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Design & Technology

Electronics

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

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