Why do you have to connect the inputs pins to ground? Isnt it enough with the buton?

 

Why do you have to connect the inputs pins to ground? Isnt it enough with the buton?

In using a CMOS gate, ALL PINS must be connected to a valid voltage level at all times. Leaving an input floating will allow it to assume ANY level between Vcc and GND. The result is erratic and unpredictable behavior.

 

HC in the model number tells us it is CMOS part, read this: https://electronics.stackexchange.com/questions/187721/connecting-unused-logic-gates

 

Welcome to AAC!

When the switches are opened, the inputs would be floating. The logic state of floating inputs is indeterminate.

 

Thanks so much. Your explanation even explains why the LED just flickered on and off.

 

I have seen ICs oscillate with open inputs.

 

Letting CMOS logic inputs float can also cause large power dissipation.

 

Another question, if the button is pressed woudn't there be a shortcircuit passing through the 10k resistor?

 

Another question, if the button is pressed woudn't there be a shortcircuit passing through the 10k resistor?

We don't consider a desired connection a short.

The inputs of CMOS logic gates can be tied to VCC or GND without problem.

Without the 10k resistor, the switch would create a short between VCC and GND.

 

OK.
Thanks again.

 

Another question, if the button is pressed woudn't there be a shortcircuit passing through the 10k resistor?

A short is considered to be a zero or very low resistance connection.
The switch could be considered to be that when pressed.
But the 10kΩ resistor limits the current to less than a mA for the applied 9V, so is not considered to be a short.