I found this circuit in a website
I would like to use this with arduino to measure my motorcycle's engine RPM
Please suggest me what modifications should I make in this circuit so that I can take interrupts from it to the arduino board
moreover I am unable to find out what are the pin nos. of the NE555 define in this circuit..
It needs little modification - just route the output signal to Arduino pin 2 or 3 and use attachInterrupt().
Pinouts for chips are trivial to find if you go to google, the problem here is that the NE555's pins are given the wrong names. They should be "Trigger", "Threshold", not IN and IN+. See the monostable example at wikipedia for correctly labelled versionhttp://en.wikipedia.org/wiki/555_timer_IC#Monostable. Note that you need a few more connections to the chip.
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... 
The 555 is cheap as chips and easily available. Falling edge will work as reliably as rising - for each falling edge there must be a rising edge. The 'noise' could be due to stray inductance, capacitances or poor circuit layout or lack of decoupling - perhaps the input is too strong - try reducing the input resistor to ground somewhat and see if that alters things. If a 'scope is available that is very useful for this kind od diagnosis.