Circuits you can open and run.

A flashlight. Tap the switch to open and close the loop.

Key up, LED dark. Tap the switch to send dots and dashes.

Three LED lanes in parallel, all lit together by one switch. For the sequence, see Traffic Light Sequencer.

Two lanes: tap lane B's resistor to cycle values and compare.

Darken the room and the transistor switches the LED on.

Two keys in series. Both must be turned for the light.

The night light's divider flipped: light wakes the LED.

A 10 kΩ pot in series with a 220 Ω safety resistor. Tap the pot to turn the knob and watch the LED fade.

Open the lid (bright room) and the buzzer sounds.

Press the button: buzzer plus a headphones-on light.

Hold the button and the motor spins. Flip the battery to run it backwards.

A 10 kΩ pot in series with the LED. Tap it to turn the knob.

A diode only passes current one way. Tap it to turn it around.

Warm the thermistor (slide the temperature up) and the LED comes on.

Hold the button to charge the capacitor. Let go: the LED fades out.

A PNP switches the + side of the lamp. Close the switch to pull its base low and the LED lights.

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.

A MOSFET turns the LED fully on, yet its gate draws nothing. Hold the button, then tap the 10 kΩ to make it 100 kΩ and see the gate still win.

Two NAND gates make an AND: the LED lights only while BOTH buttons are down. The first logic chip, and the reason NAND is called universal.

The classic 555 astable. Watch the capacitor's sawtooth on the scope.

555 monostable: tap the button and the LED stays on for about a second.

A 555 at 480 Hz into a piezo. Tap the pot on the CTRL pin to change the pitch.

The traffic light with a switch per lane. You run the sequence: red, green, amber.

A 555 clock steps a 4017 counter: red, green, amber, repeat. The real thing.

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.

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.

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.

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.

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.

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 normal brightness.

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.

The Vault with a big red emergency button wired around both locks — turn BOTH keys, or hit the button, and the light comes on.

Three switches in a line — the LED only lights when all three are closed at once. AND, one step past The Vault.

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.

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.

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.

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.

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.

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.

Two push buttons, two LEDs, one battery — no sharing. Each button lights its own lamp with its own current, independent of the other.

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.

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.

Two slide switches in parallel, one shared LED — either switch alone lights the lamp. The OR gate, at Level 1.

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.

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.

Your first real tool — two probe wires that beep and light up whenever the tips touch anything that conducts.

Dial-a-Brightness for sound — one knob rides the buzzer between full alarm and library silence, with no button in the way.

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.

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.

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.

Two buttons wired so either one fires the same buzzer and light — your first OR gate.

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.

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.

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.

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.

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.

Two push buttons in series feeding a buzzer and an LED — both hands must stay pressed for the alarm to sound.

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.

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.

A doorbell with a volume knob — the pot lives in the buzzer lane, so twisting it changes the beep without touching the light.

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.

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.

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.

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.

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.

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.

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.

The Automatic Night Light with a force-on override — flip the slide switch and the lamp stays lit even in daylight, ignoring the sensor.

A thin wire across a doorway holds the alarm silent. Break the wire and the buzzer screams — until you connect it back.

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.

The Automatic Night Light with a manual override — flip the slide switch off and the lamp stays dark even in a pitch-black room.

Cup your hand over the sensor and the LED lights up — a thermometer that turns into a switch.

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.

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.

Your first motor. A switch, a coil, and a diode that catches the voltage kick when the current stops.

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.

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.

Sunrise Fan's mirror image — the room gets DARK and the fan spins. A bedside cooling breeze that starts as the lights go out.

Press the button, the fan spins. A tiny finger current commands a hundred-milliamp motor through a transistor.

Two buttons, either one fires the fan. Panic Doorbell wearing a motor — an OR gate that spins.

One button, three mouths: a spinning motor, a red LED, and a buzzer all fire together. One transistor drives them all.

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.

Sunrise Alarm with the LED swapped for a motor. The sun rises, the fan starts — an automatic wake-you-up desk breeze.

Trip Wire Alarm's brain wearing a motor. The wire is intact — the fan is silent. Snap it — the fan spins as the alarm.

Two motors from one button — a Level 9 robot drivetrain, five levels early.

A little desk fan that starts when you cup your palm over the sensor. Warm-Hand Alarm's brain, motor mouth.

Built on the Workbench.