I have made a capacitance meter with a 555 timer and an Arduino. It's mostly complete except for the digital pin that triggers the 555. I have narrowed it down to two possible solutions. Both of them use a differentiator to make sure the trigger pin on the 555 returns positive, even if the Arduino is interrupted with a timer. One solution uses Arduino digital pin (clk) to charge and discharge one leg of the trigger cap (10nf).
Is one safer than the other for the Arduino pin?
Is there a better way to do this in general? Perhaps by using a transistor to switch the cap to ground?
Are the RC values the correct size for what I'm doing?