2 Simple Capacitance Meter Circuits Explained – Using IC 555 and IC 74121

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.

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.

Accurate Capacitance Meter Circuit using IC 74HC132

Whenever the aforementioned circuit is linked to an unknown capacitor and a DMM, a DC voltage is supplied to the DMM. The output that follows shows the capacitance value on the DMM.
The DMM reading won't, however, directly display pF or uF. In accordance with the range that the circuit is adjusted to, it will only provide a value for you to understand (the circuit has two ranges for the selections).

How to Calibrate

You will require a 1000-pF and a 1uF capacitor to calibrate the circuit. As much as feasible, choose calibration capacitors whose values you have precisely measured.

Hookup PL1 and PL2 to the 2 volt scale on the DMM. Next, move S2 to the Low position and tweak R6 until the voltmeter registers zero. Attach the 1000-pF capacitor to the terminals "capacitor under test" and set R1 such that a reading of 1 volt appears on the DMM.

After this, insert the 1uF capacitor to the "capacitor under test" terminals and turn S2 to the HIGH position.

Next, tweak potentiometer R3 so that it reads 1 volts on the DMM.

As soon as calibrated, the capacitance tester circuit can be used in the above identical manner to measure capacitors of unspecified values.

Plug the circuit output to your DMM and shift the range of the metre to the 2 volt range. Whenever a capacitor is connected to J1 and J2, the DMM's screen will display micro-farads in the "low range" and picofarads in the "high range".