2.3 - Components

Light-dependent resistors (LDRs)

• A light-dependent resistor (LDR) is a resistor whose resistance depends on the intensity of light falling on it.
• In darkness, its resistance is highest.
• As the light gets brighter, its resistance falls.
• Common applications: automatic night lights, outdoor lighting, and burglar detectors.

Thermistors

• A thermistor is a resistor whose resistance depends on temperature.
• In cold conditions, its resistance is high.
• As the temperature rises, its resistance falls.
• Because resistance falls as temperature rises, it's called a negative temperature coefficient (NTC) thermistor.
• Common applications: car engine temperature sensors, thermostats, irons, and fire alarms.

Light-emitting diodes (LEDs)

• A light-emitting diode (LED) is a diode that emits light when a current flows through it in the forward direction.
• Like any diode, it only lets current flow one way, so it only lights up when connected the right way round.
• LEDs indicate the presence of a current in a circuit, so they're often used in appliances to show they're switched on.

Advantages and uses

• Uses: energy-saving lighting, traffic lights, screens, and remote controls.
• Unlike filament bulbs, LEDs have no filament to burn out.
• They use a much smaller current than traditional filament lamps.