TTL current question

Discussion in 'General Electronics Chat' started by mbohuntr, Sep 23, 2010.

  1. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
    413
    32
    I am using the outputs from either a pair of CMOS 555's, or a LM556(not CMOS) to drive a 74109AN flip flop. Is controlling the supply current of each IC enough to protect the flip flop from the output signal of the timers?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It's not the current that you worry about, but the voltage. If the '109 sees more than 5.5 volts on an input, it will probably fail. TTL is very sensitive to voltage limits.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    The nice thing is, you feed a LM555 5.0VDC, it will match TTL specs extremely closely. Given the time frame it was designed, this is probably not a coincidence.
     
  4. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
    413
    32
    I will probably run everything through a 7805 first. This project is starting to look too current hungry for a battery project.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Gee, why don't you post a schematic of what you want to do?

    I'm curious why you believe you would need to "protect" the flip-flop from the 55x's output? Are they operating from different supply voltages?

    Note that standard bjt 555's have a Darlington on their output (pin 3), so even with a light load, their outputs don't get much higher than Vcc-1.3v. You can fix that by using a 330 Ohm pull-up resistor.

    If you're operating the 555 from a different Vcc, you could use a series resistor to limit the input current to the 74xx series IC.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Boy, I got distracted during my reply and look how much I missed. :rolleyes:

    If you're worried about current, look at 74HCxx versions. They're basically a high-speed CMOS equivalent of the TTL versions. They'd work well with the CMOS 555 timer.
     
  7. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
    413
    32
    Here it is Sarge, I posted it on an earlier post, I am venturing into unknown territory outside the classroom. I appreciate the inputs and advice. I am building a homebrew camera trigger, and would like to build the first one from pieces. After this is completed, I will probably go to a picaxe setup. This works well on the simulator, but real life currents and voltage might be problematic. The switch will be replaced with an IR emmitter/detector pair as will the timer outputs to the camera. I was going to use two sets of 4AA's giving me 4.8 volts, but I may bump it up, and use a 5v regulator.
     
  8. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
    413
    32
    My apologies, I posted the wrong one!! Here is the correct one. I guess I don't understand the scripts thing, I did a screenshot of the multisim page.
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Here is the revised file outside of the .DOC.

    [​IMG]
     
Loading...