Electronics · Semester 1 — Analog · Lesson 3 · 55 min · build 2026.09.23-1644
Big idea
Series is one path. Parallel is more than one path from + to −. Series components sit on the only way home, so a single current passes through every one of them. Parallel branches each give the current another way home.
Safety — same two rules
No shorts (+ rail to − rail with nothing in between = a finger-burning wire and a knocked-out supply) and no LED without its resistor — a naked LED can pop like a firecracker. You'll be rearranging circuits all class today, so after every change, trace each path from the + rail to the − rail and check it still goes through the resistor before you reconnect power.
Kits closed. On scrap paper, draw lesson 2's switched loop from memory — supply on the rails, 220 Ω, LED, switch, node letters. Then open the box and check Build A against your drawing: fix anything the week shook loose, and prove both switch positions. (Box came up empty? Build it from your drawing, saying each node's letter as you plug it in.) Done when it clicks both ways. Leave it standing: today's circuits grow out of this one.
Predict
Compared to one LED, will two same-color LEDs in series be brighter, dimmer, or the same?
Power off. Take the switch back out and let LED 2 take over its two rows — long leg on the LED 1 side. A switch is a controlled break in the loop; you just filled the break with a component, and that's the whole series move. Check it by nodes (note node C is now between the LEDs): + rail → resistor A, resistor → LED 1 long leg B, LED 1 short leg → LED 2 long leg C, LED 2 short leg → jumper to the − rail D. Finger on the + rail: every way home passes both LEDs — no skipping either. That's series.
Why dimmer? Every component claims a share of the supply's 5 volts — its voltage drop. Think of a drop as a toll: an LED charges its toll (about 2 V for red) just to switch on. In series, the drops add up — less voltage is left over, so less current flows, and both LEDs share that same smaller current.
One quick reading skill before the next build: circuits are about to branch, and schematics need a way to say whether crossing lines are actually joined:
On the board
A junction dot means one node holding three or more legs. A row has five holes — room to spare — and when legs can't reach, a jumper merges two rows into one bigger node.
Predict
Brighter, dimmer, or the same as one LED? And: if you pull LED 2 out, does LED 1 stay lit?
Power off — small surgery, not a rebuild: pull LED 2 out, then rejoin LED 1's short leg and the − rail jumper into one node (move the leg or the jumper, whichever is closer). What's left is the plain single-LED loop — supply, 220 Ω, LED, home (the switch sits this one out). That loop is branch 1. Now the other move: give the circuit a second loop — branch 2 beside branch 1, a complete path with its own resistor:
Why the same brightness? Each branch is its own complete loop — resistor, LED, and home — sharing only nodes A and C with its neighbor (the middle nodes B and D belong to one branch each). Finger test: starting at +, you can get home skipping LED 1 entirely — or skipping LED 2. More than one way home: parallel. The branches genuinely don't interact — as long as the supply holds its 5 V steady, and ours does; that independence is parallel's superpower, and it's why your house's lights don't dim every time something else switches on. The supply, though, feels it: it feeds both branches at once — about twice the current leaves it, and its battery drains faster.
Think about it — holiday lights
Grandma's old string of lights died all at once when one bulb burned out — a series giveaway. Most modern strings survive a dead bulb, but not all the same way: some run parallel branches, and some are secretly still series, with a tiny bypass built into each bulb that heals the loop around a failure. Engineering keeps more than one fix on the shelf.
Now you're the engineer. Design this circuit on paper before touching a single part:
The spec
One power source, two resistors, two LEDs, two switches — each switch turns its own LED on and off, without affecting the other.
Hints: start from Build C. Where does a switch have to go to break one way home and not the other? Use junction dots where three legs join. When your schematic is drawn, label the nodes, count the rows you'll need — then build exactly what you drew. Final check before power: the finger test — trace every way home from the + rail to the − rail; each one must pass through a resistor. A path that skips both means a popped LED.
Staring at blank dots? Copy Build C's schematic into the box first, then figure out where the two switches go in it. Starting from a working drawing and modifying it is real engineering, not cheating.
Build B's rule — every component claims its toll, and in series the tolls add — has a breaking point, and your kit can reach it. The blue LED charges the biggest toll we stock: about 3.2 V.
Try it — red + blue
Power off. Pick one branch of whatever's standing — Build C, or your two-switch build — and give its LED a partner: cut that branch anywhere along its way home and fill the cut with the blue LED, long leg toward + (the same series move as Build B). Predict first: red's ≈ 2 V toll plus blue's ≈ 3.2 V — do they fit under the supply's 5 volts? Power up and watch that branch's budget run out. Then look at the other branch: not even a flicker. Its way home doesn't run through the traffic jam.
The math, for the curious — always optional
Voltage drops just add. Red LED ≈ 2.0 V, blue ≈ 3.2 V. Red + red = 4.0 V — under the supply's 5 V, so current flows and both light. Red + blue ≈ 5.2 V — over budget: the pair never properly turns on, and at most a trace of current sneaks through — dark, or the faintest glimmer. The leftover voltage is what drives current through the resistor. One red LED leaves 3 of the 5 volts:
Two reds leave only 1 volt:
— which is exactly why they're dimmer.
Stretch
1. Add a third complete branch to Build C — its own resistor and LED (the kit has spare 220 Ωs) — predicting brightness and battery drain first. 2. One LED, but two switches in series: when does it light? Now rebuild with the two switches in parallel: when does it light now? Congratulations — you just invented AND and OR, the atoms every computer is built from. Much more on that next semester.
On scrap paper, from memory: draw two LEDs in series and two LEDs in parallel — power source, resistors, and junction dots included (remember: in the parallel drawing, every branch owns a resistor). Circle the schematic where pulling one LED out kills the other. Hand it in at the door.
Cleanup: power off and leave your final circuit standing — the two-switch build or Build C, blue budget-breaker included if you got to §8. It rides in the box to lesson 4, which opens with surgery on it, not a rebuild — and the meter finally gets to open the books.