Resistors in Depth

A resistor’s job is to resist current by a set amount. Choose the right value and you control how much current flows, which is how you keep an LED safe, set a sensor’s sensitivity, or divide a voltage into a smaller one.

Every resistor has three numbers worth knowing. (Quick reminder from Lesson 5: on a four-band resistor, the bands are digit, digit, number of zeros, tolerance; on a five-band resistor, three digits, then zeros, then tolerance.)

• Resistance, in ohms (Ω), shown by its colour bands.
• Tolerance: how close the real value is to the marked one. Gold means within 5%; brown means within 1%.
• Power rating, in watts: how much heat it can safely get rid of. The small ones in most kits are ¼ watt.

• A potentiometer (“pot”) has a knob or screw that moves a contact along the resistance track. Many volume knobs are potentiometers. The trimmer in the Automatic Night Light is a small one.
• A light-dependent resistor (LDR) changes its resistance with light.
• A thermistor changes its resistance with temperature. The common “NTC” type has lower resistance when it’s warmer.

Sensors like these turn something in the world (light, heat) into a change in resistance a circuit can measure. Lesson 12 goes further.

In series, resistances add: 220 Ω + 220 Ω = 440 Ω.

In parallel, the total is less than the smallest one, because current has more than one path: 1 ÷ R_total = 1 ÷ R₁ + 1 ÷ R₂. Two 220 Ω resistors in parallel make 110 Ω.

Two resistors in series across a battery form a voltage divider. The voltage at the middle point is V_out = V_in × R₂ ÷ (R₁ + R₂), where R₂ is the bottom resistor. With 4.5 V and two equal resistors, the middle sits at 2.25 V. Make one of them a sensor, and the middle voltage changes with light or temperature. That’s exactly how the night light works.

A resistor turns electrical energy into heat: P = I² × R = V² ÷ R. Example: 9 V across 100 Ω gives P = 81 ÷ 100 = 0.81 W, more than three times a ¼ W rating, and that resistor would get hot enough to burn and could fail. Engineers usually choose a resistor rated for at least twice the power it will actually see.

Standard values follow the E-series: E12 has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82), spaced so that 10% tolerances just overlap. That’s why you see 220 Ω and 470 Ω, but not 200 Ω or 500 Ω, in basic kits. Precision E96 values fill the gaps for 1% parts.

2 kΩ (2,000 Ω).

500 Ω, half of one of them.

It goes down.