9. Lights

Lights such as LEDs (Light Emitting Diodes) can be connected to a computer or within equipment and respond to electrical signals which tell them to switch on and off.

Control devices such as lights, buzzers and motors are used when the computer is controlling a situation such as traffic lights.

The picture on the right is showing that the black toner is low on a colour laser printer. Note that is very common to use an icon next to the light rather than text. This is for two reasons

1. It is very compact, only a tiny space is needed for an icon compared to text
2. It is independent of language, so this printer can be sold just as well in English or Spanish countries without modification.

This light is controlled by the microprocessor inside the laser printer.

Advantages

• Very cost effective way of indicating the status of equipment (on / off / ready / offline etc)
• They can provide a warning for users with hearing loss who might not be able to hear the error beeps produced by the internal speaker.
• They come in a large range of different colours

Disadvantages

• The light may break or wear out and need replacing
• There may be a loose connection which means the light doesn't switch on even if the circuit or control program is correct
• They can only indicate simple status (on/off) - anything more complicated would need a dedicated display for example a DVD player may have LED display to show power on/off and a separate display to show disk loading status.