555 7474 heads or tails

Discussion in 'The Projects Forum' started by monkeyhead, Jan 23, 2008.

  1. monkeyhead

    Thread Starter Active Member

    Mar 5, 2007
    45
    0
    Hi There,
    Basically here is the circuit im following: http://i29.tinypic.com/11adao5.jpg

    I've built it on bread board and it worked. I then started to build it on Stripboard (veroboard)

    But when the switch is pressed its supposed to randomly select one LED and the other should be off, it did this when i was working on breadboard. I've wired everything on but when its pressed bothe leds turn off!

    I've spent hours trying to figure out why this is happening and have had no luck!

    Any ideas to why this could be happening?

    Many thanks
    Matt
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You're at the ragged edge of what TTL can handle. Assuming the flip-flop is a 7474, 6 volts is right at the edge of the turn to smoke limit for operating voltage. The logic outputs are not meant to source big loads like LED's. The not-Q output may not be going high enough to let the 7474 toggle with the clock out of the 555.

    Use a regulator like a 7805 to run the 7474 at the "proper" voltage. Use larger resistors to the LED's so the 7474 isn't pushing more than 4 ma. That would be more like 820 ohms. The LED's will be dim, but the circuit should work.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    You use 6 volts for supply? that could be the problem.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A 7805 will drop about 1.7V across itself; if you used a 6V battery, that would cause the TTL to be running at a maximum of 4.3V - not good either.

    A single diode, like an 1N4001, will drop about 0.6-0.7V across itself. If your batteries are fresh, you could use two of them in series with the batteries to drop the voltage down to around 4.8-4.7V; as the batteries become more discharged, remove one of the diodes to bring the voltage back up.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A 7474 is an ordinary TTL IC. It has a max allowed sink current of 16mA. The LEDs with their 470 ohm current-limiting resistor are a sink load of only 8mA so they are fine.
    The Q-not output sinks 8mA into an LED plus up to 1.6mA into the D-input so its max sink current is 9.6mA and it is still fine.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Hi monkeyhead,

    Did you end up using an actual 7474 standard TTL device in your breadboard or did you use one of the other logic families like a 74LS74?

    hgmjr
     
  7. monkeyhead

    Thread Starter Active Member

    Mar 5, 2007
    45
    0
    Hi thanks for the replies.
    I was wondering whether anyone knew of any schematic to strip board simulator? I figured i could check my strip board to that to help pinpoint what's going wrong! There must be some short somewhere or something not working right.
    Matt
     
  8. monkeyhead

    Thread Starter Active Member

    Mar 5, 2007
    45
    0
    Just checked and the one ive got in at the moment is 74LS74.
    Just to let you know this circuit worked perfectly on breadboard.
    Matt
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Why don't you look on the datasheet?
    A 74LS74 has half the max allowed output current of a 7474. You are past its max allowed output of 8mA. Maybe it is blown up.

    Increase the value of the current-limiting resistors for the LEDs to 560 ohms and replace the 74LS74 after you have reduced its supply voltage to 5.0V.
     
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