Blue sagging power on GP0

I’m trying to wire an LED and 220 Ohm resistor to one of the GPIO pins on the GP0 connector of the Blue.

I measure 3.3V between the GND and 3.3V pins as I should, but when I hook the resistor in series with the LED and attach them across GND and 3.3V, the voltage across GND to 3.3V sags to 1.5V and the LED almost turns on. (If I dim the lights in the room I can see a little light from it.) I get the same result whether I’m powered off the USB, a 12V supply, or a LiPo battery.

How much current can the 3.3V supply deliver? It seems I should be able to drive an LED from it. I can drive LEDs like this from the 3.3V supply (and GPIO pins) on the Black.

What’s up?


I haven’t gotten to attaching the LED and resistor across an GPIO pin. This is across the 3.3V power and GND. I hope the 12V supply can handle more than 20ma.



Fri 28-Jul-17 10:28 AM

your LED is trying to use too much current.
a IO pin can source 3ma you need very low current LEDS

Well… I found my problem. I was looking at the board wrong. I thought pin 1 was on the left, but it’s on the right! Ouch!


Does anybody know how much current the GPIOs can source/sink?

3mA typically, some can do 6mA. Check the datasheet for a per pin value.