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Phantom voltage on pin 2


I have a very simple LED setup on a breadboard and driven by a FEZ Panda. There are 8 LEDs total. Seven of them are lit up in sequence with no two LEDs lit at the same time. The eighth LED is on pin 2 setup as an OutputPort with its initial value high (true). The seven sequential LEDs light in sequence as expected. The eighth one is set with its initial value low (false), but for some reason, it’s pulsing and has a voltage on it. Shouldnt it be unlit? What could cause this phantom voltage on that pin?

I should add that the eighth LED uses it’s own current limiting resistor, while the other 7 share one.


Are you talking about D2 or Pin2. Pin 2 has a pullup resistor for I2C so has Pin 3.

Cheers Ian


Thanks for the reply, Ian. You’ve helped me out more than once with this stuff and I appreciate your time.

The port is labeled Di2 in the pinout. I see in the brochure that Di2 does in fact have a 2.2K pullup resistor. Is there something I should be doing in hardware or software to compensate for that? I understand the concept of pullup resistors when used to detect a switch in state, such as for button use but not sure how it relates to powering a LED.


open drain pins are electrically different so you will not see the same voltage on the output. You can use other pins or you can add a buffer gate that takes the signal levels and converts these to push pull output.

Why not do this the easy way and use other IOs? There are plenty of them :slight_smile:


There are definitely plenty of them. This helps me understand what’s going on a bit better.

Thanks for the response!