Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!

Design & Technology


Inputs and outputs in electronic circuits

The input is what sets an electrical circuit in action. Any of the following components can be used as inputs:

  • a switch (eg push-switch, microswitch)
  • a key pad
  • a Light Dependent Resistor (LDR [LDR: A Light Dependent Resistor, or LDR, is a type of resistor which is affected by changes in light levels. A cadmium sulphide layer causes a decrease in resistance in the light and increase in the dark. ])
  • a thermistorthermistor: An electrical device whose resistance decreases as its temperature increases.
  • a photodiode [photodiode: a light-sensitive diode; a semiconductor which detects light and emits an electric current ]
  • a phototransistor [phototransistor: a light-sensitive transistor, which switches itself on or off when it detects light ]
  • an opto-isolator [opto-isolator: Component which sends a signal between two circuits via a light-path, thus maintaining isolation between the two circuits. ]
  • a proximity switch [proximity switch: Type of sensor which can detect its relationship to a metal target without making physical contact. ] or reed switch [reed switch: A magnetically-controlled switch which is activated by detection of movement. ].

The output is what results from an electrical circuit. Any of the following components may be used as outputs:

  • an LED [LED: Stands for Light-Emitting Diode. LEDs glow when current passes through them. ]
  • a lamp
  • a buzzer
  • a piezo [piezo: component made of crystal, ceramic or polymer which changes shape when a current is applied, or emits a current when its shape is changed ]
  • a motor or stepper motor [stepper motor: Type of motor which rotates in fixed, precise and often very small steps. Used to turn the hands in some watches. ]
  • a solenoid [solenoid: Type of electromagnetic switch which creates a magnetic field when a current is applied. ]
  • a relay [relay: type of switch which which uses an electromagnetic coil to connect two or more contacts, which close when the coil is energised. Use of relays enables a very small voltage in the relay circuit to control a much larger one in a separate circuit. ]
  • a seven-segment display [seven-segment display: type of electronic feedback display for numerical data, which displays all digits using combinations of lines in seven different positions. Common in digital alarm-clocks.. ].

Logic gates

Logic gates are a family of digital devices which compare two or more inputs and give a specific output. Each logic gate (NOT, AND, NAND, OR, NOR etc) acts in a different way, and will always act so. The action of any logic gate is shown by a Truth Table, eg the AND gate will only give a high output, when all the inputs are high.

Some more common symbols are shown in the diagram below, including output components and logic gates.

Circuit component symbols

Circuit component symbols

Back to Systems and control index

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.