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Help Me! I Dont Really Understand About BJT Transistor

  1. May 29, 2014 #1
    Hyeee. This Gonna Be My First Post. Really Hope You Guys Can Help Me.

    I Dont Understand On The Operating Principles Of The Sensor Circuit Shown In Figure (d) and Figure (e) . Can Someone Describe It For Me? I Have Quiz On Thisssss..Arghh.

    Can BJT Transistor Works As Switch? How?

    Thankksssssss :D

    Attached Files:

  2. jcsd
  3. May 29, 2014 #2


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    Transistors have what is called "saturation" mode, if they are off (no base current) they are like a closed switch in that no current can get from collector to emitter. If there is enough current supplied to the base then they go into saturation in which case they act like a closed switch. They are not idea switches because there is always a limit to the amount of current you can put through the collector/emitter before they melt.

    That fact should make it obvious how the circuit works.
  4. Jun 6, 2014 #3

    rude man

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    Taking the 1st circuit as an example, if the LDR resistance is low (say 200 ohms) then the transistor is turned off. If the LDR resistance is high (say 1 megohm) then the transistor is on. The 100 ohm collector resistor reduces the power dissipation in the transistor by limiting Vce without appreciably affecting the LED current.

    Use exactly the same approach for the second circuit.
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