Systems

Inputs, outputs and processes in systems used by designers

Inputs

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.

Purpose of input devices

curriculum-key-fact
Input devices take a signal from the physical or ‘real world’ and turn it into an electronic signal that a process device, such as a microcontroller, can understand and act upon.
  • examples of real-world signals include light level, temperature and pressure
  • examples of electronic signals include voltage and current

Input devices are usually either switches or sensors.

Switches

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:

  • reed (magnetic) switch
  • toggle or rocker switch
  • tilt switch
The standard switch symbols for push-to-make and push-to-break switches alongside a photograph of the switch.

Push-to-make (PTM) and push-to-break (PTB) switch

Examples of uses for each kind of switch may include:

Type of switchUses
PTM/PTB switchConsole controller buttons, eg fire or jump
Reed (magnetic) switchWindow sensors on alarms, eg window opens and switch contacts open
Toggle switchPower switches
Rocker switchLight switches
Tilt switchTo detect if something is no longer level
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The function of a toggle switch and rocker switch can be exactly the same - it is the shape of the case that determines what the switch is called.

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 (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.

Showing the standard electrical symbol for a light-dependent resistor (LDR) sensor alongside a photograph of the component.

Light-dependent resistor (LDR)

Question

Name a suitable input device for a greenhouse cooling system.

Thermistor.