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Voltage dividers

Two resistors split a voltage — the trick behind almost every sensor.

first shows up at Level 3 · Sense

See it working

simulating…

Two equal 10 kΩ resistors split 4.5 V into two 2.25 V halves. Open it and change either resistor — the middle voltage moves as a fraction of the battery, exactly like the formula says. 🔧 Open this circuit in the Workbench →

Put two resistors in series across a battery and the voltage at the point between them is a fraction of the battery voltage — and you get to choose the fraction with the resistor sizes.

V_mid = V_battery × R_bottom ÷ (R_top + R_bottom)

So on 4.5 V, if the top resistor is 10 kΩ and the bottom is also 10 kΩ, the middle sits at exactly 2.25 V — half. Change the ratio and the middle voltage moves.

Why every sensor uses this

Most cheap sensors are just resistors that change:

  • An LDR (light-dependent resistor) has lots of resistance in the dark and little in the bright light.
  • A thermistor has resistance that shifts with temperature.

Put one of those in a voltage divider with a normal fixed resistor, and the middle voltage becomes a measurement. Cover the LDR → middle voltage swings one way. Shine a flashlight on it → swings the other way. A Sense level project turns that swinging voltage into a switch that flips an LED.

Voltage dividers are how nearly every analog sensor in the world turns a changing resistance into a voltage.

Projects that teach this

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 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 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 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 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 Switch That Uses No Current 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. L5 30 min The Night Gate A night light switched by a MOSFET. The gate takes no current, so the moment it turns on is plain voltage-divider math you can work out exactly. L5 30 min

Parts you'll meet along the way

L1 1 kΩ resistorL1 10 kΩ resistorL1 220 Ω resistorL1 3×AA battery pack (4.5 V)L1 Jumper wiresL1 Push button (tactile)L1 Red LED (5 mm)L1 Slide switchL1 Solderless breadboardL1 Yellow LED (5 mm)L2 10 kΩ potentiometerL2 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)L5 100 kΩ resistorL5 MOSFET 2N7000