Model Train 3-Light Signal

[MarkF]

Since my last post I've added in some mounting holes and rounded off the corners as well. From what I've read since then, it looks like silkscreen labels off the edge of the board don't actually matter - however, I've made some of the labels invisible and have sent an email to OSH Park to see if it actually does matter.

What's the point of putting labels on the silk layer if they're just going to be cut off?  Don't you want to be able to read the text you went to all the trouble to put them on the PCB?  It just looks unprofessional to me.

[Phoxtane]

What's the point of putting labels on the silk layer if they're just going to be cut off?  Don't you want to be able to read the text you went to all the trouble to put them on the PCB?  It just looks unprofessional to me.

Most of the large labels you can see are on the 'F.Fab' layer, which isn't used by OSH Park. The only labels that will get cut off are the ones for the 1x6 headers, but I'm not entirely sure how to go about editing the label so this doesn't happen - however, I'm okay with them getting cut off as it was very helpful making sure everything was aligned properly.

I think I'm ready to generate gerber files and send them off for fabrication! The rectangular board size is 24x32mm, which ends up being about 1.19 square inches - which, at the rate of $5/square inch I'm getting should be $5.95, or just under $2 per PCB. Not bad at all!

Make the tracks thicker. They may not need higher current capability but it makes it easier to add bodge wires and it makes is easier to etch.
Remove all acute (less than 90deg) angles as these cause acid traps.

I'm going to leave the trace width as-is for now. Future me can deal with bodge wires if necessary 

Everything in gray here is on the F.Fab layer that's currently unused.


And again without the silkscreen layers but with copper fills!

[steaky1212]

I don't know if the mounting holes are going to be big enough, but then I don't know how you're mounting it.

I really like this resource - https://blogs.mentor.com/tom-hausherr/blog/2011/03/18/pcb-design-perfection-starts-in-the-cad-library-part-13/ - and I use it whenever I put mounting holes on a PCB.

[Phoxtane]

I went ahead and increased the board size somewhat to a 32 by 32mm square; this gave me room to add some much-needed labels, as well as a slightly less cramped layout that should be easier to solder.

I've also put the feelers out to see if people are interested in buying this little gadget. I suppose it's time to show off the part of my layout I have completed!



I'm doing the whole thing in Lego - I'm a huge nerd. It can split apart into pieces for easier transport. There's an empty space under the track that I'm planning to use as a conduit for running the cables for the signals and other sensors that will help run the layout.

[Phoxtane]

I came home from work yesterday to discover that the first version of the PCBs for this project had arrived!





I can't actually use these boards for testing purposes, as when I assigned footprints to each component in KiCad, I assigned the wide version of the SOIC-16 (~8mm) footprint as opposed to the normal SOIC-16 (~4mm) footprint. As such, it's far too wide for me to actually solder the chips I have on hand to, and I'm not about to attempt bodge wires either...

I also did a high-resolution scan of the PCBs on our scanner, just so I could inspect them properly. I'll link in the high-resolution image, as well as embed a forum-friendly preview.


http://bricksafe.com/files/Phoxtane/automatic-lego-train-control-with-fpga/EPSON001.JPG

As you can see, there's not much in the way of labels - if I didn't have the schematic on hand, I'd have a very difficult time figuring out what wire connects to what pin. Thankfully, board version 2 solves these problems, and board version 2.1 improves upon board version 2. Correcting the footprint issue freed up some space for better label placement, and I made use of the empty back side of the board to add some design information and a graphic.





I've also made the decision to run this board off of 5V exclusively; it's more common and will make it much easier to interface with the devices I'm using.

I haven't done the cost calculations yet, but I suspect the board will actually remain at around the same price as before, even with its larger size, since now it only uses one value of resistor throughout the design due to increasing the supply voltage.

[rstofer]

I'm planning on using 22/4 4 conductor cable from Home Depot Security Cable.  Try something like it for your longest run and see if the voltage drop is an issue.  You're not drawing much current that would cause a large voltage drop.

If you need more conductors, just use Cat-5 network cable.

22 AWG solid wire has a resistance of 16 Ohms per THOUSAND feet.  The LED already has a 200 Ohm resistor (depending on operating voltage) so how much difference would 16 Ohms make if you used 500 feet one way?  In fact, the wire resistance is lost in the tolerance of the resistor. This assumes that the controller is on one end of the wire and the LEDs on the other.

http://www.okonite.com/engineering/dc-resistance.html

[steaky1212]

I'd still be concerned that you are going to chew through a track as it looks very close to the mounting hole (LED3 to R3) - screw heads and washers are bigger than you think.

Nylon/fiber washers and countersunk screws will "help", but really this should be designed out.

[Phoxtane]

I'd still be concerned that you are going to chew through a track as it looks very close to the mounting hole (LED3 to R3) - screw heads and washers are bigger than you think.

Nylon/fiber washers and countersunk screws will "help", but really this should be designed out.

Fair point. I'll probably be doing yet another board respin since it turns out the SMD diodes I have are a right pain to solder by hand - probably to a through-hole part.


One last question - since the voltage drop through a generic Schottky diode is ~.7V, that means the voltage presented to the rest of the circuit is 4.3V - which is the 'source' voltage I should use when calculating the resistor value for the LEDs, rather than 5V, correct?