Measuring frequency is one of the prime requirements for many applications. The most obvious method of measuring frequency is using CRO (or now a days using DSO). But this instrument is not handheld or available with all the students or hobbyists at any time. It is actually laboratory instrument and not the portable one. Also it is costlier. Also conventional CRO does not give the direct frequency value read out. One has to first set the waveform then find time/division and finely calculate time period and frequency. So it’s a long process that takes time. Another way is use frequency counter that will give us direct digital readout of frequency. In this no need of first adjusting the waveform into screen then find time / division and then calculate time period and frequency such as CRO. Just apply the signal input and get the read out of frequency. That’s why frequency counters finding their own place in measuring instruments.
RPM counter also finds its importance in so many industries as it measures current RPM of any motor. Measuring current RPM of the motor and based on that take decision like to increase RPM or decrease it, is the prime requirement. In certain application it is required to maintain the RPM of motor within desirable limits. So in that the RPM is continuously measured and taken as feedback. If this value deviates much out of the limits, immediate actions can be taken.
So here I am presenting a single circuit to measure frequency as well as RPM. This circuit can alternatively measures frequency or RPM using a switch. If switch is connected to
· direct signal input - it measures frequency
· Sensor (opto-interrupt) output – it measures RPM.
Also it measures either frequency or RPM only once or in continuous loop after every 5 second. Let us see some other features of this circuit
· measures frequency from 50 Hz to 65 KHz
· measures RPM from 5 to 9999
· uses non contact type RPM sensor
· digital output with text on 16x2 LCD
· option for continuous measurement
Let us first see the block diagram of the system
The major building blocks are wave shaping circuit, sensor, micro controller and LCD
Wave shaping circuit: - It converts any input signal to pulses (unipolar square or rectangular) wave. Its output is between 0 to 5 V. So actually it converts the input signal shape into the type of shape that can be recognized by micro controller.
Opto interrupt sensor: - it converts each revolution of motor into a pulse. So we shall directly get the results as number of revolutions = number of pulses.
Micro controller: - it counts number of pulses (either from sensor or a direct signal) per fixed time (1 sec or 10 sec) and calculates frequency or RPM and displays them on LCD
LCD: - it is used to display text messages as well as the read outs of either frequency or RPM
Along with all these there are two SPDT switch inputs that selects frequency or RPM counter and decides to take single measurement or continuous in loop. One LED is also provided to indicate counting process is on.