Sensors, components and resistors

Electronic products can sense light, heat and touch. Knowing your components lets you design circuits that respond to the world.

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Input sensors

Sensors are input components that respond to changes:
LDR (light-dependent resistor): its resistance falls as light gets brighter. Used in night lights.
Thermistor: in the common type, its resistance falls as temperature rises. Used in thermostats and fire alarms.
Switches: push switch, toggle switch, microswitch (a light touch operates it) and reed switch (operated by a magnet, used in door alarms).
Moisture sensor: detects water, for example in plant soil.

Output components

LED: gives light, uses little power. The longer leg is usually the positive (anode).
Buzzer: makes a sound. Motor: gives rotary motion. Solenoid: an electromagnet that pushes or pulls a rod (linear motion).
Ohm's law: voltage (V) = current (I) × resistance (R). So R = V ÷ I.
This helps you choose a resistor for an LED.

Resistor colour code

Resistors have coloured bands. The first two bands are digits; the third is the number of zeros (multiplier); the fourth is the tolerance (gold = ±5%).
Example: yellow, violet, brown = 4, 7, one zero = 470 Ω.
ColourValue
Black0
Brown1
Red2
Orange3
Yellow4
Green5
Blue6
Violet7
Grey8
White9
Worked example

An LED needs 2 V and 0.02 A. It is powered by a 5 V supply. What resistor value is needed in series?

  1. The resistor must take the rest of the voltage: 5 - 2 = 3 V.
  2. R = V ÷ I.
  3. R = 3 ÷ 0.02 = 150 Ω.

Answer: 150 Ω.

Key idea

LDR resistance falls as light increases; a thermistor's resistance (common type) falls as temperature rises. Reed switches work with a magnet. Ohm's law: V = I × R. Resistor bands: two digits, then the number of zeros.

Key words
LDR
Light-dependent resistor: an input sensor whose resistance falls as light gets brighter.
Thermistor
An input sensor whose resistance changes with temperature. In the common type it falls as temperature rises.
Ohm's law
Voltage (V) = current (I) × resistance (R). So R = V ÷ I.
Resistor
A component that limits the current in a circuit. Its value is measured in ohms (Ω).
Sensor
An input component that responds to a change, such as light, temperature or pressure.
LED
Light-emitting diode: an output that gives light and only lets current flow one way.

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Quick check

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Extra practice

2 more questions

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  1. What happens to the resistance of an LDR in the dark?

    1. It rises
    2. It falls
    3. It stays the same
    4. It becomes zero
    Show the answer

    Answer: It rises

    Less light, more resistance.

  2. Why is a reed switch useful on a window alarm?

    1. It senses temperature
    2. It senses light
    3. A magnet on the window opens or closes it when the window moves
    4. It is pushed by hand
    Show the answer

    Answer: A magnet on the window opens or closes it when the window moves

    Magnets operate reed switches.

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Sources and references

Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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