Model Railway Arduino Point Control – Design

As stated elsewhere, I’ve mothballed the Arduino Dalek Project, and started to cannibalize bits for a new project: Arduino control of the turnout points on an N Gauge model railway layout.

For a start, heres some information about the point motors I’m using: The PECO PL-11 side mounted motors, which attach to the side of the points, like this:

PECO PL-11 (© PECO Publications)
PECO PL-11 (© PECO Publications)

These motors operate on 16v AC and require only (and indeed cannot tolerate more than) momentary current, and are currently controlled from “passing contact switches” drawing power from a Capacitor Discharge Unit.

There are currently 6 turnouts/points, arranged in pairs as below. As it would be reckless (and in real life impossible, due to interlocking) to operate either of the pair independently, I’ve decided to activate them together.

points1Thus, at all times both A and B will either both lead straight ahead, of both lead off to the left (for sets C&D and E&F it will be right not left).

As the Arduino can only throw out 5v DC, each pair of motors will need to be driven via a 5v DPDT replay. But, as we can’t leave the current on without melting the motors, each pair will actually need two relays (one for each direction of change), driven independently from different output pins. Another complication is that the relays will have to energised and then de-energised within a specific time-frame to avoid motor damage, but this is a programming issue to be dealt with later.

PL-11-CDU-Relay WiringThe above diagram shows only the 16vAC side of the wiring. The Arduino 5vDC is shown below.

Arduino-Relay Wiring

Obviously I could have chosen to just use SPDT relays, and connect the outputs together, but due to the vagaries of how the motors work and the orientation and placement of them on the layout, I thought it was safer to design and wire it this way at the outset, rather than back-fix it later when it went wrong. Plus, I had all the wiring in place, so thought I might as well use it.

Note the lack of common Ground between the two halves of the circuit. Grounding AC and DC together isn’t a good idea. Just say no.

Anyway, here is all is again on one convenient image.

Arduino-Relay-PL-11 Wiring
Click to embiggen

So now the next stage to to take that abstract squiggle and build it as a circuit.

Note – for myself, mainly: Each “pair” takes up 2 outputs, so for the three pairs of points we need 6 lines from the Arduino, out of a total of 12 (not 14, as I’d rather not use the 0 and 1 lines (RX & TX) as this could cause issues with data transfers activating the relays – something that the Dalek had problems with at first.

To be continued… with a start on Signalling!

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