Push button latching circuit

Discussion in 'General Electronics Chat' started by rtm038, Aug 8, 2011.

  1. rtm038

    Thread Starter New Member

    Oct 18, 2009
    Hello all,
    I'm working on a 12+VDC project and need some help figuring out the circuit. Basically, I want to control the ON/OFF states of a standard Bosch automotive type (SPDT) relay with a momentary push button switch that is supplied with a switched 12+VDC feed. Here is how I want the circuit to work:

    1. Push the button and the relay turns ON

    2. Push the button again and the relay turns OFF.
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
    The easiest thing to do is go to an electrical supply store and buy a light switch that does just that. Push on, push off.
  3. elec_mech

    Senior Member

    Nov 12, 2008
    Hi Rtm038,

    Gerty's solution is certainly the simplest. If you want to use a momentary switch, you can do it with a 555 timer and a handful of components like so:


    You can also use a CD4013. I did this with 12VDC:


    Just use a pulldown resistor at the input (pin 3) to prevent accidental switching. Connect one side of the momentary switch to pin 3 and the other to 12VDC. Use D1, C7, and R14 if you want the output to stay off when you first apply power to your circuit.

    If you want to control a relay, you'll want to add an appropriately transistor between the output and the relay coils whether you use a 555 or a 4013 - they will not be able to provide enough current to energize the coils. Don't forget to add a diode across the coils to prevent back EMF from damaging the momentary circuit.

    Hope this helps!
  4. t06afre

    AAC Fanatic!

    May 11, 2009
  5. iONic

    AAC Fanatic!

    Nov 16, 2007