- The DC remains the same but the frequency/period of the signal changes. In this case you’d measure the “off” time of the signal, as it would get shorter w/higher RPMs. Again you could map pulsewidth (PW) into RPMs.
In any case you can measure what happens by doing both, revving the engine and deciding which is correct. Or get an oscilloscope and look at the signal as you rev the engine. FWIW I’m 99% certain some forum someplace has the answer to what really happens.
This is what really happens, it just increases the frequency, constant 30% DC, as rpm’s increase. I found a 5v zener on mouser(http://www.mouser.com/ProductDetail/NXP … 6Qmlu3c%3d) but I can’t seem to figure out the reverse current needed to function. I did see that there is a zener on sparkfun(https://www.sparkfun.com/products/10301) but I can’t tell if that would be better or not. Any ideas which would be a better choice in my case? I am assuming the tach. signal can supply atleast 20mA, because I have seen some guages that draw that much and I have seen some draw 8mA and in the manual it says “can power original or aftermarket tachometers”, so I assume that it can vary to about 20mA.
Now you say
The input pin is a much higher resistance than the resistor shown and so can (for practical purposes be neglected), that is the input draws very little current.
but What are you meaning by this? That I don’t need to use a resistor before the zener diode? or that the pin has a negligible resistance for what I am doing? From what I understand the resistor is just there to induce a current to make the zener flow in reverse, and the resistance is just a value that can induce it, there is no required min or max?