I'd say you want the pulse LED. I just did a Bally MPU with a bad clock buffer. One clock phase lit up all three LEDs. The other clock phase lit up just the red and green. When i checked with a DMM it said 2.5v which is what i would expect.
The real story when i got the scope out was that the clock phase that only lit up the green and red probe LEDs was a mostly flat line across near 2.5v (bad u15). No yellow pulse LED i may have spun my wheels for longer as to why the MPU was locked up.
Still really neat. Inside a sharpie... thats great. That kit becomes bad ass if someone can add the audible buzzing circuit to it. I think you need two chirpers, one for LOW led and one for HIGH led.
