NOOB Help

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.


Regards

 

You can use a 555 to generate the time delay to control a relay. The 555 requires a momentary low signal to trigger. This can be generated a CD4077 XNOR gate configured as an edge trigger circuit (generates an output pulse for either a positive or negative going input).

To do this connect a series resistor and a capacitor to ground to one of the XNOR inputs. Connect the other end of the resistor and the other XNOR input to the 12V. This will generate a momentary low to trigger the 555 when either the 12V goes high or the 12V goes low. The duration of the momentary low is approximately equal to the RC time constant of the resistor and capacitor. This RC time constant should be significantly longer than the rise/fall time of the 12V.

 

Hi

Thank you for the information/direction. I've worked with the 555 but not the cd4077 will get the cd4077 begin testing.