A breadboard has no components and nothing to go wrong, which is exactly why it is so frustrating when a circuit built on one refuses to work. Almost every one of those failures is a misunderstanding about which holes are already connected to each other.
The pattern under the plastic
Under every hole is a spring clip. Those clips are grouped into strips, and the strips run in two different directions on the same board.
- In the main area, each column of five holes is one connection. Holes a to e in column 12 are the same electrical point. Holes f to j in column 12 are a different one.
- Nothing is connected along a row. Column 12 and column 13 are separate, even though they are touching.
- The rails at the top and bottom run the long way, the whole length of the board, and are meant for power and ground.
The gap down the middle is the point
That channel exists so a DIP chip can straddle it. Drop a 14-pin IC across the gap and each pin lands in its own column of five, with four spare holes next to it to wire into — and, crucially, the pins on the left of the chip are not shorted to those on the right.
The rail that stops halfway
This one wastes whole afternoons. On many full-size breadboards the power rails are split at the centre — the left half and the right half are separate strips, often shown by a break in the printed red and blue lines.
So you connect your supply at the left end, build at the right end, and the right end has no power at all. There is nothing to see; the wire looks connected because it is.
- 1
Look for the break in the printed line
If the coloured stripe stops and restarts near the middle, the rail is split.
- 2
Fit two link wires across the break
One for the positive rail, one for the ground rail, as the very first thing you do on a new board. Do it whether or not you think the board is split — it costs two wires and removes the question forever.
- 3
Check with a meter rather than by eye
Continuity mode, one probe at each end of the rail. It beeps or it does not. Ten seconds, and it settles it.
The other faults, in order of how often they happen
- 1
No common ground
Two supplies — a USB-powered board and a separate battery for a motor, say — with their grounds not joined. Every signal between them is then measured against nothing and behaves at random. Grounds always tie together, even when the positive rails are kept apart.
- 2
Missing decoupling capacitors
Breadboard wiring has real inductance, so when a chip switches it drags its own supply down for a moment. A 100 nF capacitor between the Vcc and ground pins of every IC, as close to the chip as physically possible, is not optional on a breadboard. It fixes an enormous share of the circuits that work sometimes.
- 3
A worn hole
The clips lose their grip after a few hundred insertions, and thick or square leads accelerate it. If one connection is intermittent when you press on it, move the whole circuit a few columns along. A cheap breadboard is a consumable.
- 4
Solid core in some holes, stranded in others
Stranded wire splays inside the clip, holds badly, and leaves stray strands that short to the next column. Use 22 AWG (0.6 mm) solid core, or ready-made jumpers.
- 5
Leads too long, and the circuit is a bird's nest
Long jumper loops pick up interference and, more importantly, you cannot trace them. Cut links to length so they lie flat on the board. Debugging is mostly reading, and you cannot read a nest.
What a breadboard cannot do
- High current. The clips are good for roughly 1 A and the rails have real resistance. Motors, heaters and anything above an amp belong on screw terminals and thick wire.
- High frequency. Every pair of adjacent strips forms a small capacitor and every jumper is an inductor. Above a few megahertz the board becomes part of your circuit, and RF simply will not behave.
- Mains voltage. Never. The clip spacing is 2.54 mm and nothing about the board is insulated for it.
- Anything that must survive being moved. A breadboard is for proving a design. Once it works, transfer it to stripboard or a PCB — a project carried to a viva on a breadboard will have shaken at least one wire loose by the time it arrives.