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.
Contents
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
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.
Parts List
- 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,
- C5 = 2.2uF/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 modifies linearly based on the value of Cx, the value of R2/R3 and the frequency (established by the S1 switch position).
The final range selector switches S3 (1x) and S2 (3x) 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.
PCB Design
Another Simple Yet Accurate Capacitance Meter Circuit
When a constant-voltage is applied to a capacitor through a resistor, the capacitor charge increases in an exponential manner. But if the supply across a capacitor is from a constant current source, the charge on the capacitor exhibits an increases that is pretty much linear.
This principle in which a capacitor is charged linearly is used here in the below discussed simple capacitance meter. It is designed to measure capacitor values well beyond the range of many similar analogue meters.
Using a constant-current supply, the meter establishes the time it requires to complement the charge over the unknown capacitor to some known reference voltage. The meter provides 5 full-scale ranges of 1,10, 100, 1000, and 10,000 µF. On the 1-µF scale, capacitance values as tiny as 0.01 µF could be measured without difficulty.
How It Works.
As displayed in Figure, parts D1, D2, R6, Q1 and one of the resistors across R1 to R5 provide 5 selection for the constant current supply through the switch S1A.
When S2 is held in the indicated position, this constant current is shorted to ground through S2A. When S2 is switched in the alternate selection, the constant-current is driven into the capacitor under test, across BP1 and BP2, which forces the capacitor charge in the linear mode.
Op amp IC1 is attached like a comparator, with its (+) input pin attached to R8, which fixes the reference voltage level.
As soon as the linearly increasing charge across the capacitor under test, reaches a few millivolts higher than (-) input pin of IC1, it instantly switches the comparator output from +12 volts to -12 volts.
This causes the output of the comparator to activate a constant-current source made using the parts D3, D4, D5, R10, R11, and Q2.
In case if S2A is switched to ground, just as S2B, this results in the shorting of the capacitor C1 terminals, turning the potential across C1 to zero. With S2 in the open condition, the constant-current pasing via C1 triggers the voltage across C1 to increase in a linear fashion.
When the voltage across the capacitor under test causes the comparator to toggle, results in the diode D6 to turn reverse biased. This action stops C1 from charging any further.
Since the charging of C1 only happens until the point where the comparator output status just changes-over, implies that the voltage developed across it should be directly proportionate to the capacitance value of the unknown capacitor.
To ensure that the C1 does not discharge while meter M1 measures its voltage, a high-impedance buffer stage, created using IC2, is incorporated for the meter M1.
Resistor R13 and meter M1 constitute a basic voltmeter monitor of around 1 V FSD. When needed, a remote voltmeter could be employed provided that it features a full-scale range of under 8 volts. (In case you incorporate this kind of external meter, make sure to set R8 on the 1-µF range, so that an accuratly identified 1-µF capacitor corresponds to a 1 volt reading.)
Capacitor C2 is utilized to counteract oscillation of the Q1 constant-current supply, and R9 and R12 are employed to guard the op amps in the event the supply DC is switched off during the time when the capacitor under test and C1 are being charged, or else they could start discharging through the op amps, leading to a damage.
Parts List
PCB Designs
How to Calibrate
Prior to supplying power to the capacitance meter circuit, use a fine screwdriver to adjust the meter M1 needle precisely to the zero level.
Position an accurately known capacitor around 0.5 and 1.0 µF at +/-5%. This would function as the "calibration bench mark."
Hook up this capacitor across BP1 and BP2 (positive side to BP1). Adjust the range switch S1 to the "1" placement (meter should display 1-µF full scale).
Position S2 to disconnect the ground lead from the two circuits (Q1 collector and Cl). The M1 meter will now begin an upscale movement and settle at a specific reading. Toggling S2 back must result in the meter to fall downward at the zero volt mark. Change S2 once more and confirm the upscale reading of the meter.
Alternatively jump S2 and fine-tune R8 until you find the meter showing the precise value of the 5% of the capacitor's calibration. The above just one calibration set-up will be quite sufficient for the remaining ranges.
Fernando Guadian says
good day sir, I have 3 questions about your circuit that uses 555 timers, what is the value of C5? is D3 a simple diode? and the capacitance meter is capable of measuring from 100 to 1000 microfarads? Thanks for your attention and greetings from CD. juarez.
Swagatam says
Thank you Fernando, C5 can be a 2.2uF/25V capacitor, D3 can be a ordinary 1N4148 diode. This circuit will not measure above 100uF/25V, but it will measure all capacitors below this value.
Fernando Guadian says
Good day engineer, I really appreciate your answer and I wish you success in your projects, I have a question about your circuit that uses the 741 opam circuits, the switches S2A and S2B must be closed or open for the reading of the capacitor? this circuit if it has the range that I need to measure, thanks.
Swagatam says
Thank you Fernando, any one of the switches, either S2 or S3 must be closed, depending on the selected range by the switch S1a
Fernando Guadian says
good day engineer, I put together your circuit that uses the 741 ic’s and it doesn’t work for me, I made the connections as they are in the diagram when I turn it on and it gives me a voltage of 8 volts, when activating S2A and S2B the circuit responds like you it mentions the voltage rises or falls to 0 volts but it does not measure the capacitor, the 1k potentiometer does not move the reading, I have checked and I have not connected anything wrong, the transistors are equivalent to the 2N2905A as well as the 1N4148 diodes, do not use the other ranges only for a range from 100 to 1000 microfarads (with the 4.7k resistance to the emitter of Q1) what could I be doing wrong? Thanks a lot
Swagatam says
Hi, the circuit using IC 74121 is quite complex and elaborate, so it can be difficult for me to troubleshoot the fault in your circuit. Are you getting a varying frequency at the output of IC1 when you adjust S1a? Please confirm this….there are actually many steps and stages in the design that needs to confirmed and tested, which I cannot do without verifying your circuit practically.
I think you should try the second design which is much easier and a tested design by me.
Fernando Guadian says
hello engineer, unfortunately I don’t have a frequency meter to verify the output of IC1, this circuit caught my attention since it can measure capacitance from 100 to 1000 microfarads, which is the range I need, in another message I had asked if the second circuit is the ic’s 555 could also measure that range and you said no, finally, is there any modification to the second circuit so that it can measure that capacitance? Thank you for your attention.
Swagatam says
Hello Fernando, if you can measure capacitors upto 1000uF then that is great! However, none of the above circuits are capable of measuring capacitors above 1000uF, and there’s no way to modify them since these are analogue circuits and have limitations, beyond which they cannot operate.
Walt Sommers says
Thank, for the service you are providing.
I only wish you were there while I was working my way through my work life.
Really enjoy your help to others.
I’m an older Electronics tech ( 82 ) still interested in electronics and many other things. Best Regards
Walt Sommers
Swagatam says
Thanks Walt, I appreciate your thoughts very much. I too would have surely enjoyed your company! Please keep up the good work!
ARULOLI says
Hi sir,
Do you supply kit (pcb and components) for yours 555 based Capacitance meter?
Swagatam says
Hi Aruli, sorry I do not supply kits, but I can show a practical demo of a working circuit soon..
Andrew Borg says
Sir 33ohms or 33k? Do you have printout of circuit I mean as it should become on board? Thks.
Swagatam says
which part are you referring to?
Andrew Borg says
Hi Swag thks for response. I was going to build Brighthub circuit but noticed it’s a bit different. This circuit is more modern? Do you have a pcb for it? Thks.
Swagatam says
You are welcome Andrew, I am sorry I do not have a PCB design for this, I wish I could do it for you, however due to lack of time I am not able to do it at the moment…
Andrew Borg says
Hi Swag ok I did this circuit. The thing is that I using a volume audio meter don’t know if this is good? Cause when I tried it nothing came.
Swagatam says
Andrew, the recommended meter is 0-1V FSD moving coil meter, not sure how other meters might work…. this circuit has been thoroughly tested by me.
Abhishek Shende says
Sir, I want to ask you that can I replace fsd meter with seven segment display? If yes how can I do that . can you please tell me procedure… Thanks
Swagatam says
Abhishek, a digital meter might not work properly in the above design, only a moving coil type meter is recommended.
Subhra Sankar Misra says
IS 1V FSD METER MEANS 0-1 VOLT DC VOLTMETER OR AC VOLTMETER?
Swagatam says
it's 0 to 1V DC
mawuli dugbartey says
hi
Swagatam can digital multi-meter be use in place of the FSD meter and then tune the selection to DC voltage….thank you…
Swagatam says
I am sorry a DMM will not work correctly, because the output could be in a continuously vibrating mode which may be efficiently countered only by a moving coil meter
imanul sofian says
Dear Mr Swagatam,
Do you have a schematic about ESR meter. Technicians recommend me to check the electrolytic first everytime I come up with a dead circuit, But I dont know how to measure it. Thank you in advance for your answer.
Swagatam says
Dear Imanul, presently I do not have it in my blog, but will possibly design it and publish it soon for your reference.
AraChiya says
I need some clarity on the resister values, you have 4K7 and 4E7, is this 47K and 47 ohm?
Swagatam says
4k7 = 4.7K ohms
4E7 = 4.7 ohms
Tanmay Deshmukh says
Hello sir,
1v fsd meter is not available in my area but 6v or up to 6 v fsd are available.Have u any alternate working idea by using another ones.pls help us.
Swagatam says
hello Tanmay,
1V is the recommended one, for other higher voltages you may need to reduce R7/R8 proportionately for getting the deflections.
Ramkumar dewangan says
Hello sir, will deflection shows for almost ranges of pF, nF, uF,…..
Another: sir may u pls reffer about components in above diag. mainly upon which quantity of deflection depends?
Swagatam says
Hello Ramkumar, the range will need to be calibrated by appropriately setting or selecting the value of R3.
please refer to the following article to know more details about the components and the functioning:
easy-electronic-circuits.blogspot.in/2014/02/simple-tachometer-circuit-or-revolution.html
kashyap anil says
FSD meter is not showing any deflection. Voltage at FSD meter is 2.2 mV. Voltage at unknown capacitance 7.8 mV. What is the range of this capacitance meter ?
Swagatam says
I have provided the solution in the previous comment.
nobert singh says
Greetings
Can i use my multimeter instead of M1?
Swagatam says
sorry, no digital multimeters will not work here….
Renu Rautela says
dear sir,
i try to make this circuit in practical . but fails to get any output deflection on fsd when capacitance is applied. please tell me how to rectify the problem.
i am getting 2.2 mv in at FSD and 7.8 mV at unknown capacitance side.
please tell me the range of above meter also.
please reply soon.
I am struck.
Swagatam says
Dear Renu,
Use a 100k pot in place of R3 and adjust it for calibrating the scale of the connected meter. I have tested this circuit and it worked well for me.
Renu Rautela says
Sir,
I cant find the solution. its not working. R3 didn't help me.
i wanna share my circuit photograph with you. Please suggest some other option for rectifying my problem. I am using voltmeter as FSD meter.
What is the range of this circuit.
Give me another alternative to talk to you for rectifyng my problem.
i will be highly obiliged.
thank you for your prompt respose.
Renu
Swagatam says
which meter have you used at the output? you must use a 0-1V FSD moving coil type meter.
The frequency range is 10Hz to 10kHz
You can send the pictures to my iD hitman2008@live.in
kashyap anil says
I am using digital multimeter for this. As i want to know the correct of voltage output.
I am getting output but its showing me almost same value for most of the different capacitor.
I want to mesure nF capacitance can it be possible with your circuit?
Swagatam says
try different value capacitors and try to find the corresponding full scale range of the meter, you can also take the help of R3 by adjusting it to locate the range of the meter which corresponds to the capacitor value.
yes nF can be measured with this meter, according to me only moving coil type meters will work correctly here, i am not sure about DMMs.
Debashees Sikder says
Dear Sir,can I use this circuit for as like oscillator meter ? Thank you sir.
Swagatam says
Dear Debashees, what's an oscillator meter?? Did you mean frequency meter?
Nazriya nazim says
Thank you very much sir, but how will i know about your newly posted article ? Where should i have to search ??
I request you to construct a one having range of 1 to tens or hundreds of Mhz with seletable frequency ranges using common ics such as CD4040,4017,555 etc but excluding the use of programmable chips. The circuit should also be capable of measuring accurate frequencies. I am requesting such an accuracy since i have faced slight to serious variations of known frequency levels ( which may have occured due to variations in the duty cycle of the waves, inability to detect high voltage peaks of the signal by the ics due to poor signal conditioning etc. )
Swagatam says
Hello Nazriya, I'll inform you under this comment itself as soon as it gets posted, yes it will be very accurate.
Nazriya nazim says
Sir could you please suggest a suitable frequency divider circuit which can be used in corporation with a digital fequency meter ( which is capable of measuring a maximum of 999 Hz )to measure upto tens or hundreds of MHz by caliberations.
The divider should have this range
f/10 to f/10,000 indicated by respective frequency ratio switches
Swagatam says
Nazriya, yes it can be done either by using many 4017 ICs or a few 4040 ICs. I'll try to post a related article possibly soon in my blog.
Nazriya nazim says
Sir i have tested the basic circuit diagram without connecting any galvanometer or binary indicator. On measuring voltages by feeding a 1 Khz wave, i got approx. 0.12 avg. Voltage. On feeding 10 Khz i got 0.4V etc. I understood that voltage increases on increase in freq. But the problem is , very small voltages in small frequency. Actually i want to make such a frequency meter which can be used to measure atleast 50 hz and therefore can be used for the testing at various construction steps of inverter like projects. I don' t need a one with LED displays. I am interested in implementing the above circuit with IC LM3914 but what to do at that lowest voltage levels???? Will U PLZ HELP ME????
Swagatam says
Nazriya, the above circuit can be used to measure right from 10Hz to 10kHz….you will have to adjust R4/R5 also appropriately for the different ranges of the frequencies…the circuit will not work with a single setting for all the ranges as is true for even highly sophisticated instruments.
By the way the LM3914 just needs 1 to 5V as input for providing the desired output display, in fact you can tweak the input preset of the LM3914 even to sense voltages as low as 0.2V
Nazriya says
Sir if i am gonna use an LED dot indicator using IC LM3914, will it perfectly function as a frequency meter on better caliberations???
Actually what change is fed to the galvanometer leads in the circuit provided by you??? Is it a voltage change in accordance with frequency???? I am asking you this, because i will be able to implement the ic instead of galvanometer iff voltage fed to it is varied…… The biggest doubt having is, WHETHER THE OUTPUT VOLTAGE AVAILABLE FROM THE IC 55 WILL BE ENOUGH TO DRIVE THE IC3914?
Nazriya nazim says
Sir can i use a voltmeter found in submersible pump ( showing reading upto 300 V) be used after any suitable calliberation since 1 V FSD meter is not available in our area.
If it can't be done, can you please tell me how to employ a binary readout in place of this meter to measure frequency. I have the following ics with me
4026,4011,4017,4047,4007 and a few 555 and 556 ics and 741 opamps
Swagatam says
Nazriya, I don't think that would be feasible, because such voltmeters would have a coil with very high no of turns, not possible to modify them
A different simpler approach could be to use a LM3915 IC for getting an LED bar graph readout
Subhramanyam says
Sir i have found a simple digital freq. meter in the following link. Please just check it out…
http://www.eeweb.com/blog/circuit_projects/1hz-to-1mhz-frequency-meter-with-digital-display
But the 4 Bit microcomputer ic 4583 is not readilly available anywhere. Can i substitute it with a 4 bit single chip microcomputer ic LC6520
Swagatam says
Subhramanyam, I don't think 4583 could be interchanged with LC6520, because all IC have different specs, and application procedures.
Subhramanyam says
Dear Swagatham,
First of all i am congradulating you for this blog being a good success. The peoples like me who are just beginners in electronics can make use of many innovative ideas from here.
Now i am coming into the subject. I have read this article and impressed much. But it would have been better to modify this with a digital reader using 4 no.s of seven segment displays instead of the analog set up. Can u please help me?
Swagatam says
Dear Subramanyam,
Thank you very much! Yes It's possible to get a digital readout from the above circuit, we can do it by integrating the following circuit with it:
https://www.homemade-circuits.com/2013/05/make-this-simple-digital-voltmeter.html
avijeet agrawal says
Hello sir,
I had tried to make the above circuit for capacitor meter.i had used old vu meter for 1v fsd meter.but when i had tested it with 1uf nonelectrolyte caps its showing me a very little deflection which is not enough to clibrate the scale.and i also want to know the use of r3
(can be variable 100k )and how it is connected i.e.to use only two tip or to use all three.meter needle is giving vibrating reading.i think r8 should be reduced
Please help me
Thanks
Swagatam says
Hello Avijeet,
Try different values for C2, R3/R4, this will produce higher deflections