Well, I never really got very far with this project. All I have is a pickup stage that works off one of the magneto lines:<br /> http://tinypic.com/dfwnd4.gif <br />This gives me a measured signal like this:<br /> http://tinypic.com/dfwn4h.gif <br />The red line is 0v, the top of the waves is 12v. The period is about 37ms (1600 engine RPM). Pulse length is about 2ms.<br /><br />An outboard tachometer usually has a little switch on the back to select the number of pulses per revolution of the engine. Most J/E engines produce 5 or 6 +12v/-12v pulses per revolution. So OMC tachs can be set for 5, 6, or 10 pulses pre rev. Teleflex tachs can be set down to 2. Also, all inboard tachs (including OMC) can be set down to 2 to accomadate four cylinder engines. Naturally, their signal is a little different.<br /><br />I can get two pulses per revolution easily enough by grabbing the signal from the other magneto line and adding it. That's simple as it only requires a pair of diodes.<br /><br />Anyway, to make a long story short, Iboats member gaugeguy assures me that any tach on the market will respond properly if fed with a 6v square wave or better. He also told me that duty cycle wasn't very important, but that 50% is ideal.<br /><br />But as you can see, I've got a 5% duty cycle there which I think is too short. I divide it by two, but then it wouldn't work with any of the tachs on the market since at best I could only get one pulse per revolution. Another idea I had was to run it through a PLL multiplier, but that will make the circuit a little complex.<br /><br />Ideally, I would like to simply "streach" the pulses out, but I have no idea how to do that. Especially since they should be streached less as the RPMS go up.<br /><br />Could anybody point me in the right direction here? Perhaps the PLL route would be easiest, or is there something really obvious I'm overlooking?