Transisor serie ?

Discussion in 'General Electronics Chat' started by FroceMaster, Jan 31, 2016.

  1. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    402
    4
    Hi.
    In my Construction i need to know if this is the way to do it or ?
    When IC4 goes high the output should activate all 3 transistors, and the LED's
    When IC3 goes high the output should only activate the T2 and T3 and the LED's
    and When IC2 goes high output should only activate T3 and LED's
    Question is now,
    How many diodes do i need ?
    D1+D2 ?
    D1+D2+D5+D6+D7 ?
    D3+D4+D5+D6+D7 ?
    D3+D4 ?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Not near enough info.
    What is IC2, 3, and 4?
    What is the transistor type?
    What power do you have available?
     
  3. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    402
    4
    All is in the png, but cant seem to attach it
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    The diode gating won't work because of the 0.7V drop across each diode when conducting.
    I suggest using OR gates to do the logic as shown below.
    You can also use 74HCT4075 OR gates if you like. All I had in my library were CD4000 types.

    LED Logic.PNG
     
    FroceMaster likes this.
  5. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    402
    4
    Or like this ?
    Or.png
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,858
    767
    Are you sure that you want to used the 74HC154 and npn bjt?
    Normally when the 74HC154 has been selected then the output will be low, so the bjt driver should be using pnp.
     
  7. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    402
    4
    i know it's the wrong 74hc154, should be 74hc4514
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    That will work also.
    But that's an usual symbol for an OR gate. :confused:
     
  9. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    That's Eagle for you... They used a more typical symbol for the TTL versions. I finally got so tired of looking at them and redrew the symbols.
     
  10. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    You could use diodes as long as you don't put multiple in series as in your original schematic and consider maximum current 74HC can source.
    upload_2016-1-31_11-8-16.png
     
  11. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
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