Systems
Inputs, outputs and processes in systems used by designers
Inputs
An electronic component capable of passing information from the outside world into an electronic system for processing. allow systems to understand changes in the environment around them. Examples include a A device that measures a physical quantity and converts it into a signal to be read and interpreted. such as a A type of resistor that is affected by changes in light levels. A cadmium sulphide layer causes a decrease in resistance in the light and increase in the dark. that senses light levels for street lamps to know when it is dark, or An electrical device whose resistance decreases as its temperature increases. 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 The potential difference across a cell, electrical supply or electrical component. It is measured in volts (V). and Moving electric charges, eg electrons moving through a metal wire.
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 A switch that 'makes' a circuit when pressed. allows current to flow (or a signal to be passed on for processing) when pressed - therefore 鈥榤aking鈥 the circuit. A A switch that 'breaks' a circuit when pressed. does the reverse and 鈥榖reaks鈥 the circuit.
Other examples of switches include:
- reed (magnetic) switch
- toggle or rocker switch
- tilt switch
Image caption, Push-to-make (PTM) and push-to-break (PTB) switch
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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 |
| Type of switch | PTM/PTB switch |
|---|---|
| Uses | Console controller buttons, eg fire or jump |
| Type of switch | Reed (magnetic) switch |
|---|---|
| Uses | Window sensors on alarms, eg window opens and switch contacts open |
| Type of switch | Toggle switch |
|---|---|
| Uses | Power switches |
| Type of switch | Rocker switch |
|---|---|
| Uses | Light switches |
| Type of switch | Tilt switch |
|---|---|
| Uses | To detect if something is no longer level |
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 The opposition in an electrical component to the movement of electrical charge through it. Resistance is measured in ohms. changes with the light level. As the light gets brighter, its resistance decreases. It can therefore be used as a simple light sensor.
A An electrical device whose resistance decreases as its temperature increases. 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.
Image caption, Light-dependent resistor (LDR)
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Question
Name a suitable input device for a greenhouse cooling system.
Thermistor.