tachometer - ignition coil interfacing

[evandude]

I know the tachometer question has been asked a million times, but I don't need help building a circuit for interpreting a conditioned input signal and displaying RPM's, which is what most of the threads were about when I did a search.

I just want to know a little about interfacing a circuit with the ignition coil in my car. I heard the typical way is the negative terminal. The question I have is what kind of signal am I going to see there?
what kind of voltage is going to be present? how noisy is it? is it going to look somewhat like a square wave (if a bit messy)?
Basically I just need to know a little about it so I can build a simple circuit to interface that signal with a PIC. Obviously I don't want my PIC directly connected to it if it's putting out hundreds or thousands of volts. I can't get to my car with an oscilloscope, so I just want a starting point for a prototype circuit to try out on it.

would a resistor voltage divider, some caps to smooth out the signal, and a comparator be sufficient?

or how about a large-value resistor in series with the input, and a reverse-biased zener (5v or so?) to ground, with some caps thrown in for good measure?

thanks

 

[Russlk]

I would use the signal from the coil. It is basicly a 12 volt rectangular (not square) wave with 300 volt spikes. A low pass filter with large series resistor and zener diode load will protect the PIC.

 

[stef]

On the digital rev counters used for racing karts, a wire is wound 2-3 times around the HT wire (the one going to the spark plug) to form a crude potential transformer. I suppose the signal is then amplified via an op-amp and fed to the micro.