Two ways to connect things — and the rules of thumb that tell you what happens.
Every multi-part circuit connects its pieces in one of two arrangements, or a mix. Knowing which
is which lets you predict behavior at a glance — without algebra. This is the difference
between guessing and reasoning when you wire up several LEDs, sensors, or power connections.
The one win
Look at two components and instantly say whether current is shared (series) or split
(parallel) — and what that means for your project.
Series: one single-file path
Series means components are in a line, one after another, on a single
path. There's only one road for current to take, so it must pass through every part in turn.
Series: A then B on one loop. The same current flows through both.
Rules of thumb for series:
Same current through everything. One path means one flow rate, shared by all.
Voltage splits up among the parts (remember from
Lesson 1: each component "uses up" some pressure).
Resistances add. Two resistors in series = their values summed. More parts in
line → more total resistance → less current. (That's the only arithmetic here.)
Break one, break all. Old Christmas lights in series: one bulb dies, the whole
string goes dark — the loop is broken.
Parallel: side-by-side paths
Parallel means components are connected across the same two points,
giving current multiple roads. Each part is its own branch between the same node pair.
Parallel: A and B each bridge the same top and bottom nodes. They share voltage; current splits between them.
Rules of thumb for parallel:
Same voltage across each branch. Both ends touch the same two nodes, so each
part feels the full source voltage.
Current splits between branches and re-joins after. Add a branch, draw more
total current from the source.
Independent. Unplug one branch and the others keep working — they each have
their own path. (Your home's outlets are wired in parallel for exactly this reason.)
Rule of thumb for your projects
Power your modules in parallel: run every board/sensor's power pin to the
+ rail and every ground to the GND rail. Each gets the full voltage and
works independently. You almost never want to put whole modules in series.
The big exception: LEDs
Each LED still needs its own series resistor. Putting several LEDs in parallel on a single
shared resistor leads to uneven brightness and trouble — give each LED branch its own current limit.
I'm your teacher — ask me anything. "Should these be series or parallel?" is one of
the most useful questions you can bring me about a real build. Ask away.