Circuit to turn a momentary pushbutton into a latched on/off toggle

Discussion in 'General Electronics Chat' started by TheRogue, Aug 29, 2012.

  1. TheRogue

    Thread Starter New Member

    Aug 28, 2012
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    Hi guys, this is my first post so first off an introduction. I live in Vancouver, BC, Canada and I work as a network engineer and computer teacher, but in my free time I have been working to set up a computer museum for many years now. Now, here's why I am posting.

    I am building a tower case for my Atari STe, and the case I have chosen to use is an ATX case, and as such has a momentary pushbutton as a power switch. I am going to modify an ATX power supply to drive the STe, but an ATX power supply requires a pin on the ATX connector to be latched to ground to turn the power supply on, and opened to turn it off. So I am looking to build a simple circuit to connect to the power button, the on/off line, ground, and the +5v trickle line from the atx supply so that when I push the power button the power on line is latched to ground to power up the system and when I push it again the connection is broken and the supply shuts down. I know this can be done simply, but I'm a little rusty with my low level logic. Can someone help me out with this? Thanks guys!
     
  2. Wendy

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    Mar 24, 2008
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  3. TheRogue

    Thread Starter New Member

    Aug 28, 2012
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    Thanks for the welcome Bill! This circuit looks to be like what I am looking for but I think it can't be used as is without modification. I think that in this circuit the 555 drives the LEDs directly whereas with what I am doing I want the pushbutton to connect a separate line to ground and disconnect it with another push, not just feed +5 to a particular device, so I think (unless I am totally off) that I will have to modify it. Am I totally off here? If not how should I modify it to my needs?
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    A simple transistor, either a logic level MOSFET or a BJT (regular transistor) will make a good on/off switch.

    Modification by any other name is designing. :D
     
  5. TheRogue

    Thread Starter New Member

    Aug 28, 2012
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    Ok, so I should just replace one of the LEDs with a transistor, with base connected to where the LED would be, and the emitter and collector connected to the power on line and ground?
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    You will not get any simpler then using a SPST Push On-Push Off Switch
     
  7. TheRogue

    Thread Starter New Member

    Aug 28, 2012
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    I understand that, the only problem is I want to keep the original power button, not hack a toggle button into the case.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You can also use a flip-flop, such as shown here.
     
  9. TheRogue

    Thread Starter New Member

    Aug 28, 2012
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    I think I can use this circuit without any modification. I think I would just connect the Power On line in the place of the +5V going to JP1. Please someone correct me if I'm wrong.

    [​IMG]
    Circuit sourced from here.
     
  10. TheRogue

    Thread Starter New Member

    Aug 28, 2012
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    Sorry I left something out of the schematic before, here is the corrected one.

    [​IMG]
     
    Last edited: Aug 31, 2012
  11. Wendy

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    Mar 24, 2008
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    The first schematic is correct. You must have pin 4 connected to Pin 8.

    Basically the same circuit I showed you.
     
  12. TheRogue

    Thread Starter New Member

    Aug 28, 2012
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    Hi Bill. Yes it is pretty much the same just slightly changed. The reason I connected pin 4 to a cap and then ground is because someone told me that if you set it up that way the device will receive a reset pulse on power up and always come up in the off setting. Is this not correct?
     
  13. Wendy

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    You can believe me or not, but I'm telling you pin 4 must be connected to pin 8. This is not optional. It is fundamental to how 555's work.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    You can use a cap to ground from pin 4, but I suggest also using a 10k to 220k resistor to Vcc. That way, pin 4 will eventually wind up at near Vcc.
     
  15. TheRogue

    Thread Starter New Member

    Aug 28, 2012
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    Oh believe me Bill, if you say so I believe you 100%, I was just confirming. I will build it according to the first schematic and let you know how it turns out. Wookie, are you saying leave pin 4 attached to pin 8 but attach the cap and the resistor as well?
     
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