patrickg
Veteran Member
The alternator output before rectification is AC (alternating current). The frequency of this ac voltage is a function of RPM of alternator but since ratio of engine RPM to alternator RPM is pretty well fixed (unless belt is slipping pretty badly) you may think of it as engine RPM dependent. A simple RC (not radio control but resistance capacitance) circuit, called a high pass filter will yield a frequency (RPM) dependent ouput. You can rectify this output and drive a DC volt meter with it. Use a meter with a large deflection (say 270 degrees or so). Carefully open the meter take the dial (has the numbers on it) out and turn it around and reinstall with non-printed side out (visible.) Draw up a dial calibrated to your circuit parameters and glue it on to the meter dial. Voila a tachometer. Some folks prefer to draw an over sized blow up of the desired meter scale, photograph it and then use a print of it reduced to the correct size to make a neat dial face. Yes, kids, you may try this at home. If you have a friend or neighbor who is a ham radio type, electronics tech, or engineer they should be able to draw you a schematic diagram in a few minutes. And if they are fairly good they should be able to make a few measurements on your tractor, some calculations, and provide a parts list to take to Radio Shack. Except for meter, parts should not exceed a few bucks. Meters run from cheap trashy stuff to expensive, I'd opt for above the moiddle of the road and be interested in its being somewhat vibration and shock resistant to survive in a tractor environment. Might want some RTV to seal it up. Don't seal any dial lamp too wel,l they burn out you know. Good luck!
Patrick
Patrick