you need to alarm when df/dt is above a limit, for what you need alarm? for +ve or -ve gradiant?
i don't know what df/dt is? digital frequency divided by digital time? and also +ve or -ve?
Yenka ... i have found some circuits i thought I might be able to make work but yenka does not have the chips these circuits call for and many of the online diagrams i find do not do in yenka what they say on the site.
here is what I know:
OK, I need to take a tach wire input signal, which is a square wave that grounds an ignition coil (lots of feedback from what I read)
as the engine revs the frequency increases proportional to RPM, 4 pulses per revolution, up to 5000 revolution per minute, both with the engine accelerating and decelerating.
on each acceleration i need a circuit that checks and makes sure the engine is not accelerating too fast, I would like this to be simple without using PIC programming (whatever that is>?). and I don't mind paying, but would really prefer a simple circuit that I can learn from.