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).
The other uses the 5v rail to charge, and the Arduino pin to discharge the trigger cap.
My questions:
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?