Building an Analog Tachometer

I'm trying to construct a ciricuit which will convert a 25v peak to peak input signal into an equivalent analog DC voltage which would be proportional to an RPM level. I want to use a 555 timer setup but I'm having some trouble. Any help would be much appreciated.

Check out 555 Timer Tutorials

While not a 555 circuit, here is a data sheet for a tach driver chip that might be easier. Fewer parts too if I remember right.

LM2917N-8 pdf, LM2917N-8 description, LM2917N-8 datasheets, LM2917N-8 view ::: ALLDATASHEET :::