In this post we will talk about a couple easy yet very handy little circuits in the form of  frequency meter and capacitance meter using the ubiquitous IC 555.

How Capacitors Work

Capacitors are one of the main electronic components which come under the passive component family.

These are extensively used in electronic circuits and virtually no circuit can be built without involving these important parts.

The basic function of a capacitor is to block DC and pass AC or in simple words any voltage which is pulsating in nature will be allowed to pass through a capacitor and any voltage that’s not polarized or in the form of a DC will be blocked by a capacitor through the process of charging.

Another important function of capacitors is storing electricity by way of charging and supplying it back to an attached circuit by the process of discharging.

The above two main functions of capacitors are used for implementing a variety of crucial operations in electronic circuits which enable getting outputs as per the required specifications of the design.

However unlike resistors, capacitors are difficult to measure through ordinary methods.

For example, an ordinary multitester might have many measuring features included like an OHM meter, voltmeter, ammeter, diode tester, hFE tester etc. but might just not have the illusive capacitance measuring feature.

The feature of a capacitance meter or an inductance meter is seen to be available only in high-end type of multimeters which are definitely not cheap and not every new hobbyist might be interested in procuring one.

The circuit discussed here very effectively tackles these issues and shows how to build a simple inexpensive capacitance cum frequency meter which can be built at home by any electronic novice and used for the intended useful application.

Circuit Diagram


IC 555 based Capacitance Meter Circuit






IC 555 based Inductance Meter Circuit

How Frequency Works to Detect Capacitance


Referring to the figure, the IC 555 forms the heart of the entire configuration.

This work horse versatile chip is configured in its most standard mode that is the monostable multivibrator mode.
Every positive peak of the pulse applied at the input that is pin #2 of the IC creates a stable output with some predetermined fixed period set by the preset P1.

However for every fall in the peak of the pulse, the monostable resets and auto triggers with the next arriving peak.

This generates a kind of an average value at the output of the IC for which is directly proportional to the frequency of the applied clock.

In other words the output of the IC 555 which consists of a few resistors and capacitors integrates the series of pulses to provide a stable average value directly proportional to the applied frequency.

The average value can be easily read or displayed over a moving coil meter connected across the shown points.

So the above reading will give a direct reading of the frequency, so we have a neat looking frequency meter at our disposal.

Using Frequency to Measure Capacitance


Now looking at the next figure we can clearly see that by adding an external frequency generator to the previous circuit, it becomes possible to make the meter interpret the values of a capacitor across the indicated points, because this capacitor directly affects or is proportional to the frequency of the clock circuit.

Therefore, the net frequency value now shown at the output will correspond to the value of the capacitor connected across the above discussed points.

That means now we have a two in one circuit which can measure capacitance as well as frequency, using just a couple of ICs and some casual electronic parts. With little modifications the circuit can be easily used as a tachometer or as RPM counter equipment.

Parts List


R1 = 4K7

R2 = 47E

R3 = CAN BE VARIABLE 100K POT

R4 = 3K3,

R5 = 10K,

R6 = 1K,

R7 1K,

R8 = 10K,

R9 = 100K,

C1 = 47n,

C2 = 100n,

C3 = 100n,

C4 = 33uF/25V,

T1 = BC547

IC1 = 555,

N1---N6 = IC4049

M1 = 1V FSD meter,

D1,D2 = 1N4148

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