part · Level 1 · Light · passive
What a 10k ohm resistor is used for
10 kΩ resistor — A big speed bump, used for sensing tricks later.
What it is
A resistor limits current, sized in ohms (Ω). 10 kΩ — ten thousand ohms, bands brown, black, orange — is big enough that almost no current gets through at battery voltages: 4.5 ÷ 10 000 = 0.45 mA. Put it on an LED and you get a faint glow.
You use 10 kΩ to measure things. It is too large for powering them. Sensors like the LDR (light) and thermistor (temperature) are resistors that change with the world, and their room-condition value is around 10 kΩ. Pair the sensor with a fixed 10 kΩ and you get a voltage divider: a middle point whose voltage reports what the sensor sees.
It is also the standard pull-up — a resistor that holds a wire at + when nothing else is driving it, so a switch can pull it to − without ever shorting the battery.
How it works
Voltage divider. Two resistors in series from + to −. The voltage at the point between them is the supply multiplied by the bottom resistor's share of the total:
V_mid = 4.5 V × R_bottom ÷ (R_top + R_bottom)
Fixed 10 kΩ on top, LDR on the bottom:
- Bright room, LDR ≈ 1 kΩ: 4.5 × 1 ÷ 11 ≈ 0.41 V
- Dark, LDR ≈ 100 kΩ: 4.5 × 100 ÷ 110 ≈ 4.1 V
That swing from 0.4 V to 4.1 V is what a transistor (which wakes up near 0.7 V) or an Arduino analog pin can read. Swap the two resistors and the numbers flip — dark gives 0.4 V, bright gives 4.1 V. Same parts, opposite sensor.
Pull-up. 10 kΩ from + to a transistor base; a switch from that base to −. Switch open: the base is held up through the 10 kΩ, and with the base clamped at 0.7 V the resistor passes (4.5 − 0.7) ÷ 10 000 ≈ 0.38 mA — enough to turn a 2N2222 on. Switch closed: the base is at 0 V, the transistor is off, and the 10 kΩ wastes only 0.45 mA instead of shorting the pack. That is the Trip Wire Alarm.
Power. Even at the full 4.5 V: 4.5 × 0.00045 ≈ 0.002 W. It never gets warm.
The numbers
| Resistance | 10 kΩ = 10 000 Ω — bands: brown, black, orange |
|---|---|
| Current at 4.5 V across it | 0.45 mA |
| Divider mid with 1 kΩ LDR (bright) | ≈ 0.41 V — 10 kΩ on top |
| Divider mid with 100 kΩ LDR (dark) | ≈ 4.1 V |
| Red LED current | ≈ 0.25 mA — a faint glow — not for lighting |
See it working
The Automatic Night Light circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Mixed up with 1 kΩ. Brown-black-orange (10 kΩ) vs brown-black-red (1 kΩ) — one colour apart. Fix: measure it, or compare the third band under good light.
- Divider feeding an LED directly. The middle point can only supply well under a milliamp. Fix: the divider drives a transistor base; the transistor drives the LED.
- Divider the wrong way round. The night-light comes on in daylight. Fix: swap the sensor and the 10 kΩ — the formula tells you which way.
- Pull-up too small (220 Ω). Closing the switch dumps 20 mA for nothing. Fix: 10 kΩ; the base only needs tenths of a milliamp.
Questions people ask
- What are the colour bands for a 10k resistor?
- Brown, black, orange, then gold: 1, 0, three zeros = 10 000 Ω = 10 kΩ at ±5 %.
- Why is 10k used for a pull-up resistor?
- It is small enough to hold the pin firmly at + against noise, yet large enough that closing the switch to − wastes only 4.5 ÷ 10 000 = 0.45 mA.
- Which resistor goes with a photoresistor?
- One close to the sensor’s middle value so the divider swings widely. Hobby LDRs sit near 1 kΩ in light and 100 kΩ+ in dark, so 10 kΩ gives a 0.4 V to 4.1 V swing at 4.5 V.
Projects that use this part
- 🌙 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
- 🕯️ 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
- 🔆 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
- 🧪 The Brightness Lab — Run a real experiment: predict, measure with your eyes, and prove Ohm's law. L1 · 30 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
- 🏝️ 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
- 🌇 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
- 🌅 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
- 🙃 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
- 🌞 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
- 🕸️ 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
- 🪶 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
- 🏃 The Four-Step Chaser — A 555 ticks five times a second and a CD4017 counter walks the light from LED to LED, round and round. Hold a button and the chase freezes. L6 · 60 min
verified against: SparkFun — Voltage Dividers · SparkFun — Pull-up Resistors · SparkFun — Resistors