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The Door Buzzer


1 Transistors


Learn It

  • In this project you're going to make an electronic door buzzer, like the one shown below.

Learn It

  • To activate the buzzer and the LED, we're going to use a circuit component called a transistor.
  • In this lesson you'll learn what a transistor is and how it can be used as an electronic switch

Learn It


  • Transistors come in many shapes and sizes, but they all have one thing in common.

Research It

  • All transistors have three legs - a collector, a base and an emitter.
  • The type of transistor you're going to be using is a BC547B, which looks like this.


  • Use online resources to help you label the picture above, to show which legs are which.

Try It

  • The circuit symbol for a transistor looks like this.


  • The legs have been labelled B,C and E.
  • Now look at the simulation below. This shows a transistor connected to a 5V Power supply, with an LED.


  • Connected to the base of the transistor is a signal generator.
  • The signal generator produces a varying voltage, going from 0 up to 5V and then back down again.
  • The voltmeter will tell you the voltage being produced by the signal generator.

Badge It

  • Copy an image of the BC547B transistor and label each pin.
  • In the circuit above, try and explain what is causing the LED to switch on and off again.
  • Make sure your are using the following terms:
    1. Base
    2. Collector
    3. Emitter
    4. Signal Generator
    5. LED
    6. Current
    7. Voltage
  • Your badge will be awarded by the quality of your explanation to the circuit above.
  • The silver badge must include the diagram.
  • Silver - A reasonable attempt at an explanation of the circuit plus labelled image of BC547B
  • Gold - A correct explanation that uses the term base, collector and emitter.
  • Platinum - A detailed explanation using all the terms from the badge task