I have an early 1980's marine diesel tachometer, and I'm trying to get the needle to move. Eventually I'll want to actually use it as a tachometer, but one step at a time.
In the original engine, this tachometer was using input from a variable reluctance sensor (inductive pickup?). This sensor was isolated from the rest of the electrical system and AFAIK it essentially consisted of a coil that would generate a pulse every time a crankshaft tooth passed it. The tachometer hence has four pins: + and - for supply voltage (12VDC), and W and G for the signal. When I hook the tach up to a 12V supply I see approximately 2VDC across W and G (open circuit, W is positive). I believe the tachometer works by effectively counting polarity reversals (zero crossings) on these pins.
What I'd like to try to do is emulate the output of the original sensor using an Arduino. One stumbling block here is that I don't actually have any technical details about the original sensor, nor do I actually have one, so I'm left guessing based on other similar sensors from the era, but that's also a problem for later. For now I'd be happy if I could make the needle move at all, to at least prove that the thing works. I've already tried some simple experiments with tone(), hooking up the Arduino to the tach via a 1kohm resistor and a 0.1uF capacitor in series, but no dice - while that removed most of the DC bias when open circuit, hooking it up to the tach instead gave me a signal where low was ~2V and high ~7V.
So, what's the simplest way to emulate a variable reluctance sensor? I've seen a few references to using an op-amp with a voltage follower hooked up to a 1:1 transformer, but perhaps there is an even simpler way? Any input appreciated. I'm pretty good at programming (although unexperienced with microcontrollers) but my electrical engineering fundamentals are incredibly shaky at best.