Presently I am using the above circuit without the 555 and feeding the transistor output to arduino interrupt and using FALLING in the code..
all works fine the calculations are accurate too but sometimes noises around create to rise RPM reading as I am using falling..
So should I use another PNP transistor to convert the pulses to + and use RISING in the code...??
I somehow feel that the 555 is an overkill... 