skill

Series & parallel

Parts in a chain share voltage; parts side-by-side share current.

first shows up at Level 1 · Light

There are only two ways to hook two parts together:

Series — end to end, like train cars. The same current flows through both. They share the voltage between them.
Parallel — side by side, both connecting the same two points. They see the same voltage. They share the current between them.

When it matters

Two LEDs in series on 4.5 V: each gets about 2 V, no problem, one resistor limits current for both. Two LEDs in parallel: each still needs its own resistor, or the brightest one hogs the current and the other stays dim.
Two resistors in series: their resistances add up (100 Ω + 220 Ω = 320 Ω). In parallel: the total resistance is smaller than either one alone — more paths mean easier flow.

Every time you place a part on a breadboard, ask: is this in series with what came before, or in parallel with it? The answer changes the math completely.

Projects that teach this

Twin Reds Two red LEDs on one loop — same current, half the leftover push, and a lesson in how voltages stack when parts sit end to end. L1 15 min Tabletop Traffic Light Three LEDs on three parallel lanes, all lit by one switch — build a tabletop traffic light and discover parallel circuits. L1 25 min The Vault Two buttons, one loop — the LED only lights when BOTH are pressed at the same time. L1 15 min Do Not Disturb Sign A green "chill" LED plus a switched red "leave me alone" LED — two lanes on one battery, and a switch that only touches one of them. L1 20 min Handhold Chain Three switches in a line — the LED only lights when all three are closed at once. AND, one step past The Vault. L1 15 min Two-Door Signal Two slide switches in parallel, one shared LED — either switch alone lights the lamp. The OR gate, at Level 1. L1 15 min Signal Light A slide switch and a push button in parallel — the slide is the steady beacon, the button is the extra flash. Either lights the lamp. L1 15 min Escape Hatch The Vault with a big red emergency button wired around both locks — turn BOTH keys, or hit the button, and the light comes on. L1 20 min The Brightness Lab Run a real experiment: predict, measure with your eyes, and prove Ohm's law. L1 30 min Dial-a-Brightness Twist a knob and watch the LED fade up and down — your first "how much" input. L2 20 min Dial-a-Buzz Dial-a-Brightness for sound — one knob rides the buzzer between full alarm and library silence, with no button in the way. L2 15 min Continuity Tester Your first real tool — two probe wires that beep and light up whenever the tips touch anything that conducts. L2 20 min Panic Doorbell Two buttons wired so either one fires the same buzzer and light — your first OR gate. L2 20 min Whisper Alarm A doorbell with a volume knob — the pot lives in the buzzer lane, so twisting it changes the beep without touching the light. L2 25 min Two-Tone Doorbell Two call buttons, one doorbell: LOUD rings the buzzer and lights the porch LED; QUIET starves the buzzer and shows why it can't just be dimmed. L2 25 min Arm & Ring A doorbell that ignores you unless a hidden slide switch is armed first — series-AND with sound, plus an 'armed' LED that shows the state. L2 20 min Twin Bells One doorbell button rings two buzzers in parallel — one in your room, one in the kitchen — with a slide switch that mutes just the bedroom bell at night. L2 20 min Two-Hand Bell Two push buttons in series feeding a buzzer and an LED — both hands must stay pressed for the alarm to sound. L2 20 min Silent Alarm Three panic buttons wired in parallel, all inside an armed lane — any one of them fires the buzzer, and a small LED tells you the alarm is watching. L2 25 min Quiz Buzzer Two teams, two big buttons: yours lights your LED and rings your buzzer, theirs does the same on their side. Two independent doorbells sharing one battery. L2 20 min Dusk Chorus The Curfew Beeper with a red LED wired in parallel with the buzzer — one dusk sensor drives a chime and a lamp at the same time. L3 30 min Warm Hand Chorus Fever Beeper with a red LED added in parallel with the buzzer — one warm-hand sensor drives a chime and a lamp at the same time. L3 30 min First Spin Your first motor. A switch, a coil, and a diode that catches the voltage kick when the current stops. L4 15 min Sunrise Fan Sunrise Alarm with the LED swapped for a motor. The sun rises, the fan starts — an automatic wake-you-up desk breeze. L4 25 min Panic Fan Two buttons, either one fires the fan. Panic Doorbell wearing a motor — an OR gate that spins. L4 20 min Party Spinner One button, three mouths: a spinning motor, a red LED, and a buzzer all fire together. One transistor drives them all. L4 25 min Twin Wheels Two motors from one button — a Level 9 robot drivetrain, five levels early. L4 25 min

Parts you'll meet along the way

L1 1 kΩ resistor L1 10 kΩ resistor L1 220 Ω resistor L1 3×AA battery pack (4.5 V)L1 Green LED (5 mm)L1 Jumper wires L1 Push button (tactile)L1 Red LED (5 mm)L1 Slide switch L1 Solderless breadboard L1 Yellow LED (5 mm)L2 10 kΩ potentiometer L2 Active buzzer (3–5 V)L3 Light sensor (LDR)L3 NPN transistor (2N2222)L3 Temperature sensor (10 kΩ thermistor)L4 DC motor (3 V)L4 Diode (1N4001)