# Safely using the ground pin

I have a `cobbler` connected to my `Pi` and a breadboard:

if I use multiple `GPIO` pins and the `3.3V` pin in a circuit can I simply just use a single ground pin to provide the negative connection safely? My circuit will contain multiple `20mA` `LEDs` and some resistors

Or will it become overloaded? (with current or voltage?)

• If you connect more than one LED in series to 3.3V you will probably be very disappointed by the result as the "forward voltage drop" will total more than 3.3. If you do it in parallel the 3.3V rail on later models (+/2/3/0) can provide 500 mA or more. raspberrypise.tumblr.com/post/144555785379/… Aug 5, 2016 at 14:15

All the ground pins (and the 5V and 3.3V pins) are connected together. There are only multiple pins for historical/convenience reasons.

Only a single connection is required. (On devices with high currents there are reasons for multiple pins, but this is irrelevant for the limited current of the Pi.) There are a few cases where multiple connections are desirable - e.g. to minimise crosstalk.

You mention 20mA LEDs. Strictly GPIO are rated at 16mA. That is not to say they won't deliver more, but is inadvisable - even if poorly documented.

You can use just one GND pin for all leds. For each LED, use a GPIO as (+) source and a 330 ohms resistor in series with each LED.

• A 330 Ohm resistor is overkill. This follows a version of Ohm's law, whereby resistance = voltage / desired current. The voltage is minus the forward drop of the LED, which will probably be at least 1.5V. Using that and 15 mA as the desired current, 1.8 / 0.015 = 120 Ohms. You can confirm this using one of the many "led resistance calculators" available online. If the forward voltage drop is more than 1.5V, the required resistor could be even smaller, but if you start with 120 Ohm and are satisfied with the brightness, stop there. Aug 5, 2016 at 14:21
• As an illustration, I've used 47 Ohm resistors with bright white leds, no problems (but to be on the safe side when I've soldered them in I stay at 100). I've used no resistor at all with two LEDs in series and guess what -- it doesn't work, but not because the LEDs burn (they don't). Aug 5, 2016 at 14:24