sr flip flop problems

Discussion in 'General Electronics Chat' started by leewillz, Jan 3, 2010.

  1. leewillz

    Thread Starter Member

    Jan 11, 2009
    17
    0
    hey i have had a few problems trying to make my circuit work, its basically an alarm system, so i want it to latch when an input comes from a comparitor circuit i have made, i am using a 6v power supply but i have put a resistor in series with it and the voltage going into the circuit now it 4.98V near enough 5v, the chip i am using to make the SR flip flop is a 7400 IC but the problem i have is when a logic 1 is at the comparitor end the light comes on but it doesant stay on so its not latching, ive made and re made this circuit around 10 times now and really need to get it working on the bread board before building it on vero board.
    thanks Lee
    p.s the outputs of the 7400 are just 2 LED's for Q and Q-BAR
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You need to post the schematic.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    Can you post your schematic?
    You can use the "manage attachments" button to upload it to the forum.
    Preferably do it in .png format as .jpg can be messy.

    Greetings,
    Bertus
     
  4. leewillz

    Thread Starter Member

    Jan 11, 2009
    17
    0
    i dont have one on this computer so ill draw one up now and post it asap
     
  5. leewillz

    Thread Starter Member

    Jan 11, 2009
    17
    0
    here is the circuit i am making, hope this helps
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    282
    There are a few problems that jump out. The 2b section inputs float when the switch is open. They need to be pulled up to Vcc with a resistor of about 20K ohms. Left to float, the inputs will tend to float up to the logic operating point and cause high frequency oscillations.

    Your LED's need a 300 ohm resistor in series with them. A LED has no resistance when turned on and can pass enough current to destroy itself or the IC driving it.

    S-R flip flops usually work with one input pulled high and the other pulled low. Look ar the data sheet for a 74LS244.
     
  7. leewillz

    Thread Starter Member

    Jan 11, 2009
    17
    0
    thanks il make those changes now
     
  8. leewillz

    Thread Starter Member

    Jan 11, 2009
    17
    0
    ok ive made the changes now both inputs come on at the same time and it seems the LDR is in control of the intensity of the Q-BAR LED and neither switch off they just vary in intensity.
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    Can you check the state of the inputs and outputs?
     
  10. leewillz

    Thread Starter Member

    Jan 11, 2009
    17
    0
    found the problem, my comparator circuit is not giving a logic "0" as it gives out 1.8v as logic 0 because of saturation on the op amp
     
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