Input devices allow systems to understand changes in the environment around them. Examples include a sensor such as a light-dependent resistor (LDR) that senses light levels for street lamps to know when it is dark, or thermistors that detect when it is too hot or cold in a room.
Input devices are usually either switches or sensors.
Switches allow current to flow through them when the contacts inside are joined together. They are usually named after how they work. For example, a push-to-make switch allows current to flow (or a signal to be passed on for processing) when pressed - therefore ‘making’ the circuit. A push-to-break switch does the reverse and ‘breaks’ the circuit.
Other examples of switches include:
Examples of uses for each kind of switch may include:
|Type of switch||Uses|
|PTM/PTB switch||Console controller buttons, eg fire or jump|
|Reed (magnetic) switch||Window sensors on alarms, eg window opens and switch contacts open|
|Toggle switch||Power switches|
|Rocker switch||Light switches|
|Tilt switch||To detect if something is no longer level|
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.
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 (NTC - negative temperature co-efficient), but some thermistors can work in the opposite way (PTC - positive temperature co-efficient) 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.
Pressure sensors produce a signal that varies depending on the amount of pressure placed on them. They are useful for security systems. For example, a pressure pad can detect if somebody walks on top of it and then trigger an alarm. Some types can also be used to measure the pressure of gases and liquids.
Name a suitable input device for a greenhouse cooling system.