Bad circuit ideas but I don't know why

Discussion in 'General Electronics Chat' started by shaqywacky, Feb 12, 2011.

  1. shaqywacky

    Thread Starter Active Member

    Apr 1, 2009
    The book I'm reading gives a list of bad circuit ideas but doesn't explain why they are bad, it just give circuits that you shouldn't do.

    After reading them I noticed one that I used all the time.

    [​IMG]The picture in the attachment is, what it called, a logic gate indicator and it is a bad circuit apparently.

    My question is why is it bad and what it is the correct way to show the gates output when it still needs to be given to another gate?
  2. Jaguarjoe

    Active Member

    Apr 7, 2010
    The first nand is an inverter, so both inputs are high and it's output will be low and so the anode of the LED will also be low therefore never lighting. The second nand has a low and a high input so it's output wants to be high but it is hard wired low which could terminally upset it.
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Never tie a logic OUTPUT to a power rail. In your case, ground/negative.

    Unused INPUTS should be tied to either the high or low power rail.
  4. shaqywacky

    Thread Starter Active Member

    Apr 1, 2009
    It's not the logic of the circuit that I'm concerned about, It's how to display the output of a gate whose output needs to also go to another gate. I attached a picture of exactly what the book showed. It says that the output goes to another gate(That's what I was trying to show with the first picture).

    @ Thatoneguy, I actually didn't know that either, so I'll try to keep it in mind later.
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    The LED loads the output of the driving gate. Depending on the type of gate, the current through the LED may or may not be satisfactory, but I don't think that's the main point.

    The issue here is that the gate output voltage may be reduced by the LED loading. The voltage seen by the following gate may not therefore have the correct logic levels. At the very least, noise immunity may be reduced, and in fact the circuit may not work at all.
  6. shaqywacky

    Thread Starter Active Member

    Apr 1, 2009
    So should I use a transistor to switch the LED? Sorry that my question is kind of obvious, but I'm working on something and I would like to completely rule out that this is the problem/solution.
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    It's certainly not recommended that standard CMOS logic be used to drive an LED without any limiting resistance. Standard CMOS has very low current drive capability. Use the interposed transistor option or perhaps a dedicated driver chip.

    Adjuster's comments are important.
  8. Wendy


    Mar 24, 2008
    There are a few circumstances where the internal resistance of a CMOS output can drive an LED without a current limiting resistor, but they are exceedingly rare. It is more correct to say you should never use a LED without a current limiting resistor or equivalent.

    If you have trouble understanding LEDs I would highly recommend reading chapters 1 and 2 of LEDs, 555s, Flashers, and Light Chasers.