There are two ways to connect two parts: one after the other, so the current goes through both (series), or side by side, so the current splits between them (parallel). The choice changes how bright your LEDs are and what happens when one fails. In this lesson you build both on the same breadboard and compare them.
What you'll learn
- What series and parallel circuits are
- How voltage is shared in series and current is shared in parallel
- Why one broken part turns off a whole series circuit
- Why each LED in parallel needs its own resistor
What you need
- UNO R3 board and USB cable (UNO R3 Starter Kit)
- 4 red LEDs
- 3 resistors, 220 Ω
- 1 extra jumper wire
- Breadboard and jumper wires (UNO R3 Starter Kit)
Series and parallel
| Series | Parallel | |
|---|---|---|
| How it's connected | One after the other, in a single loop | Side by side, each with its own path |
| Current | The same current goes through every part | The current splits; each branch takes its own share |
| Voltage | The 5 V is shared between the parts | Every branch gets the full 5 V |
| If one part fails | Everything goes off | The others keep working |
| In real life | Old fairy lights, where one bad bulb turns them all off | The lights and sockets in your house |
Wiring
On the left: LED 1 and LED 2 in series with one resistor. On the right: LED 3 and LED 4 in parallel, each with its own resistor.
| From | To |
|---|---|
| UNO 5V | Bottom + rail (red wire) |
| UNO GND | Bottom − rail (black wire) |
| + rail | Columns 10, 30 and 40, row j |
| Series: resistor 220 Ω | Column 10 (row h) to column 14 (row h) |
| LED 1 long leg (+) / short leg (−) | Column 14 / column 15, row f |
| Column 15, row j | Column 19, row j (yellow wire) |
| LED 2 long leg (+) / short leg (−) | Column 19 / column 20, row f |
| Column 20, row j | − rail |
| Parallel: resistor 220 Ω | Column 30 (row h) to column 34 (row h), LED 3 in columns 34 and 35, row f |
| Resistor 220 Ω | Column 40 (row h) to column 44 (row h), LED 4 in columns 44 and 45, row f |
| Columns 35 and 45, row j | − rail |
Try it
- Plug in the USB cable. LEDs 3 and 4 (parallel) are bright. LEDs 1 and 2 (series) are dimmer.
- Why? In series, each LED takes about 2 V, so only 1 V is left for the resistor: 1 V ÷ 220 Ω = 4.5 mA. In parallel, each branch gets the full 5 V: 3 V ÷ 220 Ω = 13.6 mA each.
- Unplug, pull LED 1 out and plug in again. LED 2 goes off too: the series loop is broken.
- Put LED 1 back and pull out LED 3 instead. LED 4 keeps shining: its branch is still a complete loop.
- Unplug, move the black wire from 20j to 22j, and add a third red LED with its long leg in 20g and its short leg in 22g. All three series LEDs go dark: three LEDs need about 6 V, more than the 5 V you have.
Why one resistor per LED?
You might think one resistor could be shared by two LEDs in parallel. But no two LEDs are exactly the same: the one that needs slightly less voltage takes most of the current, gets brighter and hotter, and can burn out. Then the other one gets all the current. One resistor per LED keeps each one safe.
Try this next
- Build a series chain of a red and a yellow LED. Do they still light? Add up their voltages: about 2.0 + 2.1 = 4.1 V.
- Try a red and a blue LED in series: 2 + 3 = 5 V leaves nothing for the resistor, so they stay dark or barely glow.
- Add a third parallel branch. It is just as bright as the others.
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.
- The series LEDs don't light at all: check the yellow wire between columns 15 and 19, and make sure both LEDs point the same way (long legs to the left).
- The series LEDs are as bright as the parallel ones: the short wire may be in the wrong place, making a second path round one LED. Compare with the table.
