How to cut traces and wire jumpers on Arduino Shield PCB
February 11, 2014 6:28 PM   Subscribe

I'm a total hardware engineering noob who's getting into Arduino development. I have an Electric Imp connected to the Arduino via a shield, which works great. However, I've added another shield that conflicts with the pins that are used by the Electric Imp for software serial IO. The Imp shield says that I can cut a trace and wire a jumper to use hardware serial on two open ports instead. I've never done this before – can someone give explicit steps as to what I need to do?

- This thread mentions what is needed, but I don't know how to read a schematic.

- Someone has done what I think is the jumper part here but no mention of cutting the traces. It's a very similar project to mine.
posted by krunk to Computers & Internet (5 answers total)
If you don't know how to read a schematic, this is a great way to start learning! Here's the one for the Electric Imp shield; SJ1 and SJ2, mentioned in the thread you linked, are the symbols in the top-right. They have similar symbols to all the other components, but they actually just correspond to the sections labeled RX and TX on the underside of the board, in the bottom-left of this photo.

The narrow trace connecting two of the pads in the RX section connects the Imp to Arduino pin 8, which is apparently the RX pin for the Arduino's software-emulated serial port. (So it's labeled RX, even though the Imp itself is transmitting on that pin.) The third pad is connected to Arduino pin 0 and nothing else. It sounds like you want to switch the serial connection from pins 8/9 to pins 0/1, right? Then all you need to do is:

1. Cut the two traces with a knife -- an X-Acto or similar craft knife, or even a pocket knife, should work fine. Depending on the thickness of the trace and soldermask, you might need to sort of scratch it out.

2. Check with a multimeter that there's no continuity between the pads, to make sure you've cut all the way through.

3. Put a blob of solder (aka "solder bridge") between the center "RX" pad and the pin-0 pad. Don't go overboard; use just enough solder to nicely cover both pads. Repeat for "TX" and pin-1.

4. Test continuity again between Arduino pins 0/1 and Imp pins P5/P7 to make sure the soldered connection is good.

If you have problems making the solder bridge, you can always do what the person in your last link did and use wires instead, but this way is a little neater. Good luck!
posted by teraflop at 6:56 PM on February 11 [1 favorite]

(balls! teraflop beat me to an answer I was composing. Yeah, what they said ...)

Note that connecting 0 & 1 to a shield will mean that you'll have to take the shield off to program the Arduino underneath. Unos only have one hardware serial port, unfortunately.
posted by scruss at 6:59 PM on February 11

Yes, this is exactly what I needed! Thanks to both of you for the quick responses. The shield is now working perfectly, although as scruss mentioned I have to remove the shield to program the Arduino. More annoyingly, I can no longer use the serial monitor for debugging – is there a good way of getting around this?

My project didn't seem to be working at first, but after reprogramming it a few times it sprung to life, which seems to be the case with Arduino more often than not.

Thanks again!
posted by krunk at 7:17 PM on February 11

If you want to get your hardware serial back, you could move the Electric Imp software serial to different, open pins. This would be a manual wiring job, though, and assumes your other shield doesn't eat up the pins you'd need.
posted by Standard Orange at 11:05 PM on February 11

Yeah, it would be nice if there was an easy way to select which ports to use. I guess that's the downside to using shields.

I suppose if I always had the Imp as the topmost shield I could solder wires from Imp pins P5/P7 and attach them to the stackable header on the Arduino pins that I need? Would I have to reverse the trace that I cut?
posted by krunk at 6:19 AM on February 12

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