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I am using a 555 in my project. I am not using the Discharge pin neither the Treshold pin.
Do I really have to put this pins to ground?
In my project the 555 is working perfectly without these pins conected.
The rather broad and general advice offered to me by the experts has been to make the pins that are not used either '0' or '1' by tying them to ground or V+ - which one depends on the IC. While this may not clear everything up it does suggest that unless there is information to the contrary, that you shouldn't just ignore it. Like you, I know that it might make a difference.
I can't recall an application that doesn't use the threshold input, but I suppose you could use trigger to set the output high and reset to bring it back low, in which case the threshold would not be needed. If you're truly not using it, connect it to pin 1. You can either leave the discharge pin floating or connect it to pin 1 if it isn't used. Either way works.
Pin 1 is usually, but not necessarily, connected to circuit ground.
If you look at the internal schematic of a 555 timer you will see that its a window comparator , with upper and lower trip points, that directly control a S/R flip-flop. Pin 6 is the direct connection to the upper comparator and pin 2 is the connection to the lower.
In your case to insure proper operation you should ground pin 6.
Pin 7 can be left floating since its just a transistor switch. Pin 5 is a direct connetion to the upper voltage divider so it too can be left "floating", but it is usually connected to ground thru a small value capacitor for stability.
So in effect it appears you are using pins 2 and 4 to control the internal flip-flop. Temporarily grounding pin 4 will set the output low, while then grounding pin 2 will set the output high.