The LM2917 requires an AC frequency as input, the 12v pulses going to the coil are square waves, not true AC, just on (12v) and off (0v). The circuit at the piclist site has the RC components (C1 and R1 through R3) to smooth out the square wave into more of a curvy sine wave approximation which the LM2917 can understand.
For the frequency you just use the equation they give out.
RPM and frequency are direcly related but you need to take into account the number of cylinders (ie ignition pulses per revolution) and if it is 4 stroke or 2 stroke.
Freq = ((# of cylinders)*(RPM))/120
[120 is for 4 stroke, 2 stroke would be 60]
For a screaming B16 VTEC/Honda 4cyl:
- At idle (850 RPM) freq would be 28Hz
- At 7250 RPM the freq would be approx 242Hz
Unless you really want the challenge/reward, perhaps the ready-made route would be best, they certainly are cheap enough now and probably use a low power digital microcontroller like a PIC chip to do it all in software which could allow them to do extra things like flash the LED if the RPM threshold is passed for a certain time to better attract attention.
Using a premade light you could modify the LED to a different colour or size which is most likely a lot easier than completely from scratch.
