on off on gate /2 way switch

Discussion in 'The Projects Forum' started by Losgat, Oct 8, 2012.

  1. Losgat

    Thread Starter New Member

    Jul 3, 2012
    Hi All

    I have a project that requires a switch to be closed for 2sec and then open/released after 12v is applied to the circuit. (easy part) The problem is that the same switch must be closed for 2sec and then open/released when the 12v is removed from the circuit.

    Would this be the right way to go? (cd4077 and 555 timer)


  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Can you not just put your switch to pin2 of the 555 timer, and ground, or just use an npn transistor to invert the switch signal, then the collector to pin 2, dont see the need for the 4077??
    Last edited: Oct 8, 2012
  3. crutschow


    Mar 14, 2008
    Because the OP wants the 555 to trigger when the switch is both pushed and released (if I understand correctly).

    One way to do that is with one exculsive-or gate, a resistor, and a capacitor. The push button goes directly to one input and to the other input through an RC series resistor and capacitor to ground. This will give a pulse output for both the push and release of the switch with a pulse-width approximately equal to the RC time-constant. If the 2s accuracy is not critical you could possibly use this to provide the 2s delay and eliminate the 555.

    Note: For the debounce to work properly, the signal to the gates should be taken from the junction of R1 and R2.