Transistors and Resistance

Discussion in 'General Electronics Chat' started by RhysGM, Nov 25, 2012.

  1. RhysGM

    Thread Starter New Member

    Nov 4, 2012
    10
    0
    Hello, I have a transistor (BC547B) that is being used as a switch, connected to the base I have an 9v battery and an LDR with a maximum resistance of 1m ohms. However this is still not enough not to switch off the transistor. Can someone tell me how I can work out what resistors I need, so the transistor is on in the light and off in the dark?

    Links for the components I'm using;

    http://www.rapidonline.com/Electron...468/?sid=b45ead51-5d50-4c3d-9f34-1e06a4f70d0c

    http://www.rapidonline.com/Electronic-Components/Light-Dependent-Photoresistor-58-0134


    Thanks for any help you can give.
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    You need to add this components
     
    • 0.1.PNG
      0.1.PNG
      File size:
      8 KB
      Views:
      27
  3. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    Put a resistor from the base to ground. Experiment with the value of the resistance starting with a high value such as 470kΩ and working your way down.

    Edit: Jony130 beat me to it. That circuit should help.
     
  4. RhysGM

    Thread Starter New Member

    Nov 4, 2012
    10
    0
    I put in a 1m ohm and the circuit remained on :confused:
     
  5. RhysGM

    Thread Starter New Member

    Nov 4, 2012
    10
    0
    Does R2 and R1 need to be the same?

    I'm actually trying to replicate this project.

    www.madlab.org/schematics/StopThief.pdf

    I think I bought the wrong LDR there were 2 one with a max of 20mΩ and another with 1mΩ.


    I'm not sure what the 2 10k resistors are doing in your diagram and I think they are similar to the LDR and the 47k and preset in my project, however I'm not sure how to balance the equation.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    R1 don't need to be 10K it can be from 470R to 10K.
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You said 1m ohm and 2m ohm which are milli-ohms (0.001 ohms and 0.002 ohms).
    You should have said 1M and 2M which are megohms (1 million ohms and 2 million ohms).
     
  8. RhysGM

    Thread Starter New Member

    Nov 4, 2012
    10
    0
    Thank you for the correction. Do you know of a formula that I can use to work out what how much resistance I need to use?
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Simply use ohm's Law:
     
Loading...