Simplest soft siwtch for relay circuit.

Discussion in 'General Electronics Chat' started by stoopkid, Oct 1, 2012.

  1. stoopkid

    Thread Starter Member

    Mar 3, 2011
    136
    1
    I'm looking for a simple circuit to switch a relay on and off with a low power momentary button. I could use an attiny85 if I need to but I figure there must be some way to do it with a 555 timer or a couple of transistors. I can't seem to find the right keywords to turn anything up in a search.

    Thanks.
     
  2. BSomer

    Member

    Dec 28, 2011
    433
    106
    Yes, there is a simple way to do it with a few transistors, resistors, and a capacitor... See attached image. For more info on how it works and some other circuits similar in operation go here.

    In the comments below the video there have been some posts from others that showed some other working methods.
     
    Last edited: Oct 14, 2012
  3. crutschow

    Expert

    Mar 14, 2008
    12,973
    3,219
    You could have the PB connected to the clock input (with a suitable de-bounce circuit) of a flip-flop set to toggle, such as a CD4013 with the D input connected to the /Q output. The FF output would control the relay driver transistor.

    Also here's a toggle FF circuit using a 555.
     
  4. stoopkid

    Thread Starter Member

    Mar 3, 2011
    136
    1
    I am trying that circuit with a 3906 instead of a P-chan mosfet, because I don't have one. I end up with the switch always staying on and when I hold the button down it flashes... But still ends up on either way. Is this because of the BTJ or is there something else that could cause this?

    Thanks
     
  5. stoopkid

    Thread Starter Member

    Mar 3, 2011
    136
    1
    I got it working by changing R1 to 3.1k and changing the cap to 47uF. It could use a bit more tweaking but I really need it to not turn on when it is initially powered up. Is there any way to fix that?

    THanks
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,848
    767
    You can connecting a 1uf in parallel with B,E of Q2.

    And change the Title
    From
    Simplest soft siwtch for relay circuit.
    To
    Simplest soft "switch" for relay circuit.
     
  7. stoopkid

    Thread Starter Member

    Mar 3, 2011
    136
    1
    Thanks, that works perfectly. I'd be happy to change the title if I knew how. It doesn't appear to be available when editing the post.
     
  8. ScottWang

    Moderator

    Aug 23, 2012
    4,848
    767
    Probably the system setup the limited days of edit function for Title, since the day of your first posted.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    Source and drain are swapped on your PMOS tranny.
     
  10. BSomer

    Member

    Dec 28, 2011
    433
    106
    Sorry. That one was an older drawing I made and have since changed it. I just happened to upload the wrong one. It is now corrected though. :)
     
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