What is SJ in the following image in Eagle?
Solder jumper link to short out R1 I think.
what is its advantage over a simple jumper? I got the circuit from here:
Don't need a header or anything, you just have two pads to solder if you want to.
A disadvantage would be that you actually have to solder/desolder it. So it's usually used as one-time jumper. To set an address or some setting perhaps.
You simply put a blob of solder that joins the two pads. Use this instead of a switch or a "pins and shunt". It is used for a one-time turn on. Not easily turned off, but you can if you want to, badly enough.
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Huh? Easily turned off with a bit of solderwick...
The Sparkfun EAGLE library has several solder jumper parts defined, apparently creating both "normally open" and "normally closed" versions depending on the size of the opening in the solder paste layer. I'm not sure I'd trust that to work going from one PCB fab to another, but it's a neat idea...
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