If you go to Darkith's website he basically lays it out for you. Two sets of two LEDs wired in parallel with each set going through a resistor in series to limit the voltage to each set of LEDs. I replaced the Heng Long Sensor in the tower with a 38 Mhz sensor. Everything works great. I used an old DBU board that I had screwed up some of the wire tracing on for this one, but I guy could get some perf board and wire it up as well.

Mato sells the Sensor for $11, and then another $5 for LED's and resistors, and you have a reasonably priced battle unit.