The role of sensors in control

Sensors are used to measure physical quantities such as temperature, light, pressure, sound, and humidity. They send signals to the processorprocessor: shorthand for central processing unit (CPU). For example:

• A security alarm system may have an infraredinfrared: in communications, an infrared beam modulates (switches on/off) to a receiver that interprets the modulations as data sensor which sends a signal when the beam is broken.
• A heat sensitive sensor in the corner of a room may detect the presence of a person.
• Temperature sensors could be used to control the heating in a large building.
• Magnetic sensors are used to detect metal and can be placed in roads to monitor traffic flow.

Other physical quantities that can be transmitted directly to the computer's processor include:

• rainfall/water levels
• radiation level
• pH level
• oxygen level

Analogue to digital conversion

Data such as pressure, light and temperature is analogueanalogue: continuous data, eg the second hand on an analogue watch displays each second and the space in-between datadata: information without context, eg a list of students with numbers beside their names is data, when it's made clear that those numbers represent their placing in a 100 metre race, the data becomes information. Computers can only work with digitaldigital: data measured at discrete intervals, eg a digital watch typically moves from displaying one second to the next without displaying the values in-between data.

An interface box or analogue to digital converter (ADC) is needed to convert the analogue data from the sensors into digital data the computer can process.