Coutesy of SgtWookie from thread http://forum.allaboutcircuits.com/showthread.php?t=28010&page=2 Sorry, some noob questions: 1) Is it possible to vary RPM w/ a pot in place of R3? 2) Is the wire loop the output? 3) The cap you mentioned (220uF) can go anywhere between pin 1 (on the 555) and pin 8 (on the 555)? I'm trying to simlulate an RPM signal for an aftermarket ECU (Megasquirt) for a Miata - the tacho signal itself is a simple squarewave of (I believe) 0-12v amplitude, with one pulse per ignition event. If I try to read this on a multimeter, I just see a very low (and unsteady) voltage. I'd rather not buy the Jimstim and I think it would be fun to make my first circuit. I'd like to test RPM on the ECU but also make a LED tachometer w/ shift light.