Robotics · Electronics basics · Grades 5–6
Breadboard: rows
The LED lights in one hole but not in the next one — how are the holes connected inside the board?
The situation
The first robotics club session. On the table is a white board full of holes — a breadboard — plus a battery, a resistor, a red LED and a few wires. The teacher showed the circuit and you pushed the parts into the holes — the LED does not light. Move one wire a single hole over and it lights up! Why? What is inside the board, and which holes are connected to each other? Look inside the board, place the parts and find and fix the most common club mistakes yourself.
Topic
Breadboard: how the rows are connected. The first club session: metal strips inside the holes — which holes are connected to each other
Key ideas
- A strip of five holes
- The gap in the middle
- Power rails
Formulas
The five holes of one column (a–e) are joined inside by a single metal strip; f–j are on another strip. The gap in the middle separates them.
When the circuit is built correctly, the resistor limits the current: A = 12 mA. Without a resistor the current reaches an ampere — the LED burns out (in more detail in Grade 7).
If two LEDs are connected in parallel (anodes on one strip, cathodes on another), the current is shared between them — with one resistor each gets half.
Theory
Why we use a breadboard
A breadboard (breadboard) is for building circuits without soldering. You push the legs of the parts into the holes and, if needed, pull them out and move them elsewhere. The spacing between the holes is 2.54 mm — exactly the pitch of the legs of chips and modules. But the board does not “connect all the holes”: there is a clear rule inside it.
Inside the board: five holes — one strip
If you look inside the board (“Look inside”), under every column lies a metal clip strip: holes a, b, c, d, e are on one strip — they are connected. f, g, h, i, j are on another strip. The neighbouring column (for example 12 and 13) is not connected. So for two parts to be connected, their legs must be in the same half of the same column.
The gap in the middle
The groove in the middle of the board — the gap — separates rows e and f: e12 and f12 are not connected. This is on purpose: chips sit across the gap, one row of legs goes to the a–e side and the other to the f–j side, and they do not short. Beginners often think e and f are “one column”, and the circuit does not work.
Power rails
The two long lines at the edges are rails: the red line is “+”, the blue one is “−”. All the holes along a rail are connected (grouped in 5s, but it is one strip). The battery’s “+” wire goes into the “+” rail and the “−” wire into the “−” rail — after that it is easy to take power from anywhere. Careful: on long boards (830 holes) the rail is split in the middle — the two halves must be joined with a wire, otherwise one half stays without power.
Classic mistakes
1) Both legs of the LED are on one strip (c12 and a12) — the strip joins them, the current bypasses the LED and it does not light. 2) A part is “connected” across the gap (e12 ↔ f12) — in fact the circuit is broken. 3) A wire is pushed into a rail with no power. 4) The LED is the wrong way round — the long leg (anode) must be on the “+” side. 5) An LED with no resistor — the current exceeds the limit 30 times over and the LED burns out at once. 6) The “+” and “−” wires on one strip — a short circuit, and the battery gets hot.
About the model
The resistance of the strips is ignored (in reality a few milliohms). The LED model is piecewise: no current up to 1.9 V, then 2 Ω; the normal current is 20 mA, the limit is 30 mA — above that the LED burns out in time (at 1 A, instantly). The battery is 3 × AA = 4.5 V with an internal resistance of 0.45 Ω; in a short circuit ≈ 10 A flows and the battery gets hot (a real battery does too, but here it will not explode). Only one leg fits in each hole.
Did you know?
- The solderless breadboard was developed in 1971 by the American Ronald J. Portugal (E&L Instruments). “Breadboard” means a board for cutting bread: in the early 20th century radio hobbyists built circuits by nailing parts to a wooden bread board from the kitchen — hence the name.
- The spacing of the holes on the board is 2.54 mm — exactly 0.1 inch. This pitch is inherited from the first DIP chips of the 1960s: that is why today’s Arduino boards, sensor modules and chips all fit a breadboard.
Inside the experiment
A real board in a hand-inked isometric drawing: turn it, place the parts, wire the circuit, write and upload the code — current and voltage are computed at once, and a wrong connection burns a part. Inside are missions, theory, a quiz and a guided tour.
More experiments in this section
- The electric circuit: a torch — Grades 5–6
- LED and resistor: Ohm’s law — Grade 7
- Resistors: the colour code — Grade 7
- Voltage divider and potentiometer — Grade 7