LEDs are in almost every project, from the power light on your TV to traffic lights. They are easy to use once you know two things: they only work one way round, and they always need a resistor. In this lesson you put one LED in the right way and one the wrong way, and you calculate the resistor for any LED.
What you'll learn
- What an LED is and why it only lets current through one way
- How to tell the + leg from the − leg
- Why an LED burns out without a resistor
- How to calculate the right resistor for an LED
What you need
- UNO R3 board and USB cable (UNO R3 Starter Kit)
- 2 red LEDs
- 2 resistors, 220 Ω
- Breadboard and jumper wires (UNO R3 Starter Kit)
What is an LED?
LED stands for light-emitting diode. A diode is a part that lets current through in one direction only, like a one-way valve in a water pipe. An LED is a diode that gives off light when current flows through it.
An LED only works one way round. The long leg is + (anode) and the short leg is − (cathode). If the legs have been cut, look at the rim of the LED: the side with a small flat edge is −.
Why a resistor?
An LED is not like a light bulb. Once the voltage across it reaches about 2 V (for a red LED), it lets through almost any current. Connected straight to 5 V, it takes far more current than the 20 mA it can handle, and it burns out in a moment. The resistor sets the current to a safe value.
Resistor = (supply voltage − LED voltage) ÷ current. For a red LED on 5 V with 15 mA: (5 − 2) ÷ 0.015 = 200 Ω. The nearest value in your kit is 220 Ω, which gives about 13.6 mA. Bigger is always safe; the LED is just a little dimmer.
| LED colour | Voltage it takes | Resistor on 5 V |
|---|---|---|
| Red | about 2.0 V | 220 Ω |
| Yellow | about 2.1 V | 220 Ω |
| Green | about 2.2–3.0 V | 220 Ω |
| Blue and white | about 3.0–3.2 V | 220 Ω (a little dimmer) |
Wiring
LED 1 is the right way round. LED 2 is deliberately the wrong way round: its long leg goes to the − side.
| From | To |
|---|---|
| UNO 5V | Bottom + rail (red wire) |
| UNO GND | Bottom − rail (black wire) |
| + rail | Columns 20 and 34, row j |
| Resistor 220 Ω | Column 20 (row h) to column 24 (row h) |
| LED 1 long leg (+) | Column 24, row f |
| LED 1 short leg (−) | Column 25, row f |
| Resistor 220 Ω | Column 34 (row h) to column 38 (row h) |
| LED 2 short leg (−) | Column 38, row f (backwards on purpose) |
| LED 2 long leg (+) | Column 39, row f |
| Columns 25 and 39, row j | − rail |
Try it
- Plug in the USB cable. LED 1 lights. LED 2 stays dark: it is backwards, so it blocks the current.
- Unplug, turn LED 2 round (long leg in 38f, short leg in 39f) and plug in again. Now both light. A backwards LED doesn't get damaged at these voltages; it just doesn't work.
- Look closely at the LED with a torch: inside you can see two small metal parts. The bigger one, shaped like a cup, is usually the − side.
Don't try an LED without a resistor. It might survive a second, but it gets weaker every time, and it can crack.
Try this next
- Replace LED 2 with a blue LED. It lights, but it's dimmer than you might expect, because it takes about 3 V for itself.
- Work out the resistor you would need for a red LED on a 9 V battery at 15 mA: (9 − 2) ÷ 0.015 = 467 Ω, so the kit's 560 Ω would be a good choice.
- Put the RGB LED from the kit on the board and find out which legs are red, green and blue. The longest leg is the common −.
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.
- Neither LED lights: check that the resistors go from the + rail to the column where the LED's leg is, and that each LED's other leg reaches the − rail.
- An LED is very dim and turning it round doesn't help: it may have been damaged earlier without a resistor. Try a new one.
