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latching momentary switch to relay circuit

Discussion in 'The Projects Forum' started by dmc0162, May 3, 2008.

  1. dmc0162

    Thread Starter Active Member

    Dec 13, 2007
    alright here is what I need to do:

    I am using a bosch relay to control a 12v output to a solenoid in my car. The solenoid has a resistance of 4 ohms. I want to be able to push a momentary switch to control when the relay switches. The problem is, when I push the momentary button, I do not want to have to hold it it down. Therefore, I need to make a latch in the circuit so that pushing the button switches the relay to one position or the other.

    I CANNOT USE the kind of push button switches that click on, and then you have to click it again to switch it off, they MUST BE momentary switches.

    The only other thing I can think of would be to use mechanically latching switches like on old tape players where one button pops up when you push another, but I can't find these switches ANYWHERE

    Thanks for your help. I basicaly need to know if there is an easy way to build a latching circuit for this, and maybe some could even draw me a quick schematic. Thanks again
  2. scubasteve_911

    AAC Fanatic!

    Dec 27, 2007
    I'm not really sure what the electrician-equivalent to a toggle flip-flop is, but that's what you need to do.

    A t-type or toggle type flip-flop basically does exactly what you want. You apply a logic rising edge, then the output (Q) changes to an opposite state. Then, when you apply another, it changes, etc..

  3. SgtWookie


    Jul 17, 2007
    See the attached. Your relay, two pushbutton switches (one NC, one NO), and a diode.

    S2 doesn't have to be on the ground side of the relay coil; it could break the keep-alive path anywhere from past where the output of the switched contacts is taken to power the load, to the + side of the relay's coil, as in the 2nd schematic.
  4. MATTY B

    Active Member

    Jan 26, 2008
    get a headlight relay for a 70 superbeetle. It is a latching relay and will do what you want in one relay and one switch.