A breadboard lets you build circuits without soldering: you push the legs of parts into the holes, and metal clips inside connect them. Once you know which holes are connected to which, you can build almost any circuit on it. In this lesson you find the hidden connections and build an LED circuit that uses both halves of the board and both power rails.
What you'll learn
- Which holes are connected inside a breadboard
- What the gap down the middle is for
- How the power rails along the edges work
- How to read hole positions like "column 14, row c"
What you need
- UNO R3 board and USB cable (UNO R3 Starter Kit)
- 1 red LED
- 1 resistor, 220 Ω
- 4 extra jumper wires
- Breadboard and jumper wires (UNO R3 Starter Kit)
The hidden connections
Turn the breadboard over and look at the back. Under the sticker are metal clips. On the front this is how they connect the holes:
- Columns of five. The numbered columns (1 to 63) are split into two halves. In each half, the five holes in one column (rows a to e, or rows f to j) are connected. Two parts whose legs are in the same group of five are connected.
- The middle gap. The channel down the middle separates the top half from the bottom half. Column 14 rows a to e is not connected to column 14 rows f to j. Chips with legs on both sides sit across this gap.
- Power rails. The long strips along the top and bottom edges run the whole length of the board, marked + (red) and − (blue). On some boards the rails are split in the middle; if a part far along the rail gets no power, add a short wire across the break.
Holes are named by column and row: 14c means column 14, row c. The lessons always use this naming.
Wiring
The current in this circuit goes on a long trip: from the bottom + rail, through the resistor in the bottom half, over the gap with a wire, through the LED in the top half, and back to the top − rail, which is joined to the bottom − rail.
| From | To |
|---|---|
| UNO 5V | Bottom + rail (red wire) |
| UNO GND | Bottom − rail (black wire) |
| Bottom + rail | Column 10, row j (red wire) |
| Resistor 220 Ω | Column 10 (row h) to column 14 (row h) |
| Column 14, row g | Column 14, row d, across the gap (yellow wire) |
| LED long leg (+) | Column 14, row e |
| LED short leg (−) | Column 17, row e (bend the legs apart a little) |
| Column 17, row a | Top − rail (black wire) |
| Bottom − rail, far right end | Top − rail, far right end (black wire) |
Try it
- Build the circuit and plug in the USB cable. The LED lights.
- Unplug and take out the yellow wire across the gap. Plug in again: the LED stays dark, because the two halves of column 14 are not connected. Put it back.
- Unplug and move the LED's short leg from 17e to 14a, so both legs are in column 14. Plug in: the LED stays dark. Both legs are now in the same group of five, so the strip connects them to each other and the LED gets no voltage across it. Put it back in 17e.
- Take out the black wire that joins the two − rails. The LED goes out: the top rail is a separate strip and isn't connected to GND by itself.
Try this next
- Move the whole circuit 10 columns to the right. It still works, because only the groups of five matter, not where they are.
- Power the top + rail as well, with a red wire from the bottom + rail at the far end, so you have power along both edges.
- Make a small card with the breadboard's connections drawn on it and keep it next to your board.
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.
- A part is very hard to push in: new breadboards are tight. Hold the leg close to the board and push straight down. Don't force it, or the leg bends.
- Some holes on the rail don't work: your board may have rails split in the middle (look for a gap in the red and blue lines). Bridge the gap with a short wire.
