| | Data Logging : Sensors | 
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Data that is to be recorded by a data logging system is gathered using sensors. A sensor is a device which measures a physical quantity such as the temperature and then sends a voltage to the computer representing the quantity. Sensors can be either analogue or digital :
Digital : Sensors are either on or off. For example a switch of pressure pad.
Analogue : Sensors can measure a range of values. The size of the voltage the sensor produces is determined by the size of the quantity that the sensor has measured. For example if a cold temperature is measured a small voltage will be sent, whilst if a hot temperature is measured a larger voltage will be sent.
There are lots of different sensors available, each of which is designed to measure a specific quantity. Examples of sensors include :
| Sensor | Type | Quantity Measured | Application |
| Temperature sensor | Analogue | How hot/cold it is. | Monitoring the temperature of a science experiment. |
| Light sensor | Analogue | How light/dark it is. | Turning street lights on when it is dark. |
| Pressure Pad | Digital | If a pad is being pressed. | Detecting cars arriving at a car park barrier. |
| pH sensor | Analogue | The acidity of a liquid. | Monitoring river pollution. |
| Button | Digitial | If the button is being pressed. | Obtaining a ticket at a car park entry barrier. |
| Light Gate | Digital | Detects an object passing through the gate. | Measuring the speed or acceleration of objects. |
| Passive Infra Red (PIR) | Digital | Detects when an object moves into an area. | Activating a burglar alarm if someone enters a room. |
If you are designing a data logging system you will need to decide which sensors to use and where to put them. Many data logging systems will use lots of sensors at the same time.
If you need to use analogue sensors then as well as choosing the type of sensor(s) you want to use you must also make sure the sensor(s) take measurements over ranges that are suitable for the system you are creating. For example you may want to use a temperature sensor and have to make a choice between sensor A that measures temperature in the range -10ºC to 40ºC and sensor B that measures temperature in the range 0ºC to 120ºC. Sensor A would be best for measuring the temperature at which water melts and sensor B would be best for measuring the temperature at which water boils.
GCSE ICT Companion 04 - (C) P Meakin 2004