part · Level 1 · Light · passive
When to use a 1k ohm resistor
1 kΩ resistor — A bigger speed bump — about five 220s stacked.
What it is
A resistor limits current; its size is in ohms (Ω). 1 kΩ is one kilo-ohm, 1000 Ω — about four and a half of your 220 Ω resistors in a row. The bands are brown, black, red: 1, 0, two zeros.
Where 220 Ω is an LED's bodyguard, 1 kΩ is the "just a trickle" value. It shows up in three jobs on this site: making an LED deliberately dim, feeding a transistor's base a small, polite current, and forming half of a voltage divider with a thermistor.
No polarity, no heat to speak of. At 4.5 V straight across it, it passes 4.5 ÷ 1000 = 4.5 mA and dissipates 4.5 × 0.0045 ≈ 0.02 W — a tenth of its 0.25 W rating.
How it works
Dim LED. Red LED at 2.0 V on the 4.5 V pack: 2.5 V across the resistor, 2.5 ÷ 1000 = 2.5 mA. Compare 11 mA with 220 Ω. The Brightness Lab is exactly this side-by-side.
Transistor base. A 2N2222 turns fully on with a few tenths of a milliamp into its base, and its base pin looks like a diode fixed at about 0.7 V. If the divider feeding it sits at 1.0 V, the base resistor sees 1.0 − 0.7 = 0.3 V, so 0.3 ÷ 1000 = 0.3 mA flows in. The transistor multiplies that by roughly 200, so it could pass 60 mA — far more than the 11 mA LED or 30 mA buzzer needs, which means it is switched hard on. Without the 1 kΩ the divider would be dragged straight down to 0.7 V and the sensor could never lift it further.
Divider bottom. With a thermistor (10 kΩ at room temperature) on top and 1 kΩ on the bottom: V_mid = 4.5 × 1 ÷ (10 + 1) ≈ 0.41 V. Warm the bead to about 4 kΩ: 4.5 × 1 ÷ 5 = 0.9 V. That crossing of the 0.7 V line makes the Fever Beeper switch.
Quiet buzzer. 1 kΩ in series with an active buzzer that behaves like ~150 Ω: 4.5 ÷ 1150 ≈ 3.9 mA, enough for a whisper instead of the 30 mA shout.
The numbers
| Resistance | 1 kΩ = 1000 Ω — bands: brown, black, red |
|---|---|
| Red LED current at 4.5 V | ≈ 2.5 mA — clearly lit, soft |
| Base current from a 1.0 V divider | ≈ 0.3 mA — (1.0 − 0.7) ÷ 1000 |
| Divider with 10 kΩ on top | 0.41 V — 4.5 × 1 ÷ 11 |
| Power at 4.5 V across it | ≈ 0.02 W — rated 0.25 W |
See it working
The The Brightness Lab circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Bands confused with 10 kΩ. Brown-black-red is 1 kΩ; brown-black-orange is 10 kΩ. Fix: the third band colour is the giveaway — red = two zeros, orange = three.
- Skipping the base resistor "because it works". It does, until the transistor or the sensor divider overheats or misbehaves. Fix: 1 kΩ between divider and base, every time.
- Using 1 kΩ where 220 Ω was meant. LED is dim and you think it is broken. Fix: check the bands.
Questions people ask
- What are the colour bands of a 1k resistor?
- Brown, black, red, then gold: 1, 0, two zeros = 1000 Ω = 1 kΩ at ±5 %.
- Will a 1k resistor light an LED?
- Yes, dimly. On 4.5 V with a 2.0 V LED it passes (4.5 − 2.0) ÷ 1000 = 2.5 mA — visible in a normal room, faint in sunlight.
- Why is there a 1k resistor on the transistor base?
- To set a small, safe base current — about (1.0 − 0.7) ÷ 1000 ≈ 0.3 mA — and to stop the base from pulling the sensor divider straight down to 0.7 V.
Projects that use this part
- 🧪 The Brightness Lab — Run a real experiment: predict, measure with your eyes, and prove Ohm's law. L1 · 30 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
- 🌙 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
- 💨 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
verified against: SparkFun — Resistors · SparkFun — Transistors · SparkFun — Voltage Dividers