RPM Measurement Using a Microcontroller - LM2907 Simulation Method

A way to measure RPM by simulating an LM2907 is demonstrated using an Arduino and Adafruit 16x2 LCD display. This technique uses one digital pin and one timer. The timer generates an interrupt every 4 ms. The interrupt routine checks for a change on the input pin. If there is a change, a constant value is added to an accumulator or "bucket." In either case, pin change or not, the accumulator is multiplied by a value less than one. The value in the accumulator represents the RPM of the rotor generating the signals. The advantages of this technique include: regularly spaced interrupts, no division operations that could result in a divide by zero error, no timing problems with RPMs near zero, provides a cap on the maximum RPM, and no subtraction operation that could result in a negative value. It appears to be a robust way to measure RPM. It is best to use signals with approximately 50% duty cycle. Variations in the duty cycle might cause problems if used to measure frequencies near the theoretical maximum.