Rotor tach

[C. Beaty]

I found some back issues of the Sunstate Rotor Club newsletter and came across this rotor tach in the August ’78 issue.

I’ll bet Al “Electric” Hammer could explain why a calibration control is unnecessary but he’s never around when you actually need him.

[Al_Hammer]

Chuck , it appears that the idea is that the capacitor C1 will charge fully on each switch closure since the RC time constant is very small. The charge on the cap Q in coulombs = Capacitance (C) * Voltage (E) or .56 uF * 5.4 volts= ~ 3 micro coulombs per switch closure.
When the switch opens, this charge is dumped into the other, larger cap which averages the pulses. Actually, the exact value of C2 doesn't matter- it just holds whatever charge is dumped into it.
For a full scale meter current of 50 uA (= 50 microcoulombs per second) we need 50/3 = 16.66 charging pulses per second. With two magnets, this is 16.6/2= 8.33 revs per second.
8.33 * 60 = 500 rpm full scale.
Accuracy won't be affected at all by changes in R1 since the cap will always charge to full voltage in the time allotted. The 5 % tolerance on C will mean about a 5% accuracy tolerance as long as the battery is charged.

PS: and I'm sure you didn't need me to figure this circuit out- but thanks for pretending to ask. :)

[C. Beaty]

Thanks, Al. I knew you could do it!

PS: You know a whole lot more about Mr. Koolohm than I.