The UNO has a row of power pins next to the analog pins. Each one has its own job, and knowing them saves you from many mistakes later. In this lesson you power two LEDs from two different pins and see the difference, and you learn how much current the board can safely give and how to use a battery instead of USB.
What you'll learn
- What the 5V, 3.3V, GND and VIN pins do
- How the UNO turns the USB or battery voltage into 5 V
- How much current each power pin can give
- How to run the UNO from the 9 V battery
What you need
- UNO R3 board and USB cable (UNO R3 Starter Kit)
- 2 red LEDs
- 2 resistors, 220 Ω
- 9 V battery and battery snap (for the last step)
- Breadboard and jumper wires (UNO R3 Starter Kit)
The power pins
| Pin | What it gives | Use it for |
|---|---|---|
| 5V | 5 volts | Most parts in your kit: LEDs, buttons, sensors, the LCD. Up to about 400 mA in total from USB |
| 3.3V | 3.3 volts | Parts that need 3.3 V, such as the RFID reader. Only small currents, about 50 mA |
| GND | 0 volts | The return path for every circuit. The UNO has three GND pins, and they are all connected |
| VIN | Voltage in | Connect a 7–12 V battery here (with − to GND) to run the UNO without USB |
Inside the UNO, small chips called voltage regulators turn the incoming power into a steady 5 V and 3.3 V. That's why the LED stays equally bright whether the USB comes from a computer or a phone charger.
Wiring
| From | To |
|---|---|
| UNO 5V | Bottom + rail (red wire) |
| UNO GND | Bottom − rail (black wire) |
| + rail | Column 20, row j |
| Resistor 220 Ω | Column 20 (row h) to column 24 (row h) |
| LED 1 long leg (+) / short leg (−) | Column 24 / column 25, row f |
| Column 25, row j | − rail |
| UNO 3.3V | Column 34, row j (orange wire) |
| Resistor 220 Ω | Column 34 (row h) to column 38 (row h) |
| LED 2 long leg (+) / short leg (−) | Column 38 / column 39, row f |
| Column 39, row j | − rail |
Try it
- Plug in the USB cable. Both LEDs light, but LED 2 on 3.3 V is dimmer. Ohm's law explains it: (5 − 2) ÷ 220 = 13.6 mA on 5V, but (3.3 − 2) ÷ 220 = 5.9 mA on 3.3V.
- Both circuits share the same − rail. That's normal: all circuits on the board return to the same GND.
- Unplug the USB cable. Clip the 9 V battery into the snap, then connect the snap's red wire to VIN and its black wire to GND. Both LEDs light again, with the same brightness as on USB, because the regulator still makes 5 V.
- Disconnect the battery when you are done, so it doesn't run flat.
Important: the 9 V battery goes only to VIN and GND. Never connect it to the 5V pin or straight to your breadboard circuit: 9 V on the 5V pin can damage the UNO.
Try this next
- Add a third LED with a resistor on the + rail. All three run from the same 5V pin.
- Find the three GND pins on your UNO: two next to 5V and one next to pin 13.
- Work out how many LEDs at 13.6 mA the 5V pin could power: 400 ÷ 13.6 = about 29. In practice keep well below that.
Troubleshooting
- Nothing lights up: check that the USB cable is plugged in (the UNO's ON LED is lit), that the long leg of the LED goes to the + side, and that every wire sits in the column the table says. Wiggle each wire gently; a wire that isn't pushed all the way in is the most common fault.
- LED 2 doesn't light: check that the orange wire is in 3.3V and not in IOREF or RESET, which are right next to it.
- Nothing works on the battery: check that red goes to VIN and black to GND, and that the battery isn't flat. If your snap ends in a round plug, you can plug it into the UNO's round power socket instead.
