Summary

Holds 2 leds directed at the extruder tip. Leds are held in a mount attached to the fan. Powered from the printrboard

Should fit any other printer with the same fan as the printrbot simple, but will be less-than-useful unless the fan is in a similar position.

Instructions

Two white LEDs are held in a holder attached to the fan. 5V Power is drawn from one of the extension headers on the printrboard

There are two parts.
1) A mount for 2 leds which attaches using 2 of the fan screws
2) A little box holding on/off switch and 100 ohm resistor.

You'll need some resistor but the exact value needs to be calculated based on your LED characteristics. There are various LED resistor calculators on the internet. In my case I wired them in parallel (My LEDs have a 3.6V drop, so series wasn't an option).

led power

You'll need 5V from somewhere. The eagle files show 2 suitable points on the expansion headers. See also
http://reprap.org/wiki/File:PrintrboardRevB_Front.jpg:

JP2/EXP1 -- on the edge 13=+5V 14=GND i.e. next to 6 pin icsp header, 5v on edge
JP11/EXP2 -- central 1=GND 2=+5V
I found the pins on EXP1 to be more accessible.

Puting it together

insert the LEDs into the 2 outer holes. glue them in if necessary.
wire the LEDs cathode-cathode, anode-anode
add flying multicore cable to anode and cathode on the side nearest the printrboard.
flip up the fan holder. remove the 2 screws which are now at the top.
use the screws to attach the mount, and tighten. flip back down.
the cathode wire goes to the selected GND pin of the printrboard
the anode wire goes thru your resistor and switch to the selected 5V pin

Optionally, these wires go via the small box within which you can add a piece of stripboard, a switch, & resistor. The box has a groove to let it be ziptied to a vertical of the circuitboard frame.

Possible improvements

1) Cover the base of the leds, to avoid back glare.
2) The box for the electronics is small, and not really necessary. You might not even want a switch.

Caveat
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Check with multimeter at all stages. Don't short anything. If you blow up your board, I'm not responsible.

printrbot simple led mount