1 kΩ resistor color code

Where you'll meet it

A transistor switch on a 5 V Arduino pin

A pin cannot drive much on its own, so it switches an NPN transistor (such as a BC547 or 2N2222) instead. The 1 kΩ resistor sits between the pin and the base and limits the base current. The load sits between +5 V and the collector, and the emitter goes to ground.

I_B = (5 V − 0.7 V) / 1 kΩ ≈ 4.3 mA

4.3 mA is easy for the pin, far below the 20 mA it should give.

For a switch that turns fully on, a common rule of thumb is to give the base about a tenth of the load current: 4.3 mA × 10 ≈ 43 mA, so loads up to about 40 mA. A BC547 is rated 100 mA.

From a 3.3 V board: (3.3 V − 0.7 V) / 1 kΩ ≈ 2.6 mA, good for loads up to about 26 mA.

The resistor turns (4.3 mA)² × 1 kΩ ≈ 0.018 W into heat, so a ¼ W part is plenty.

0.7 V is the usual base-emitter estimate; a transistor switched fully on often drops 0.8 to 0.9 V, so the real base current is a little lower (about 4.1 to 4.2 mA). Check your transistor's datasheet. A motor or relay coil also needs a flyback diode across it, and check its current first: many 5 V relay coils draw more than 40 mA.

On SMD parts

Look for 102, 1001 or 01B

• 102102 = 10 × 10² = 1,000 Ω (3-digit code).
• 10011001 = 100 × 10¹ = 1,000 Ω (4-digit code, usually ±1% parts).
• 01B01B = 100 × 10 = 1,000 Ω (EIA-96: 01 means 100, B means ×10).

1 kΩ has an EIA-96 code because 100 is the first value in the E96 series.

Check it with a meter

Read 950–1,050 Ω

1. Take the resistor out of the circuit.
2. Set the meter to Ω, range 2 kΩ (or auto).
3. Touch one probe to each lead. Keep your fingers off the metal: your body is a resistor too.
4. Anything from 950 to 1,050 Ω is a good ±5% part. A 5-band ±1% part should read 990 to 1,010 Ω.

A meter has a small error of its own, so a reading a hair outside the range does not prove the part is bad. Measure again with firm contact before you swap it.