In this post we will talk about a couple of 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.
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 below we can clearly see that by adding an external frequency generator (IC 555 astable) 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.
- R1 = 4K7
- R3 = CAN BE VARIABLE 100K POT
- R4 = 3K3,
- R5 = 10K,
- R6 = 1K,
- R7 1K,
- R8 = 10K,
- R9, R10 = 100K,
- C1 = 1uF/25V,
- C2, C3, C6 = 100n,
- C4 = 33uF/25V,
- T1 = BC547
- IC1, IC2 = 555,
- M1 = 1V FSD meter,
- D1,D2 = 1N4148
Capacitance Meter using IC 74121
This simple capacitance meter circuit provides 14 linearly calibrated capacitance measuring ranges, from 5 pF to 15 uF FSD. S1 is employed as a range switch, and operates in collaboration with S4 (s1/x10) and S3 (x l) or S2 (x3). The IC 7413 operates like an astable oscillator, together with R1 and C1 to C6 which act like the frequency determining elements.
This stage activates the IC 74121 (a monostable multivibrator) so that it generates an asymmetric square wave with a recurring frequency whse value is decided by the parts R1 and C1 to C6 and with a duty cycle as decided by R2 (or R3) and Cx.
The typical value of this square -wave voltage changes linearly as the duty cycle is changed, which in turn is modifies linearly based on the value of Cs, the value of R2/R3 (s10/x I) and the frequency (established by the S1 switch position).
The final range selector switches S3j..-xl) and 52 (x3) basically insert a resistor in series with the meter. The configuration around the pins 10 and pin 11 of the IC 74121, and for the Cx must be as short and stiff as is feasible, to ensure that stray capacitance here is minimal and without fluctuations. P5 and P4 are employed for independent zero calibration for low capacitance ranges. For all higher ranges, calibration done by oreset P3 is just sufficient. F.s.d. calibration is rather straightforward.
Do not initially solder C6 in circuit rather attach it over the terminals marked Cx for the unknown capacitor. Put S1 in position 3, S4 in position x1 and S2 closed (s3); this gets set up for the ranges of 1500 pF f.s.d. Now, C6 becomes ready to be applied as a calibration bench mark value. Next, pot P1 is tweaked until the meter deciphers 2/3 of f.s.d. Then, S4 could be moved to position ' x 10', S2 held open and S3 is closed (x1 ); this compares to 5000 pF f.s.d., while working with C6 as the unknown capacitor. The result for these complete set up should provide 1/5 of fs.d.
On the other hand you can procure an assortment of accurately known capacitors and use these across the Cx points and then adjust the various pots for fixing the calibrations on the meter dial appropriately.