a latch of your choice – I used a suitcase latch that allowed little movement, but that’s not the way it’s supposed to close. You can probably find something better if you look hard enough. It is designed to be closed during operations, and can only be unlatched from inside the layout room.

20151002 4

Video showing operation at that end:

To finish it off, I laid track over the bridge, left it to cure overnight, and used a dremel with an ultra thin cutoff wheel to cut it at the gaps – A 90° cut at the hinge end and a 45° cut at the other end. This allows the rails to meet very precisely and trains roll uninterrupted.

On to the electrical:

Since it’s hidden trackage as you can’t see it from Grand Forks or west staging, I wanted some electrical protection so trains wouldn’t run in accidentally. To this end, here’s the circuit diagram (for those who can read it!)

Swing gate electrical diagram

The effect is this: When the gate is open, the limit switch (as shown in the above photo) is open, leaving the relay unpowered and in normal position. This leaves power only going to a pair of red LEDs, which are automotive T10 lights positioned at either end of the gate. The power for the red (north) rail on the gate, as well as a 50cm section on either side, is routed through a normally open contact that energizes it only when the gate is closed and the relay is energized. At the same time, the contacts for the red lights open and the green lights close. The black rail is always powered because it’s much easier that way (I’d need to buy another relay otherwise) and you only need to open one side to stop a train.

Gate open:

20151002 6

Gate closed:

20151002 5

This means there’s quite a few wires running through the duckunder above, I think 9 conductors total! Here’s the central panel with the relay – Can you tell I did it in a shorter time than it took to write this post?

20151002 1

Very important: The wires going from the terminals into the swing gate are stranded. Don’t use solid, as it is very likely to break with use.

Swing Gate in operation:

My apologies for any repetition you notice between the video and this article!

Not shown in the video are these two views. It’s important that operators do not back their train into an open gate, since the isolated section is only a few carlengths from a scale 900′ drop. To that end, I made the lights very visible from both entrances to the bridge.

Grand Forks looking toward west staging:

20151002 2

West staging looking toward Grand Forks:

20151002 3

As you can see, the lights are very visible. This could be disruptive in photos, as you can well imagine! If you’ve read my earlier posts, you may recall that the entire power for the layout is controlled by a switch beside the lights, so the whole system comes on automatically – DCC, turnouts, accessories, and swing gate. This is why I used the relay I did, what’s known colloquially as an ice cube relay. You can simply pull out the relay and all circuits using its contacts will be opened, turning off both red and green lights as well as track power. (base here and ice cube here.) It’s very important that you get the correct control voltage, as they are common in both 12VDC and 120VAC.

Well, that wraps it up. Please comment if you have any questions about this or if my end-of-a-long-week-sleep-deprived-ramble was confusing. Next project is laying west staging, and then I may be on to scenery.

Cheers for now!