Physics

1.3 - Components

6 min

Circuit symbols

Meters and resistors

LDRs and thermistors

note 1 of 3

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.

A curve falling steeply then levelling off, on axes labelled resistance (Ω) and light intensity, its high end marked dark and low end bright, beside a boxed circuit symbol labelled LDR.

note 2 of 3

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.

A curve falling steeply then levelling off, on axes labelled resistance (Ω) and temperature (°C), its high end marked cold and low end hot, beside a boxed circuit symbol labelled thermistor.

note 3 of 3

Diodes

A diode is a component, usually made from a semiconductor such as silicon, that lets current flow freely in one direction only.
In the reverse direction it has a very high resistance, so almost no current flows.

Forward and reverse bias

Forward bias - The direction in which current can flow.
Reverse bias - The direction in which the diode has high resistance and does not conduct.
A diode doesn't conduct until the forward voltage reaches a specific threshold value.

A current-voltage graph with I up the vertical axis and V along the horizontal, the line flat at zero for negative and small positive V, then curving sharply upwards.

Uses

Used in electronic circuits, such as radio receivers.
Used to turn alternating current (ac) into direct current (dc).

key terms

TermDefinition

Light-dependent resistor (LDR)A resistor whose resistance decreases as the intensity of light falling on it increases.

ThermistorA resistor whose resistance decreases as its temperature increases.

DiodeA component that lets current flow through it in one direction only.

Forward biasThe direction of potential difference in which a diode allows current to flow.

Reverse biasThe direction of potential difference in which a diode has a very high resistance and does not conduct.

end of notes

Key points to remember

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