My immediate problem is turning a long pulse into a short -ve pulse to trigger a 555. I'm using a FET to invert a slowly varying voltage,and plan to use a capacitor/resistor passive differentiator to turn the low going output signal into a low going spike. Two questions. 1 - What kind of values will give me a 10-100 ms spike? 2 - in primers on differentiators, they show a negative going spike - probably not good to take 555 pin 2 below ground - is this a problem and if so, how should I fix it.