User interface

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
$\mid$	Start
$\bigcirc$	Stop

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