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Input devices allow systems to understand changes in the environment around them. For example, a sensor like a light-dependent resistor (LDR) that senses light levels for street lamps that know when it is dark, or thermistors that detect when it is too hot or cold in a room.

Purpose of input devices

• examples of real world signals include light level, temperature and pressure
• examples of electronic signals include voltage and current

Sensors

Sensors can be used to detect changes in light level, temperature and pressure. They are used in a wide range of products, from night lights to security alarms and central heating systems.

Types of sensors

A light dependent resistor (LDR) is a special type of resistor whose resistance changes with the light level. As the light gets brighter, its resistance decreases. It can therefore be used as a simple light sensor.

A thermistor works in a similar way except it responds to changing temperature levels. Usually its resistance decreases as the temperature increases but some thermistors can work in the opposite way where resistance increases as temperature increases. An example of a use for a thermistor is in a heating system. It can be used to check the temperature in a room and trigger the turning on of a heater if it is too cold.