The obvious application of a frequency meter is measuring the rate at which a certain variable changes. For instance, if a magnetic or optical detector is tied to a wheel, then the frequency of generated pulses corresponds to the speed a vehicle moves - in this case the frequency meter would function as an analog speedometer. Fluid flow can be rather precisely measured in a similar way - by immersing a freely revolving propeller into the flow and then measuring the frequency of pulses a magnetic or optical sensor mechanically connected to it produces. Results of experiments performed on a home brewed air flow meter are presented in another article within this web site.

But practical uses for reliable frequency meters such as this one span a much wider range of analog real world physical properties such as temperature, humidity, light intensity, distance etc. The reason for this is that it is usually rather easy to construct an oscillator, the frequency of which is in some way dependent on the physical property being measured. For example, there are fine linear relative humidity sensors produced in a form of variable capacitors, such that their capacitance depends on relative content of H₂O in air. The preferred method of utilysing one of them is to construct an oscillator so that its frequency depends primarily on sensor capacitance. Since such sensors are often mounted outside, one needs a frequency-to-voltage converter (i.e. frequency meter) in order to display the measured rH to the end user.

An important property of using the described two step approach to measurement instead of a single step ones like directly converting e.g. a photodiode current to analog voltage is that digital pulses produced by oscillators are very robust when one needs to transfer them over long distances - if signal shape is of no concern but the only property that carries useful information is pulse frequency, such signal will be highly immune to not simply electronic noise and variations in transfer channel properties (wire resistance etc.) but as well to all kinds of external interferences. Further, in contrast to analog ones, digital signals can easily be passed through inexpensive optocouplers if electrical insulation is desirable. As an illustration, take a look at Light Intensity and Direction sensor project elsewhere on these pages.

Another reason for using measurement oscillators instead of amplifiers is that certain kinds of sensors can be degraded if connected to even the smallest amount of direct voltage. The already mentioned humidity sensitive capacitors are only one family of such touchy components. If a sensor like that is to be exploited, the only practical way of doing so is inserting it into some kind of an oscillator, so that only a certain alternating voltage exists between the two component leads. Another interesting example of DC non-tolerant device is Rain Alarm Sensor which measures electrical resistance between two sets of several mutually interleaved unshielded PCB traces. Any amount of DC current between the two sets of wet copper conductors would eventually lead to electrochemical degradation and sensor failure. So instead of measuring the surface resistance via a kind of an OP AMP based DC Ohm metering circuit, we connected the sensitive surface to a relaxation oscillator formed out of a single CD40106 gate and used the frequency of generated digital pulses as an indication of rain.

Read more on this in the final part of this article...