# User interface

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A User interface allows a user to interact with a machine. User interfaces mainly provide two things:

• input The user can change things; he or she can change how the machine works, or give more information to the machine.
• output After the user has given some input, the machine will do something, and then provide some output
An example of a user interface with pushbuttons

Many machines can be very dangerous. A machine should have a user interface that can be handled easily, even if the person operating the machine has panicked. The user interface should therefore be intuitive, and simple to use. An example of such a user interface is that of the kill switch. A kill switch must shut off the machine at all costs - the idea is to avoid injury or harm to people. This is very different from shutting off the machine at the end of the shift, or when it is no longer needed.

According to EN ISO 13850, the kill switch has to be red on a yellow background.

The colors used to mark different states are close to those used by signals used on the road.

Display
Color Meaning Notes
Red Danger Alerting of possible danger or of states which make it very important to act immediately
Yellow Something is not normal If nothing is done, the situation may become dangerous.
Blue Something needs to be done The person operating the machine needs to do something
Green Everything is normal Used to show safe conditions, also used to start a new process.
White Neutral Confirmation, also used for things that cannot be expressed by red, yellow, blue or green.
Operating panel
Color Meaning What it does Notes
Red Operate in an emergency Kill switch, stop, also used for fighting fire Must not be used for stating/putting the machine into operation
Yellow Something needs to be done to get back to normal Re-start, Operation to avoid anormal condition or unwanted change. Must not be used for either starting or stopping a machine.
Blue Start something new Start, Reset
Green Start the usual/common procedure Start from a safe state Must not be used for stopping/switching off
White meaning underermined Start/On (preferred), Stop/Off
Grey Start/On, Stop/Off
Black Stop/Off (preferred), sometimes Start/On

There may be additional symbols, for example:

Symbol What it does
${\displaystyle \mid }$ Start
${\displaystyle \bigcirc }$ Stop

In many cases, such symbols are better, because some people are color blind. They need to be explained, like warnings, though.