Engineers don't draw pictures of breadboards. They draw circuit diagrams (also called schematics): simple symbols joined by lines that show what is connected to what. Once you can read them, you can build circuits from any book, website or datasheet. In this lesson you learn the symbols for every part in this path and build a circuit from a diagram.
What you'll learn
- The symbols for a battery, resistor, LED, push button and potentiometer
- How lines and dots show connections
- How to turn a circuit diagram into a breadboard layout
- How to check your build against the diagram
What you need
- UNO R3 board and USB cable (UNO R3 Starter Kit)
- 1 push button
- 1 potentiometer, 10 kΩ
- 1 red LED and 1 green LED
- 2 resistors, 220 Ω
- Breadboard and jumper wires (UNO R3 Starter Kit)
The symbols
- Lines are wires. Where two lines meet with a dot, they are connected. Lines that cross without a dot are not connected.
- The power supply is drawn as a long and a short line: the long line is +. In this path, the UNO's 5V and GND are the power supply.
- The LED symbol is a triangle pointing the way the current flows, with a bar on the − side and two small arrows for the light.
- Positions don't matter. A diagram shows connections, not where parts sit. The breadboard layout can look completely different and still be the same circuit.
Read the diagram
Follow the current from + in the diagram above: from 5V it goes through the push button to a junction (the dot). There it splits into two parallel branches. One goes through a 220 Ω resistor and the red LED to GND. The other goes through the potentiometer, a second 220 Ω resistor and the green LED to GND.
Before you build, predict what it does. Then check: pressing the button lights both LEDs, and the knob changes only the green one.
Wiring
One way to build the diagram on the breadboard. Each dot in the diagram becomes one column on the board.
| From | To |
|---|---|
| UNO 5V | Bottom + rail (red wire) |
| UNO GND | Bottom − rail (black wire) |
| Push button | Across the gap: legs in 12e, 14e, 12f and 14f |
| + rail | Column 12, row j |
| Column 14 (the junction) | Resistor 220 Ω to column 18 (row h), and a yellow wire from 14i to 30j |
| Red LED long leg (+) / short leg (−) | Column 18 / column 19, row f |
| Potentiometer | Legs in columns 30, 31 and 32, row f |
| Resistor 220 Ω | Column 31 (row h, the wiper) to column 35 (row h) |
| Green LED long leg (+) / short leg (−) | Column 35 / column 36, row f |
| Columns 19 and 36, row j | − rail |
Try it
- Build the circuit. Then check it the way engineers do: point at each line in the diagram and find the matching connection on the board. Tick it off on paper.
- Plug in the USB cable. Nothing lights until you press the button.
- Hold the button and turn the knob: the green LED dims, the red one stays bright.
- Draw your own diagram of the series LEDs from lesson 7, without looking back at the lesson. Then compare.
Well done
You now know the basics that every project builds on: voltage, current and resistance, the breadboard, LEDs, resistors, switches and potentiometers, and how to read a circuit diagram. When you finish this path, continue with the first lesson of Arduino basics, where you write your first program.
Try this next
- Add a second push button to the diagram so the green LED has its own button, then build it.
- Search online for "LED flashlight circuit diagram" and find the symbols you now know.
- Redraw the circuit in this lesson in a different layout, with the same connections. Show it to someone and ask them to check that it is the same circuit.
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.
- Both LEDs are on all the time: the push button is turned the wrong way. Turn it 90 degrees.
- The knob also dims the red LED: the red LED's resistor is connected after the potentiometer instead of at the junction. In the diagram, the red branch starts at the dot, before the pot.
