the basic idea is that you have a chip(driver chip) that gets a signal from a wire(tach wire). when that chip detects a certain A/C Voltage(thats what the tach wire sends) it turns on a corresponding LED. sorry i could not be any more specific but I have never had to build one of these. the hardest part should be getting the chip(one that reads A/C voltage) and programming it. you need to know the A/C voltage output at specific RPMs to correctly program the chip. if you look online for "semiconductor chip" and your city name you may find some places that you could call to see if they offer a LED driver chip that takes A/C voltage as an input.