skill

Ohm's law

Voltage = current × resistance. One equation, endless circuits.

first shows up at Level 1 · Light

See it working

simulating…

The LED eats about 2 V, so the resistor sees 4.5 − 2 = 2.5 V. I = V ÷ R = 2.5 ÷ 220 ≈ 11 mA. Open the circuit and tap the resistor to try 100 Ω or 470 Ω — the current moves exactly how the equation says. 🔧 Open this circuit in the Workbench →

Ohm's law is the one equation you'll use forever:

V = I × R

V — voltage, in volts. How hard the battery pushes.
I — current, in amps. How much electricity is actually flowing.
R — resistance, in ohms (Ω). How hard the part fights back.

Rearrange it for whichever piece you're solving for:

I = V ÷ R — "how much current will flow?"
R = V ÷ I — "what resistor do I need?"

Real example — picking an LED resistor

Every LED project on this site does the same calculation. Say you have a 4.5 V battery pack, a red LED that "eats" 2 V across itself, and you want about 10 mA through it:

Voltage left for the resistor: 4.5 V − 2 V = 2.5 V
Resistor size: R = 2.5 V ÷ 0.010 A = 250 Ω
Nearest common part: 220 Ω → real current ≈ 11 mA. Perfect.

That's it. Every "why 220? why not 470?" question has a real answer, and Ohm's law is how you get it. No guessing.

Try the Ohm's law calculator — pick which of V, I or R you want, enter the other two, and it runs the circuit live.

Projects that teach this

First Light Build a real flashlight circuit and learn why every circuit is a loop. L1 15 min 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 Two Greens Twin Reds with green LEDs — bigger voltage bites, less leftover for the resistor, and a great lesson in how far a 4.5 V battery can actually push. L1 15 min Night Pin One 10 kΩ, one red LED, one slide — the simplest dimmer. A single big resistor runs the LED at 0.27 mA: invisible in daylight, perfect for a dark room. L1 10 min Bike Torch One yellow LED, two switches: a slide for a battery-friendly low beam, a push button for a bright flash, both lanes feeding the same lamp in parallel. L1 15 min Signal Pair Two push buttons, two LEDs, one battery — no sharing. Each button lights its own lamp with its own current, independent of the other. L1 15 min Dim Candle A 1 kΩ and a 10 kΩ in series throttle a red LED to a candle glow. Flip the bypass slide — the 10 kΩ gets shorted and the LED pops to a clear, steady glow. L1 15 min Three Sisters Three red LEDs side by side from one battery — each with its own 220 Ω — proving parallel lanes do their own Ohm's law and the battery adds the bill. L1 15 min One for All Three LEDs on ONE shared 220 Ω — the shortcut that fails. Red and yellow dim-split the current; green sits dark. Lesson: each LED needs its own resistor. L1 15 min Step Up Three 1 kΩ lanes in parallel into one red LED, each with its own slide switch. One slide = candle, two = glow, three = full lamp. The lanes stack. L1 15 min Longer Loop Three 1 kΩ speed bumps stacked end-to-end on one lane feed a single red LED. Resistors in series ADD up — the LED is throttled to a near-dark 0.87 mA. L1 15 min Bright and Dim Two red LEDs on one battery — one through 220 Ω (bright), one through 1 kΩ (candle). Same LEDs, two currents. The resistor sets each lane's brightness. L1 15 min Rainbow Loop Red, yellow and green LEDs on three parallel 220 Ω lanes under one slide. One flip lights all three, each lane running its own Ohm's law. L1 15 min Reserve Beacon A 10 kΩ in series with the LED keeps a dim always-on pilot glow. Flip the bypass slide — it shorts the 10 kΩ, and the pilot pops to full First-Light brightness. L1 15 min Short Cut First Light with a slide switch wired straight across the LED. Flip the slide and the LED goes dark even though the loop stays complete. 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 Pocket Beeper Level 2's answer to First Light: battery, push button, buzzer. Three parts, one loop, one loud beep — no resistor and no LED to explain. L2 10 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 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 Loud/Polite Doorbell One doorbell button, one buzzer, and a slide switch that shorts across a 1 kΩ resistor — flip it to swap between a startle-loud beep and a scratchy whisper. L2 20 min Duet Doorbell One button, two buzzers in parallel — one wired direct at full 30 mA, one starved through a 220 Ω to about 12 mA — voicing together at two different loudnesses. L2 20 min Vault Chime Three slide switches in a line, a buzzer and a red LED hanging off the end — the chime only sings and glows when every single switch is closed. L2 20 min Hidden Mute A doorbell with a secret slide switch tucked inside just the buzzer lane: flip it and the ring button still lights the LED, but the sound stays off. L2 15 min Power Pilot A green pilot LED that glows the whole time the battery is plugged in, next to a red alarm LED and buzzer that only fire when the button is pressed. L2 15 min Shared Knob One pot sits above the merge of two parallel LED lanes (red + yellow). Twist it and BOTH LEDs fade together — the knob throttles the shared rail. L2 15 min Automatic Night Light A light that switches itself on when the room gets dark — no hands, no code. L3 35 min Sunrise Alarm The night light — flipped. Swap two parts and the LED wakes up with the sun instead. L3 25 min Cookie Jar Alarm The Sunrise Alarm's brain wired to a buzzer instead of a lamp — quiet in the dark jar, screaming when the lid comes off. L3 25 min Trip Wire Alarm A thin wire across a doorway holds the alarm silent. Break the wire and the buzzer screams — until you connect it back. L3 25 min Warm Hand Alarm Cup your hand over the sensor and the LED lights up — a thermometer that turns into a switch. L3 30 min Reading Light The Automatic Night Light with a knob on top — turn it to dial in how dark the room has to get before the lamp fires. Your first tunable threshold. L3 30 min Vacation Nightlight The Automatic Night Light with a manual override — flip the slide switch off and the lamp stays dark even in a pitch-black room. L3 25 min Study Lamp The Automatic Night Light with a force-on override — flip the slide switch and the lamp stays lit even in daylight, ignoring the sensor. L3 25 min Curfew Beeper The Automatic Night Light's brain with a buzzer where the LED used to sit — beeps the moment the room goes dark, so you never miss sunset. L3 30 min Fever Beeper The Warm Hand Alarm's brain wired to a buzzer instead of a lamp — silent in a normal room, beeping the second a warm palm covers the sensor. L3 25 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 Dawn Chorus Cookie Jar Alarm with a red LED added in parallel with the buzzer — one light sensor drives a chime and a lamp the moment the sun hits. L3 30 min Trip-Wire Chorus Trip Wire Alarm with a red LED added in parallel with the buzzer — one thin wire holds the whole chorus silent; snap it and both mouths fire. L3 30 min Night Doorbell A doorbell that only works at night. The ring button fires buzzer and LED together — but only once the LDR sees darkness. In daylight the button does nothing. L3 30 min Alley Lamp Automatic Night Light with a push-button override. Dark room OR button press fires the lamp — a wired-OR built straight onto the transistor's base. L3 30 min The Upside-Down Transistor A PNP transistor switches the + side of a lamp and turns on when you pull its base DOWN. Same button, same LED, whole circuit flipped. L3 30 min Puff Button First Spin with the slide swapped for a push button. Press for a spurt of breeze, let go and it stops — a dust puffer for a keyboard or camera lens. L4 10 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 Safety Blender A slide lid switch and a push run button, both in series with the motor. Fan only spins when the lid is on AND your finger is on the button. L4 15 min Handheld Fan Press the button, the fan spins. A tiny finger current commands a hundred-milliamp motor through a transistor. L4 20 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 Dial-a-Spin Dial-a-Brightness with a motor instead of an LED. Twist the knob and watch the fan speed slide from full spin to a slow crawl. L4 20 min Trip Wire Fan Trip Wire Alarm's brain wearing a motor. The wire is intact — the fan is silent. Snap it — the fan spins as the alarm. L4 25 min Sun Beacon Sunrise Fan with a second lane — a yellow LED lights and a motor spins together the moment daylight hits the sensor. One brain, two mouths. L4 25 min Dusk Fan Sunrise Fan's mirror image — the room gets DARK and the fan spins. A bedside cooling breeze that starts as the lights go out. L4 25 min Warm Hand Fan A little desk fan that starts when you cup your palm over the sensor. Warm-Hand Alarm's brain, motor mouth. L4 25 min The 3.3-Volt Rule A zener diode holds 3.3 V for the LED no matter what the 9 V battery wants. Flick the switch to add a second load and watch the zener share. L5 30 min The Zener Battery Tester A zener in series with an LED makes it ignore the first 3.3 V. A fresh 9 V lights it; a run-down battery barely can. A sharper gauge than a bare LED. L5 25 min