Every circuit you will ever build comes down to three ideas: voltage, current and resistance. They sound difficult, but you can picture them with water in a pipe. In this lesson you learn what each one means, build a circuit that is one complete loop, and change each of the three to see what happens to an LED.
What you'll learn
- What voltage, current and resistance are, and their units: volts, amps and ohms
- Why electricity needs a complete loop to flow
- How more resistance means less current
- Ohm's law, the simple formula that links all three
What you need
- UNO R3 board and USB cable (UNO R3 Starter Kit)
- 1 red LED
- Resistors: 220 Ω and 10 kΩ
- Breadboard and jumper wires (UNO R3 Starter Kit)
The water pipe picture
| Electricity | Unit | Water in a pipe |
|---|---|---|
| Voltage: how hard electricity is pushed | volt (V) | The water pressure from a tank high up |
| Current: how much electricity flows | amp (A), or milliamp (mA) = 1/1000 A | How much water flows through the pipe each second |
| Resistance: how hard it is for electricity to flow | ohm (Ω) | A narrow part of the pipe that slows the water down |
The UNO's 5V pin is like a tank that pushes with 5 volts. GND (ground) is where the water ends up: 0 volts. Current only flows if there is a path all the way from 5V back to GND. That path is a circuit, a word that comes from "circle".
Ohm's law
The three are linked by one short formula: current = voltage ÷ resistance, or I = V ÷ R. Double the voltage and the current doubles. Double the resistance and the current halves.
An LED takes about 2 V of the 5 V for itself, so about 3 V is left for the resistor. With 220 Ω: 3 V ÷ 220 Ω = 0.0136 A, which is 13.6 mA. That's a good, bright current for an LED. With 10 kΩ: 3 V ÷ 10,000 Ω = 0.3 mA, about 45 times less.
Wiring
This time you wire the circuit straight to the UNO's pins, without the power rails, so you can see the loop: out of 5V, through the resistor and the LED, and back into GND.
| From | To |
|---|---|
| UNO 5V | Column 20, row j (red wire) |
| Resistor 220 Ω | Column 20 (row h) to column 24 (row h) |
| LED long leg (+) | Column 24, row f |
| LED short leg (−) | Column 25, row f |
| Column 25, row j | UNO GND (black wire) |
Try it
- Close the loop. Plug in the USB cable. The LED lights: current flows from 5V, through the resistor and the LED, back to GND.
- Break the loop. Pull the black wire out of GND. The LED goes out at once, even though 5V is still connected. No loop, no current. Put the wire back.
- More resistance. Unplug, swap the 220 Ω resistor for 10 kΩ (stripes brown, black, orange, or brown, black, black, red), and plug in again. The LED is much dimmer: more resistance, less current.
- Less voltage. Put the 220 Ω resistor back and move the red wire from 5V to the UNO's 3.3V pin. The LED is dimmer again: less push, less current.
With a multimeter
If you have a multimeter, set it to DC volts (V⎓) and touch its probes across the resistor: you should read about 3 V. Across the LED you read about 2 V. Together they add up to the 5 V from the UNO. The multimeter guide shows how to hold the probes and pick the right setting.
Try this next
- Try a 560 Ω resistor. Use Ohm's law to guess the current before you plug in: (5 − 2) ÷ 560 = about 5 mA.
- Break the loop in a different place, for example by pulling the LED out. Does it matter where the loop is broken?
- Draw the loop on paper with arrows showing which way the current flows.
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 LED is just as bright with 10 kΩ: check the stripes; you may have picked a 220 Ω resistor again. The next lessons show how to read them.
- The LED doesn't light on 3.3V: blue and white LEDs need about 3 V themselves, so 3.3 V is barely enough for them. Use a red LED for this test.
