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I'm having some trouble driving LEDs from my Raspberry Pi, so I started poking around with my multimeter, and the voltages seem very wrong.

Compared the the ground port, the 5V0 ports show 4.6 V, which seems normal-ish. The 3V3 ports, however, seem to ramp sawtooth-wave style from 0 V to 390 mV (if I put my multimeter in AC mode, it reads either around .1 or .06 V.

Using WiringPi's gpio readall command on a newly rebooted Pi shows that GPIO 0 is set to IN and reads 0. My multimeter says this pin is at .14 V. If I run gpio mode 0 out then gpio write 0 1, the readall command says that the pin is set to OUT and 1, but my multimeter still reads .14 V. The opposite is the case for GPIO 2: it starts off at 1 (and shows 4.6 V on my multimeter) and I cannot turn it off.

Edit: the test points (labelled TP1 and TP2 on the board) show 4.8 V.

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    You don't say where you are measuring the voltages. Start off with the test points. If your 5v is actually measuring 4.6v then it is out of spec (although the Pi may still work). If you are measuring the GPIO pins then you should have a load resistor (~10k) – Milliways Oct 28 '13 at 5:38
  • I'm measuring the GPIO pins. Should I connect the load resistor between the GPIO and ground? I've edited my question with more info. – Cajunluke Oct 28 '13 at 14:48
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    Digital multimeters have a very high impedance ~10MΩ and are very susceptible to interference, particularly in a noisy environment. When measuring a circuit which can be in an unknown state (input, output, tri-state) you need to swamp this interference - basically connecting a resistor across the multimeter. Connecting from GPIO and ground is OK. At least you will know what the actual voltage is. The supply voltage at 4.8V is low, but just within spec. – Milliways Oct 28 '13 at 23:02
  • @Milliways I don't really see any difference between when I use the 10kΩ resistor and when I don't. I'm using a breakout board for the GPIO pins, and I see the 5V there (as in the question), but when I just put my jumper cables into the ribbon that came with it, I see the 3.3V source but not the 5V source. (Here I'm largely guessing as to what pin is what, but I can test the end four pins [1, 2, 25, 26] and find the voltages and ground.) – Cajunluke Oct 30 '13 at 3:35
  • Specifically, when I put the 10kΩ resistor between pins 1 and 9 and measure that voltage, my multimeter shows 0V. (Could it be my multimeter is wrong?) – Cajunluke Oct 30 '13 at 3:37
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The solution seems to have been to get new LEDs; I apparently blew up the ones I was using.

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