I am trying to work out a tachometer for a sawmill. The circular sawblade runs at 500 RPM, or 8.3 RPS. The blade is 42 inches in diameter.
I have built a prototype using the LM2917M-8 chip . I use 12 magnets on one of the pulleys that drive the saw blade, so I get about 100 pulses per second.
For my prototype, I mounted the 12 magnets on a 6 in diameter cast iron ring, and set that up on my lathe which I know runs at 325 RPM. The DC output is about 0.25V for that speed. The DC output seems to be fairly linear with speed.
I'm trying to figure out an analog meter circuit that would have a zero reading speed of, say 350 RPM, and a full scale reading of, say 700 RPM so the 500 RPM speed is in the middle of the meter scale.
The application note for the LM2917M-8 gets me started and I've got the prototype to make an output at the 325 RPM speed, but I'm stuck now to get the rest of the idea done.
The frequency - voltage curves on the datasheet, seem to be much higher frequencies than I am using ( eg 1000 - 5000 Hz)
So, I need help to figure out what meter movement I should use ( I can easily get 50 to 100 microamp meter movements) , and how I should get the meter zero scale to be 350 RPM and the full scale to be 700 RPM.
I am new to the forum.
miro