3.6 - 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.

Sensing circuits with LDRs and thermistors

• An LDR or thermistor can be used in a sensing circuit to control another component as surroundings change.
• The supply's potential difference is shared between components based on their resistances: the larger a component's resistance, the larger its share of the potential difference.

Example - Thermistor controlling a fan

• The thermistor is in series with a fan, with a fixed resistor connected in parallel with the fan.
• As the temperature rises, the thermistor's resistance falls, so it takes a smaller share of the supply potential difference.
• The potential difference across the fan therefore rises, making the fan turn faster.

---

Example - Bulb across an LDR

• A bulb connected in parallel across an LDR shares the LDR's potential difference.
• In the dark, the LDR's resistance is high, so the potential difference across it is high and the bulb shines brightly.
• The bulb gets brighter as the surroundings get darker.

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.

Uses

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