ground of circuit board vs ground??

[charlieonehorse]

In the 3pdt wiring diagram one node is labeled "to ground" and another is coming off the LED and says "to ground of circuit (black lead of 9v)"

are these the same place?

I am building a random number generator and having trouble finishing the wiring.

here is the diagram i am using http://www.diystompboxes.com/pedals/images/3PDTbwiring.jpg


p.s. why is the 1k resistor to the LED needed?

[Earthscum]

the grounds are basically the same. It's a practice of not creating ground loops... others here are better to explain that. Basically, what they are showing you is that you wire your LED ground to the ground that is closest to the source ground (in this case, the battery clip). I've found that this basically helps eliminate some of the "pop" from the LED, but for all intent here, this is the same ground. One goes directly to the board, and the other gets wired into wherever the battery ground lead gets soldered into.

The resistor is there to keep from frying your LED with too much current. I find 470 ohm-2.7k (depending on how bright ya want it) to be good. Basically, with a 9V across a 1k resistor, you get .009A, or 9mA, of current going through the LED. If you used a 470R (ohm), the current through the LED would be about 19-20mA, which is really close to alot of LED max ratings.

I may be off... LED Vf (forward voltage drop), say 1.6V... 9-1.6=7.4/1000= 7.4mA, and a 470R would be 15.7mA...

Either way, concept is basically the same... keep from frying your LED.

[GibsonGM]

Basically, what Earthscum said.  The reason is that you end up with what's called a 'star ground'.  This is a central ground location with each 'subcircuit' ground leading back to it...sort of like the hub of a wheel, with each subcircuit on a spoke.   
If you draw different currents thru different parts of a circuit, you can end up with 'noise' on the ground lines, which can be reflected as noise in your signal. 
Another issue is that altho the resistance of a ground is very low, it is still a resistance, and can cause stray currents to flow, also resulting in noise.
So separating them is a good idea.   Search "star ground" for more info!