I tried two circuits below, none of them seems to be working (I used 74LS04 and 74LS00), the output doesn't change when I press the switch.
I found that the voltage at point "A" doesn't drop below 0.8V (stays around 2V) when the switch is pressed. My guess the reason is that: when the switch is pressed, the capacitor starts to discharge, but there is a also a current flow out of the logic gate to ground thru R2, caused a voltage drop across R2, so that the voltage at A can not reach logic "0".
Any other ways to make it work? Thanks much!