How to calculate resistor value in transistor light detector circuit? Thanks in advance.......

Discussion in 'The Projects Forum' started by Thinn, Feb 10, 2015.

  1. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1
    Hello fri,

    Is there anybody explain me how to calculate resistor value in this circuit? I'm really new in circuit theory. When Vin input is 14V , Vout need to be zero voltage and when Vin input is around 5V, Vout need to be 12V.

    Thanks and regards,
    Thinn
     
    • ckt.pdf
      File size:
      11.6 KB
      Views:
      54
  2. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1
    Here is the .png version for my circuit.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    put a zener diode of 4.3V in series with R1, make R1 a 1K.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,544
    1,251
    What is the coil resistance of the 12V relay?

    ak
     
  5. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1

    How's about other resistor? Please can you explain me with an example equation. Thanks!
     
  6. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1
    coil resistance???? Do you means R4 or something else???? I'm really new in circuit. That's why I don't know what you mean.
     
  7. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1
    Please, anybody guide me to how to calculate resistor value in this circuit..........Thanks in advance.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    The transistor will turn on when it gets 0.7V across the B/E junction, so if you want it to turn off below 5v, you have to get rid of 4.3v,easiest way is a zener diode,otherwise you're using resistor dividers, just divide the base current into 4.3v and it will give you the value of R1.

    If you dont understand that i suggest you take up knitting instead!
     
    Thinn likes this.
  9. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1

    Thanks for your help! I think I got why I need to use zener diode. But I still don't know how to calculate other resistor value.
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    By inspection, R3=0, R4= infinite.

    No answer is possible for R1 and R2 without knowing the resistance of the relay coil. How much current is that dang thing supposed to draw?
     
    Thinn likes this.
  11. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1
    Thanks ErnieM :)
    Now, I got the realy coil resistance 400 ohm. Also current draw is nearly 2A. Please let me know the calculation.
     
  12. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1

    Realy coil resistance is 400 ohm.
     
  13. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    12V relay 400 Ohms.
    I=V/R=12V/400 Ohms=30mA.
    Where is the 2A came from?

    I think you only using one bjt is enough to drive the relay, moving the relay to replace R2, and take the R3, R4 and second bjt away.

    If the relay need 30mA, then we can offer it some more as 40mA, so if we want to make the bjt to get into the saturation region then the gain should be 10.
    Ib=Ic/gain=40mA/10=4mA
    R1=(Vin-Vbe)/Ib=(14V-0.7V)/4mA=3325 ohms, use 3.3K.
     
    Last edited: Feb 13, 2015
    Thinn likes this.
  14. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1
    Thank you very much for your help.
    2A is came from load current (LED). I'm really new in ckt, that's why may I ask more question :) If I take out second bjt, Is the relay is on when Vin is around 5V?
     
  15. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    You just follow the last formula to calculate the R1 is ok.
     
    Thinn likes this.
  16. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1
    Thanks! I'll try it again and back to you :)
     
  17. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1
    Hello Scott Wang, I was testing my ckt again using your formula, relay is making a sound continuously when the light is on. I don't know what wrong with relay.
    Load draw 2A. system voltage is 12V. R1 is 3.3K ohm. I used 2n2222 transistor. Zener diode is 4.7V. Please guide me is there something wrong in my ckt?
     
  18. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Are you used one or two bjts?
    Sounds like the current of relay is not enough, it needs reduce the resistor of base.
    Could you attach the circuit, and label the voltage and the values of resistor?
     
    Thinn likes this.
  19. Thinn

    Thread Starter New Member

    Feb 4, 2015
    21
    1
    Here it is. I used 2 Bjts because I don't know how to hookup the relay in ckt. That's why I can't make new ckt yet. Also I put zener diode series with R1.
     
  20. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    The below have two kinds of circuits, used one bjt and two bjts.
    If you have different voltage to input then you need to using different resistor, 3.3K for 14V and 1.075K for 5V, you can try 1k and 1.2K.
    The first you can measure the current of relay how much current is it drawn.

    12Vrelay_Thinn.gif
     
    Thinn likes this.
Loading...