After reading this article,
check out the improved, modified An Even Better
The 2 line x 16 character LCD shows the calculated inductance and
frequency. The frequency might be of interest because inductors with
cores can appear to
vary in inductance with changing test
Files for this project available for downloading:
To make the complete project you will need at a minimum both of the hex
files below. You can also download the source files if you wish to
The hex file containing code for the frequency meter - U3- ATTINY2313
or AT90S2313 2313LCmeter_070217A.hex
WINAVR C Compiler Source file 2313LCmeter_070217A.c
The WINAVR Project Folder
The hex file containing code for the LCD driver chip - U4- ATTINY2313
The assembly source code for the LCD driver chip LDCbutons040904C.asm
The assembly source code for the LCD driver chip updated for newer
The include file needed by the LCD driver chip
There must be half a dozen similar LC meters on the web. Why didn't I
just download one of them and use it? I suppose part of the
answer is that I wanted to do some of it myself. As nice as these other
projects are, I wanted to make an LC meter that others could copy and
get working without very much trouble. That meant that "exotic"
components need to be kept to a minimum, and to me, that means no
relays or hard-to-find special switches. It also means that the
complete design needs to be spelled out in sufficient detail to allow
anybody with basic assembly skills to put it together and make it work.
This instrument requires two precision components: A precision
a precision inductor. You only need to start with one precision
component, either the reference capacitor or the reference inductor,
and using this meter, you can select or adjust the other precision
In my case, I used a pretty high accuracy BK Precision
meter and sorted through piles of inductors and capacitors to find
that had the lowest error. I then used those parts, a 1 millihenry
inductor and a 0.01 microfarad capacitor, in this meter.
The basis of this project is several similar projects on the world wide
web and some magazine articles before the world wide web was a common
of information interchange. Unfortunately, I am not able to determine
originator(s) of the concept, but I suspect that it is as old as radio.
Another project on my web site, LC
Determination by Resonant
, measures the resonant frequency of an
L/C circuit, but the hardware stops at the frequency measurement. It
does not proceed to calculate the unknown inductance or capacitance.
To convert from frequency to inductance or capacitance requires a
pocket calculator or spreadsheet, and is
fine if only taking a few measurements per day. Beyond that, the manual
labor seems a bit much, and the time to complete a measurement and
calculation becomes burdensome. As I found
myself spending more time winding transformers in my Thailand lab for
various power supplies that I longed for a meter that would read out
directly in inductance and capacitance, such as I had in my
lab in Arizona.
The only difficulty in getting the instrument to display inductance and
capacitance directly was in organizing the assembly language code,
and probably more importantly, properly scaling the operations. I had
looked into it, and it seems to be too much trouble. It would take
about as much time and energy to add this feature to the LC meter as it
would to learn to use a C compiler. So, that's why this project is
written in C.
Please excuse the fact that in some ways the source code looks more
like assembly than C. This will improve with time.
It was a true
pleasure to type:
and have the calculation performed and the result returned with high
precision. It is a small difference from organizing assembly language
code to do the same, but it is such small things that changes the way
we do things. Similarly, debugging was easier in C than in
assembly language because I needed to look at and manipulate large
numbers during debugging.
Basic Theory of Operation
An LC oscillator oscillates at the resonant frequency of a parallel LC
resonant circuit. When measuring an inductor, a precision capacitor is
switched in to the circuit. When measuring a capacitor, a precision
inductor is switched into the circuit.
If the Q of the resonant circuit is about 10 or greater, the
measurement error contributed by this factor will be less than 1%. Q is
the comparison of the losses in the circuit, often the result of
resistance in the circuit, with the reactance of the inductor and
capacitor. The Q of good quality capacitors is usually not a problem -
just keep away from ceramic and electrolytic capacitors and you
shouldn't have a problem. The Q of the inductor is the one to watch out
for. The first inductor that I used had a Q of 3 when measuring a 0.47
uf capacitor, and that made the error nearly 20%!
Q of the inductor is of greatest concern at the lowest operating
frequency. In this application, this corresponds to the situation in
which the highest value capacitor is being measured. A 1 mH
inductor resonates with a 1 uf capacitor at 5.035 kHz. The reactance of
the inductor is 6.28 x 5.035 kHz x 1mH = 31.6 Ohms. In order to keep
the error contributed by the Q of the coil to less than 1% of the
measured value, the Q must be 10, so the resistance of the resonant
circuit must be less than 1/10 of the reactance, or 3.16 Ohms. You can
check the resistance of inductors you are considering for use as L2
with an Ohmmeter to find its resistance.
Before measuring, the circuit's offset reactance needs to be measured
and removed, using a zero set procedure. Turn the meter on for a few
seconds, to allow enough time for the analog circuitry and the readings
to settle, then if measuring an inductor, short the Lx/Cx terminals
together, or if measuring a capacitor, leave the terminals open.
Press the ZERO SET button and hold it down for a couple of measurement
cycles. The LED blinks one time each measurement cycle. Release the
ZERO SET button, and after the current measurement cycle is completed,
the meter should read zero.
A convenient alternative means of shorting the Lx/Cx terminals during
ZERO SET for an inductance measurement is to press and hold the SHORT
button during the ZERO SET procedure.
• Inductance from 0.1 uH to 50 H with 0.1 uH resolution. Note:
High resistance in inductors contributes to measurement error.
• Capacitance from 1 pf to 1 uf with 1 pf resolution. (Note:
Resistance of the reference inductor limits accuracy with large
• The pilot light blinks once during the transmission of the results
• Resonant frequency is displayed in Hz.
• Measurement interval is approximately 1.1 seconds.
The actual range of the firmware extends to 300 Microfarads and 300
Henries, but with the component values in the present oscillator
circuit, the oscillator is not capable of operating that slow. For
values above about 1 microfarads and 50 Henries, I suggest increasing
the values of C1, C2, C3, and L2, and increasing the frequency
measurement time from one second to 10 seconds or more.
Accuracy is, for the most part, dependent upon the accuracy of the
reference inductor, the reference capacitor, and the 4 MHz clock
oscillator for U3. Resolution is not the same as accuracy. Note that
you could measure a 25 Henry inductor and display will show it in 100
nanohenry increments (as an example: 25074164.6). The accuracy of the
measurement could probably only be on the order of 1%, and take note of
the frequency: at 25 Henrys, the oscillator only runs at 318 Hz and is
sampled for only one second, so the granularity of the measurement,
that is the smallest change in the measured value that can be detected,
is found by taking larger of the difference in frequencies 1 Hz above
1 Hz below 318 Hz, compared to 318 Hz.
Inductance corresponding to 317 Hz = 25.232 H +248 mH
Inductance corresponding to 318 Hz = 25.074 H
Inductance corresponding to 319 Hz = 24.917 H
The resistance of the inductor will affect the inductance measurement
slightly. I have some pc mounted power supply chokes with very poor Q -
about 100 oHms for the 10 millihenry inductor. Its measurement seems to
be off about 5%. Capacitors tend to have much higher Q than inductors,
and unless you are making your own using low Q dielectric, it would not
be worth worrying about how the Q of capacitors affects the measurement.
This meter assumes a lumped inductor and a lumped capacitor coupled
directly against each other. If there is a lot of capacitance
distributed over the inductance, such as in a delay line or
transmission line, the resulting measurement will not be meaningful in
terms of inductance of capacitance. This means that this is no the
appropriate tool to measure the inductance of that 75 meter diameter
lowfer loop antenna behind your house.
On the subject of long test leads, I have noticed that my meter is
susceptible to interference from my computer when I tried to use the
meter right next to the computer. This is not a great surprise since
the computer has two intentional transmitters operating (Bluetooth and
WI-Fi), and of course some lesser incidental noise sources. Even if the
meter were to be shielded, the test terminals, and any test leads
connected to them can pick up noise.
and Assembly Overview
The circuit is composed of a comparitor-based oscillator (U1), a
microcontroller that measures the frequency and calculates the
inductance or capacitance from the frequency (U3), and a
microcontroller that takes the output from the first controller to
drive a Liquid Crystal Display (U4).
Physically, the meter breaks down into these subassemblies:
The 2 line X 16 character liquid crystal display,
the Oscillator board,
the microcontroller board, and
the front panel wiring harness that ties it all together.
Everything but the battery is mounted on the top of the case. The
oscillator is permanently mounted onto the front panel by virtue
of its being soldered onto the banana plugs. Copper was cut away
to form an isolated pad for the ungrounded banana plug. The wiring
onto the oscillator board, the buttons, the L/C switch, and the battery
clip. The microcontroller board at the top, plugs directly into the
LCD, which is covered by the board in this picture.
2 line X 16 character liquid crystal display