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Voltage dividers

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

first shows up at Level 3 · Sense

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 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

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 Red LED (5 mm)L1 Slide switchL1 Solderless breadboardL2 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)