Trouble with push on push off latch circuits

Discussion in 'The Projects Forum' started by treahuggs, Sep 28, 2013.

  1. treahuggs

    Thread Starter New Member

    Sep 11, 2013
    4
    0
    Hey guys, I have been having trouble with various push button latch circuits. even the 555 timer posted in the forums didn't work with me. In any case I found this circuit on a blog that addresses some of the problems with the talkingelectronics circuit -
    (seen here)
    [​IMG]

    So here is the address of the circuit I am trying and its schematic-
    http://fakeeequips.wordpress.com/2011/09/25/transistor-latching-circuits-improvement/
    [​IMG]

    The issue I am having with the breadboarded circuit is that when the push button is pressed and released the LED will just flash once instead of latching on. If I take the emitter lead from q1 away from ground the LED will stay on (dont think its relevent but just including more information). I have gone over the circuit looking for misplaced wires or missing points but everything looks good to me. I can supply a picture of my built circuit if wanted (though I dont use cut to length wires so it looks messy).

    Please help, this is the last component I need to finish my project which I will be glad to show off when it is done.
     
  2. Nykolas

    Member

    Aug 27, 2013
    87
    31
    The logic in the original circuit, although overly complex, is OK. The gain (hfe) in the transistors you chose is wrong.
    Try the circuit attached (use the remaining gates in ll to get higher output current. E
     
  3. treahuggs

    Thread Starter New Member

    Sep 11, 2013
    4
    0
    Sorry, I also forgot to mention that the circuit needs to use transistors. Apart from being the last component for my own personal project this circuit falls into a project for a class of mine also. So two wins if I can get it to work.

    Thanks for the circuit you attached though, ill keep it saved.
     
  4. absf

    Senior Member

    Dec 29, 2010
    1,490
    371
    I simulated the second circuit on Proteus and it works if R7 is changed from 100K to 10K. I also added R8 470 ohm to limit the current of the LED. But it would also work without it.

    The original circuit is designed for 12V and I saw that the resistance value for R4 and R2 are, IMO, too high for Q3 to work properly. As you can see from the sim, the current through Q4 2N3904 is only 15mA. So if you need more current for your load, you need higher supply voltage or change some of the resistor values of your circuit.

    Allen
     
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  5. doug08

    Member

    Jan 30, 2011
    153
    2
  6. treahuggs

    Thread Starter New Member

    Sep 11, 2013
    4
    0
    Well this is embarrassing... I still cant get it and I have tried to use circuit maker, OrCAD, and that program you used Proteus and still couldnt even simulate it right. This is not what a senior ET student in college should be having trouble with...

    Im going to tear down the circuit and try it again I suppose. Thanks for the help and I welcome more suggestions.
     
  7. treahuggs

    Thread Starter New Member

    Sep 11, 2013
    4
    0
    OK guys thanks to your help I got the circuit working for the most part. There are just two issues I have encountered now. One is that the latching works great the first couple times but then it seems to latch itself on. When I press the button again (slow or fast) the LED will blink once instead of turning off. I have played around with the resistor values and the 1k seems to do the trick. My load will be using the 9 volts to power everything in the circuit. What is in the circuit is an LM386 amp (powering a 8ohm 2.5 in speaker. [soon to be an 8ohm 2' speaker for final build]), 5v regulator (going to a bluetooth module), and then an LED for power status.

    The second issue I am having which might go away after the circuit is built off the breadboard is I have A LOT of noise from the latch circuit that is breadboarded. Is that the issue or should a circuit like this not be used in a music application?

    Any ideas why the circuit only likes to work only for a while at first?

    Thanks for the help guys! here is the updated circuit, I cleaned it up out of respect for asking for help from you all.
    [​IMG]
     
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